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Tuesday, March 26, 2024

Top 50 Java Programs from Coding Interviews

Interview Questions

Describe and compare fail-fast and fail-safe iterators. Give examples.

The main distinction between fail-fast and fail-safe iterators is whether or not the collection can be modified while it is being iterated. Fail-safe iterators allow this; fail-fast iterators do not.

Fail-fast iterators operate directly on the collection itself. During iteration, fail-fast iterators fail as soon as they realize that the collection has been modified (i.e., upon realizing that a member has been added, modified, or removed) and will throw a ConcurrentModificationException . Some examples include ArrayList , HashSet , and HashMap (most JDK1.4 collections are implemented to be fail-fast).

Fail-safe iterates operate on a cloned copy of the collection and therefore do not throw an exception if the collection is modified during iteration. Examples would include iterators returned by ConcurrentHashMap or CopyOnWriteArrayList .

ArrayList , LinkedList , and Vector are all implementations of the List interface. Which of them is most efficient for adding and removing elements from the list? Explain your answer, including any other alternatives you may be aware of.

Of the three, LinkedList is generally going to give you the best performance. Here’s why:

ArrayList and Vector each use an array to store the elements of the list. As a result, when an element is inserted into (or removed from) the middle of the list, the elements that follow must all be shifted accordingly. Vector is synchronized, so if a thread-safe implementation is not needed, it is recommended to use ArrayList rather than Vector.

LinkedList , on the other hand, is implemented using a doubly linked list. As a result, an inserting or removing an element only requires updating the links that immediately precede and follow the element being inserted or removed.

However, it is worth noting that if performance is that critical, it’s better to just use an array and manage it yourself, or use one of the high performance 3rd party packages such as Trove or HPPC .

Why would it be more secure to store sensitive data (such as a password, social security number, etc.) in a character array rather than in a String?

In Java, Strings are immutable and are stored in the String pool. What this means is that, once a String is created, it stays in the pool in memory until being garbage collected. Therefore, even after you’re done processing the string value (e.g., the password), it remains available in memory for an indeterminate period of time thereafter (again, until being garbage collected) which you have no real control over. Therefore, anyone having access to a memory dump can potentially extract the sensitive data and exploit it.

In contrast, if you use a mutable object like a character array, for example, to store the value, you can set it to blank once you are done with it with confidence that it will no longer be retained in memory.

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What is the ThreadLocal class? How and why would you use it?

A single ThreadLocal instance can store different values for each thread independently. Each thread that accesses the get() or set() method of a ThreadLocal instance is accessing its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or transaction ID). The example below, from the ThreadLocal Javadoc , generates unique identifiers local to each thread. A thread’s id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

What is the volatile keyword? How and why would you use it?

In Java, each thread has its own stack, including its own copy of variables it can access. When the thread is created, it copies the value of all accessible variables into its own stack. The volatile keyword basically says to the JVM “Warning, this variable may be modified in another Thread”.

In all versions of Java, the volatile keyword guarantees global ordering on reads and writes to a variable. This implies that every thread accessing a volatile field will read the variable’s current value instead of (potentially) using a cached value.

In Java 5 or later, volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex.

Using volatile may be faster than a lock, but it will not work in some situations. The range of situations in which volatile is effective was expanded in Java 5; in particular, double-checked locking now works correctly.

The volatile keyword is also useful for 64-bit types like long and double since they are written in two operations. Without the volatile keyword you risk stale or invalid values.

One common example for using volatile is for a flag to terminate a thread. If you’ve started a thread, and you want to be able to safely interrupt it from a different thread, you can have the thread periodically check a flag (i.e., to stop it, set the flag to true ). By making the flag volatile, you can ensure that the thread that is checking its value will see that it has been set to true without even having to use a synchronized block. For example:

Compare the sleep() and wait() methods in Java, including when and why you would use one vs. the other.

sleep() is a blocking operation that keeps a hold on the monitor / lock of the shared object for the specified number of milliseconds.

wait() , on the other hand, simply pauses the thread until either (a) the specified number of milliseconds have elapsed or (b) it receives a desired notification from another thread (whichever is first), without keeping a hold on the monitor/lock of the shared object.

sleep() is most commonly used for polling, or to check for certain results, at a regular interval. wait() is generally used in multithreaded applications, in conjunction with notify() / notifyAll() , to achieve synchronization and avoid race conditions.

Tail recursion is functionally equivalent to iteration. Since Java does not yet support tail call optimization, describe how to transform a simple tail recursive function into a loop and why one is typically preferred over the other.

Here is an example of a typical recursive function, computing the arithmetic series 1, 2, 3…N. Notice how the addition is performed after the function call. For each recursive step, we add another frame to the stack.

Tail recursion occurs when the recursive call is in the tail position within its enclosing context - after the function calls itself, it performs no additional work. That is, once the base case is complete, the solution is apparent. For example:

Here you can see that a plays the role of the accumulator - instead of computing the sum on the way down the stack, we compute it on the way up, effectively making the return trip unnecessary, since it stores no additional state and performs no further computation. Once we hit the base case, the work is done - below is that same function, “unrolled”.

Many functional languages natively support tail call optimization, however the JVM does not. In order to implement recursive functions in Java, we need to be aware of this limitation to avoid StackOverflowError s. In Java, iteration is almost universally preferred to recursion.

How can you swap the values of two numeric variables without using any other variables?

You can swap two values a and b without using any other variables as follows:

How can you catch an exception thrown by another thread in Java?

This can be done using Thread.UncaughtExceptionHandler .

Here’s a simple example:

What is the Java Classloader? List and explain the purpose of the three types of class loaders.

The Java Classloader is the part of the Java runtime environment that loads classes on demand (lazy loading) into the JVM (Java Virtual Machine). Classes may be loaded from the local file system, a remote file system, or even the web.

When the JVM is started, three class loaders are used: 1. Bootstrap Classloader: Loads core java API file rt.jar from folder. 2. Extension Classloader: Loads jar files from folder. 3. System/Application Classloader: Loads jar files from path specified in the CLASSPATH environment variable.

Is a finally block executed when an exception is thrown from a try block that does not have a catch block, and if so, when?

A finally block is executed even if an exception is thrown or propagated to the calling code block.

Output can vary, being either:

When designing an abstract class, why should you avoid calling abstract methods inside its constructor?

This is a problem of initialization order. The subclass constructor will not have had a chance to run yet and there is no way to force it to run it before the parent class. Consider the following example class:

This seems like a good start for an abstract Widget: it allows subclasses to fill in width and height , and caches their initial values. However, look when you spec out a typical subclass implementation like so:

Now we’ve introduced a subtle bug: Widget.cachedWidth and Widget.cachedHeight will always be zero for SquareWidget instances! This is because the this.size = size assignment occurs after the Widget constructor runs.

Avoid calling abstract methods in your abstract classes’ constructors, as it restricts how those abstract methods can be implemented.

What variance is imposed on generic type parameters? How much control does Java give you over this?

Java’s generic type parameters are invariant . This means for any distinct types A and B , G<A> is not a subtype or supertype of G<B> . As a real world example, List<String> is not a supertype or subtype of List<Object> . So even though String extends (i.e. is a subtype of) Object , both of the following assignments will fail to compile:

Java does give you some control over this in the form of use-site variance . On individual methods, we can use ? extends Type to create a covariant parameter. Here’s an example:

Even though longs is a List<Long> and not List<Number> , it can be passed to sum .

Similarly, ? super Type lets a method parameter be contravariant . Consider a function with a callback parameter:

forEachNumber allows Callback<Object> to be a subtype of Callback <Number> , which means any callback that handles a supertype of Number will do:

Note, however, that attempting to provide a callback that handles only Long (a subtype of Number ) will rightly fail:

Liberal application of use-site variance can prevent many of the unsafe casts that often appear in Java code and is crucial when designing interfaces used by multiple developers.

What are static initializers and when would you use them?

A static initializer gives you the opportunity to run code during the initial loading of a class and it guarantees that this code will only run once and will finish running before your class can be accessed in any way.

They are useful for performing initialization of complex static objects or to register a type with a static registry, as JDBC drivers do.

Suppose you want to create a static, immutable Map containing some feature flags. Java doesn’t have a good one-liner for initializing maps, so you can use static initializers instead:

Within the same class, you can repeat this pattern of declaring a static field and immediately initializing it, since multiple static initializers are allowed.

If one needs a Set , how do you choose between HashSet vs. TreeSet ?

At first glance, HashSet is superior in almost every way: O(1) add , remove and contains , vs. O(log(N)) for TreeSet .

However, TreeSet is indispensable when you wish to maintain order over the inserted elements or query for a range of elements within the set.

Consider a Set of timestamped Event objects. They could be stored in a HashSet , with equals and hashCode based on that timestamp. This is efficient storage and permits looking up events by a specific timestamp, but how would you get all events that happened on any given day? That would require a O(n) traversal of the HashSet , but it’s only a O(log(n)) operation with TreeSet using the tailSet method:

If Event happens to be a class that we cannot extend or that doesn’t implement Comparable , TreeSet allows us to pass in our own Comparator :

Generally speaking, TreeSet is a good choice when order matters and when reads are balanced against the increased cost of writes.

What are method references, and how are they useful?

Method references were introduced in Java 8 and allow constructors and methods (static or otherwise) to be used as lambdas. They allow one to discard the boilerplate of a lambda when the method reference matches an expected signature.

For example, suppose we have a service that must be stopped by a shutdown hook. Before Java 8, we would have code like this:

With lambdas, this can be cut down considerably:

However, stop matches the signature of Runnable.run ( void return type, no parameters), and so we can introduce a method reference to the stop method of that specific SomeBusyService instance:

This is terse (as opposed to verbose code) and clearly communicates what is going on.

Method references don’t need to be tied to a specific instance, either; one can also use a method reference to an arbitrary object, which is useful in Stream operations. For example, suppose we have a Person class and want just the lowercase names of a collection of people:

A complex lambda can also be pushed into a static or instance method and then used via a method reference instead. This makes the code more reusable and testable than if it were “trapped” in the lambda.

So we can see that method references are mainly used to improve code organization, clarity and terseness.

How are Java enums more powerful than integer constants? How can this capability be used?

Enums are essentially final classes with a fixed number of instances. They can implement interfaces but cannot extend another class.

This flexibility is useful in implementing the strategy pattern, for example, when the number of strategies is fixed. Consider an address book that records multiple methods of contact. We can represent these methods as an enum and attach fields, like the filename of the icon to display in the UI, and any corresponding behaviour, like how to initiate contact via that method:

We can dispense with switch statements entirely by simply using instances of ContactMethod :

This is just the beginning of what can be done with enums. Generally, the safety and flexibility of enums means they should be used in place of integer constants, and switch statements can be eliminated with liberal use of abstract methods.

What does it mean for a collection to be “backed by” another? Give an example of when this property is useful.

If a collection backs another, it means that changes in one are reflected in the other and vice-versa.

For example, suppose we wanted to create a whitelist function that removes invalid keys from a Map . This is made far easier with Map.keySet , which returns a set of keys that is backed by the original map. When we remove keys from the key set, they are also removed from the backing map:

retainAll writes through to the backing map, and allows us to easily implement something that would otherwise require iterating over the entries in the input map, comparing them against allowedKey , etcetera.

Note, it is important to consult the documentation of the backing collection to see which modifications will successfully write through. In the example above, map.keySet().add(value) would fail, because we cannot add a key to the backing map without a value.

What is reflection? Give an example of functionality that can only be implemented using reflection.

Reflection allows programmatic access to information about a Java program’s types. Commonly used information includes: methods and fields available on a class, interfaces implemented by a class, and the runtime-retained annotations on classes, fields and methods.

Examples given are likely to include:

Annotation-based serialization libraries often map class fields to JSON keys or XML elements (using annotations). These libraries need reflection to inspect those fields and their annotations and also to access the values during serialization.
Model-View-Controller frameworks call controller methods based on routing rules. These frameworks must use reflection to find a method corresponding to an action name, check that its signature conforms to what the framework expects (e.g. takes a Request object, returns a Response ), and finally, invoke the method.
Dependency injection frameworks lean heavily on reflection. They use it to instantiate arbitrary beans for injection, check fields for annotations such as @Inject to discover if they require injection of a bean, and also to set those values.
Object-relational mappers such as Hibernate use reflection to map database columns to fields or getter/setter pairs of a class, and can go as far as to infer table and column names by reading class and getter names, respectively.

A concrete code example could be something simple, like copying an object’s fields into a map:

Such tricks can be useful for debugging, or for utility methods such as a toString method that works on any class.

Aside from implementing generic libraries, direct use of reflection is rare but it is still a handy tool to have. Knowledge of reflection is also useful for when these mechanisms fail.

However, it is often prudent to avoid reflection unless it is strictly necessary, as it can turn straightforward compiler errors into runtime errors.

Nested classes can be static or non-static (also called an inner class). How do you decide which to use? Does it matter?

The key difference between is that inner classes have full access to the fields and methods of the enclosing class. This can be convenient for event handlers, but comes at a cost: every instance of an inner class retains and requires a reference to its enclosing class.

With this cost in mind, there are many situations where we should prefer static nested classes. When instances of the nested class will outlive instances of the enclosing class, the nested class should be static to prevent memory leaks. Consider this implementation of the factory pattern:

At a glance, this design looks good: the WidgetParserFactory hides the implementation details of the parser with the nested class WidgetParserImpl . However, WidgetParserImpl is not static, and so if WidgetParserFactory is discarded immediately after the WidgetParser is created, the factory will leak, along with all the references it holds.

WidgetParserImpl should be made static, and if it needs access to any of WidgetParserFactory ’s internals, they should be passed into WidgetParserImpl ’s constructor instead. This also makes it easier to extract WidgetParserImpl into a separate class should it outgrow its enclosing class.

Inner classes are also harder to construct via reflection due to their “hidden” reference to the enclosing class, and this reference can get sucked in during reflection-based serialization, which is probably not intended.

So we can see that the decision of whether to make a nested class static is important, and that one should aim to make nested classes static in cases where instances will “escape” the enclosing class or if reflection on those nested classes is involved.

What is the difference between String s = "Test" and String s = new String("Test") ? Which is better and why?

In general, String s = "Test" is more efficient to use than String s = new String("Test") .

In the case of String s = "Test" , a String with the value “Test” will be created in the String pool. If another String with the same value is then created (e.g., String s2 = "Test" ), it will reference this same object in the String pool.

However, if you use String s = new String("Test") , in addition to creating a String with the value “Test” in the String pool, that String object will then be passed to the constructor of the String Object (i.e., new String("Test") ) and will create another String object (not in the String pool) with that value. Each such call will therefore create an additional String object (e.g., String s2 = new String("Test") would create an addition String object, rather than just reusing the same String object from the String pool).

There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work .

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Review these 50 questions to crack your Java programming interview

by javinpaul

A list of frequently asked Java questions from programming job interviews.

Hello, everybody! Over the past few years, I have been sharing a lot of Java Interview questions and discussion individually. Many of my readers have requested that I bring them together so that they can have them in the same spot. This post is the result of that.

This article contains more than 50 Java Interview questions covering all important topics like Core Java fundamentals, Java Collection Framework , Java Multithreading and Concurrency , Java IO , JDBC , JVM Internals , Coding Problems , Object-Oriented programming , etc.

The questions are also picked up from various interviews and they are, by no means, very difficult. You might have seen them already in your phone or face-to-face round of interviews.

The questions are also very useful to review important topics like multithreading and collections. I have also shared some useful resources for further learning and improvement like The Complete Java MasterClass to brush up and fill gaps in your Java skills.

So what are we waiting for? Here is the list of some of the most frequently asked Java questions in interviews for both beginner and experienced Java developers.

50+ Java Interview Questions for 2 to 3 years Experienced Programmers

So, without wasting any more of your time, here is my list of some of the frequently asked Core Java Interview Question s for beginner programmers. This list focuses on beginners and less experienced devs, like someone with 2 to 3 years of experience in Java.

1) How does Java achieve platform independence? ( answer ) hint: bytecode and Java Virtual Machine

2) What is ClassLoader in Java? ( answer ) hint: part of JVM that loads bytecodes for classes. You can write your own.

3) Write a Java program to check if a number is Even or Odd? ( answer ) hint: you can use bitwise operator, like bitwise AND, remember, even the number has zero at the end in binary format and an odd number has 1 in the end.

4) Difference between ArrayList and HashSet in Java? ( answer ) hint: all differences between List and Set are applicable here, e.g. ordering, duplicates, random search, etc. See Java Fundamentals: Collections by Richard Warburton to learn more about ArrayList, HashSet and other important Collections in Java.

5) What is double checked locking in Singleton? ( answer ) hint: two-time check whether instances is initialized or not, first without locking and second with locking.

6) How do you create thread-safe Singleton in Java? ( answer ) hint: many ways, like using Enum or by using double-checked locking pattern or using a nested static class.

7) When to use the volatile variable in Java? ( answer ) hint: when you need to instruct the JVM that a variable can be modified by multiple threads and give hint to JVM that does not cache its value.

8) When to use a transient variable in Java? ( answer ) hint: when you want to make a variable non-serializable in a class, which implements the Serializable interface. In other words, you can use it for a variable whose value you don’t want to save. See The Complete Java MasterClass to learn about transient variables in Java.

9) Difference between the transient and volatile variable in Java? ( answer ) hint: totally different, one used in the context of serialization while the other is used in concurrency.

10) Difference between Serializable and Externalizable in Java? ( answer ) hint: Externalizable gives you more control over the Serialization process.

11) Can we override the private method in Java? ( answer ) hint: No, because it’s not visible in the subclass, a primary requirement for overriding a method in Java.

12) Difference between Hashtable and HashMap in Java? ( answer ) hint: several but most important is Hashtable , which is synchronized, while HashMap is not. It's also legacy and slow as compared to HashMap .

13) Difference between List and Set in Java? ( answer ) hint: List is ordered and allows duplicate. Set is unordered and doesn't allow duplicate elements.

14) Difference between ArrayList and Vector in Java ( answer ) hint: Many, but most important is that ArrayList is non-synchronized and fast while Vector is synchronized and slow. It's also legacy class like Hashtable .

15) Difference between Hashtable and ConcurrentHashMap in Java? ( answer ) hint: more scalable. See Java Fundamentals: Collections by Richard Warburton to learn more.

16) How does ConcurrentHashMap achieve scalability? ( answer ) hint: by dividing the map into segments and only locking during the write operation.

17) Which two methods you will override for an Object to be used as Key in HashMap ? ( answer ) hint: equals and hashcode

18) Difference between wait and sleep in Java? ( answer ) hint: The wait() method releases the lock or monitor, while sleep doesn't.

19) Difference between notify and notifyAll in Java? ( answer ) hint: notify notifies one random thread is waiting for that lock while notifyAll inform to all threads waiting for a monitor. If you are certain that only one thread is waiting then use notify , or else notifyAll is better. See Threading Essentials Mini-Course by Java Champion Heinz Kabutz to learn more about threading basics.

20) Why you override hashcode, along with equals() in Java? ( answer ) hint: to be compliant with equals and hashcode contract, which is required if you are planning to store your object into collection classes, e.g. HashMap or ArrayList .

21) What is the load factor of HashMap means? ( answer ) hint: The threshold that triggers the re-sizing of HashMap is generally 0.75, which means HashMap resize itself if it's 75 percent full.

22) Difference between ArrayList and LinkedList in Java? ( answer ) hint: same as an array and linked list, one allows random search while other doesn't. Insertion and deletion easy on the linked list but a search is easy on an array. See Java Fundamentals: Collections , Richard Warburton’s course on Pluralsight, to learn more about essential Collection data structure in Java.

23) Difference between CountDownLatch and CyclicBarrier in Java? ( answer ) hint: You can reuse CyclicBarrier after the barrier is broken but you cannot reuse CountDownLatch after the count reaches to zero.

24) When do you use Runnable vs Thread in Java? ( answer ) hint: always

25) What is the meaning of Enum being type-safe in Java? ( answer ) hint: It means you cannot assign an instance of different Enum type to an Enum variable. e.g. if you have a variable like DayOfWeek day then you cannot assign it value from DayOfMonth enum.

26) How does Autoboxing of Integer work in Java? ( answer ) hint: By using the valueOf() method in Java.

27) Difference between PATH and Classpath in Java? ( answer ) hint: PATH is used by the operating system while Classpath is used by JVM to locate Java binary, e.g. JAR files or Class files. See Java Fundamentals: The Core Platform to learn more about PATH , Classpath , and other Java environment variable.

28) Difference between method overloading and overriding in Java? ( answer ) hint: Overriding happens at subclass while overloading happens in the same class. Also, overriding is a runtime activity while overloading is resolved at compile time.

29) How do you prevent a class from being sub-classed in Java? ( answer ) hint: just make its constructor private

30) How do you restrict your class from being used by your client? ( answer ) hint: make the constructor private or throw an exception from the constructor

31) Difference between StringBuilder and StringBuffer in Java? ( answer ) hint: StringBuilder is not synchronized while StringBuffer is synchronized.

32) Difference between Polymorphism and Inheritance in Java? ( answer ) hint: Inheritance allows code reuse and builds the relationship between class, which is required by Polymorphism, which provides dynamic behavior. See Java Fundamentals: Object-Oriented Design to learn more about OOP features.

33) Can we override static method in Java? ( answer ) hint: No, because overriding resolves at runtime while static method call is resolved at compile time.

34) Can we access the private method in Java? ( answer ) hint: yes, in the same class but not outside the class

35) Difference between interface and abstract class in Java? ( answer ) hint: from Java 8 , the difference is blurred. However, a Java class can still implement multiple interfaces but can only extend one class.

36) Difference between DOM and SAX parser in Java? ( answer ) hint: DOM loads whole XML File in memory while SAX doesn’t. It is an event-based parser and can be used to parse a large file, but DOM is fast and should be preferred for small files.

37) Difference between throw and throws keyword in Java? ( answer ) hint: throws declare what exception a method can throw in case of error but throw keyword actually throws an exception. See Java Fundamentals: Exception Handling to learn more about Exception handling in Java.

38) Difference between fail-safe and fail-fast iterators in Java? ( answer ) hint: fail-safe doesn’t throw ConcurrentModificationException while fail-fast does whenever they detect an outside change on the underlying collection while iterating over it.

39) Difference between Iterator and Enumeration in Java? ( answer ) hint: Iterator also gives you the ability to remove an element while iterating while Enumeration doesn’t allow that.

40) What is IdentityHashMap in Java? ( answer ) hint: A Map , which uses the == equality operator to check equality instead of the equals() method.

41) What is the String pool in Java? ( answer ) hint: A pool of String literals. Remember it's moved to heap from perm gen space in JDK 7.

42) Can a Serializable class contains a non-serializable field in Java? ( answer ) hint: Yes, but you need to make it either static or transient.

43) Difference between this and super in Java? ( answer ) hint: this refers to the current instance while super refers to an instance of the superclass.

44) Difference between Comparator and Comparable in Java? ( answer ) hint: Comparator defines custom ordering while Comparable defines the natural order of objects, e.g. the alphabetic order for String . See The Complete Java MasterClass to learn more about sorting in Java.

45) Difference between java.util.Date and java.sql.Date in Java? ( answer ) hint: former contains both date and time while later contains only date part.

46) Why wait and notify method are declared in Object class in Java? ( answer ) hint: because they require lock which is only available to an object.

47) Why Java doesn’t support multiple inheritances? ( answer ) hint: It doesn’t support because of a bad experience with C++, but with Java 8, it does in some sense — only multiple inheritances of Type are not supported in Java now.

48) Difference between checked and unchecked Exception in Java? ( answer ) hint: In case of checked, you must handle exception using catch block, while in case of unchecked, it’s up to you; compile will not bother you.

49) Difference between Error and Exception in Java? ( answer ) hint: I am tired of typing please check the answer

50) Difference between Race condition and Deadlock in Java? ( answer ) hint: both are errors that occur in a concurrent application, one occurs because of thread scheduling while others occur because of poor coding. See Multithreading and Parallel Computing in Java to learn more about deadlock, Race Conditions, and other multithreading issues.

Closing Notes

Thanks, You made it to the end of the article … Good luck with your programming interview! It’s certainly not going to be easy, but by following this roadmap and guide, you are one step closer to becoming a DevOps engineer .

If you like this article, then please share with your friends and colleagues, and don’t forget to follow javinpaul on Twitter!

Additional Resources

Java Interview Guide: 200+ Interview Questions and Answers
Spring Framework Interview Guide — 200+ Questions & Answers
Preparing For a Job Interview By John Sonmez
Java Programming Interview Exposed by Markham
Cracking the Coding Interview — 189 Questions and Answers
Data Structure and Algorithms Analysis for Job Interviews
130+ Java Interview Questions of Last 5 Years

P.S. — If you need some FREE resources to learn Java, you can check out this list of free Java courses to start your preparation. P. S. S. — I have not provided the answer to the interview questions shared in the image “ How many String objects are created in the code?” can you guess and explain?

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Java Programming Exercises, Practice, Solution

Java exercises.

Java is the foundation for virtually every type of networked application and is the global standard for developing and delivering embedded and mobile applications, games, Web-based content, and enterprise software. With more than 9 million developers worldwide, Java enables you to efficiently develop, deploy and use exciting applications and services.

The best way we learn anything is by practice and exercise questions. Here you have the opportunity to practice the Java programming language concepts by solving the exercises starting from basic to more complex exercises. A sample solution is provided for each exercise. It is recommended to do these exercises by yourself first before checking the solution.

Hope, these exercises help you to improve your Java programming coding skills. Currently, following sections are available, we are working hard to add more exercises .... Happy Coding!

List of Java Exercises:

Basic Exercises Part-I [ 150 Exercises with Solution ]
Basic Exercises Part-II [ 99 Exercises with Solution ]
Methods [ 23 Exercises with Solution ]
Data Types Exercises [ 15 Exercises with Solution ]
Java Enum Types Exercises [ 5 Exercises with Solution ]
Conditional Statement Exercises [ 32 Exercises with Solution ]
Java recursive method Exercises [ 15 Exercises with Solution ]
Math [ 27 Exercises with Solution ]
Numbers [ 28 Exercises with Solution ]
Java Inheritance Exercises [ 9 exercises with solution ]
Java Abstract Classes Exercises [ 12 exercises with solution ]
Java Interface Exercises [ 11 exercises with solution ]
Java Encapsulation Exercises [ 7 exercises with solution ]
Java Polymorphism Exercises [ 12 exercises with solution ]
Object-Oriented Programming [ 28 Exercises with Solution ]
Exercises on handling and managing exceptions in Java [ 7 Exercises with Solution ]
Java Lambda expression Exercises [ 25 exercises with solution ]
Streams [ 8 Exercises with Solution ]
Java Thread Exercises [ 7 exercises with solution ]
Java Miltithreading Exercises [ 10 exercises with solution ]
Array [ 77 Exercises with Solution ]
Stack [ 29 Exercises with Solution ]
Collection [ 126 Exercises with Solution ]
String [ 107 Exercises with Solution ]
Input-Output-File-System [ 18 Exercises with Solution ]
Date Time [ 44 Exercises with Solution ]
Java Generic Methods [ 7 exercises with solution ]
Java Unit Test [ 10 Exercises with Solution ]
Search [ 7 Exercises with Solution ]
Sorting [ 19 Exercises with Solution ]
Regular Expression [ 30 Exercises with Solution ]
JavaFX [ 70 Exercises with Solution ]

Note: If you are not habituated with Java programming you can learn from the following :

Java Programming Language

More to Come !

Popularity of Programming Language Worldwide, Nov 2023 compared to a year ago:

Rank	Language	Share	Trend
1	Python	27.99 %	+0.0 %
2	Java	15.91 %	-0.8%
3	Javascript	9.18 %	-0.3%
4	C/C++	6.76 %	+0.2%
5	C#	6.67 %	-0.3 %
6	PHP	4.86 %	-0.3 %
7	R	4.45%	+0.4%
8	TypeScript	2.95 %	+0.1%
9	Swift	2.7 %	+0.6%
10	Objective-C	2.32%	+0.2%
11	Rust	1.98%	+0.3%
12	Go	1.98%	-0.0%
13	Kotlin	1.76 %	-0.1%
14	Matlab	1.6 %	+0.0%
15	Ada	1.02%	+0.2%
16	Ruby	1.0 %	-0.1 %
17	Dart	0.99 %	+0.1 %
18	Powershell	0.93 %	+0.0 %
19	VBA	0.93 %	-0.1 %
20	Scala	0.62 %	-0.1 %
21	Lua	0.62 %	0.0 %
22	Abap	0.58 %	+0.1 %
23	Visual Basic	0.55 %	-0.1 %
24	Julia	0.35 %	-0.0 %
25	Groovy	0.31 %	-0.1 %
26	Perl	0.31 %	-0.1 %
27	Haskell	0.27 %	-0.0 %
28	Cobol	0.25 %	-0.1 %
29	Delphi/Pascal	0.18 %	+0.2 %

Source : https://pypl.github.io/PYPL.html

TIOBE Index for November 2023

Nov 2023	Nov 2022	Programming Language	Ratings	Change
1	1	Python	14.16%	-3.02%
2	2	C	11.77%	-3.31%
3	4	C++	10.36%	-0.39%
4	3	Java	8.35%	-3.63%
5	5	C#	7.65%	+3.40%
6	7	JavaScript	3.21%	+0.47%
7	10	PHP	2.30%	+0.61%
8	6	Visual Basic	2.10%	-2.01%
9	9	SQL	1.88%	+0.07%
10	8	Assembly language	1.35%	-0.83%
11	17	Scratch	1.31%	+0.43%
12	24	Fortran	1.30%	+0.74%
13	11	Go	1.19%	+0.05%
14	15	MATLAB	1.15%	+0.14%
15	28	Kotlin	1.15%	+0.68%
16	14	Delphi/Object Pascal	1.14%	+0.07%
17	18	Swift	1.04%	+0.17%
18	19	Ruby	0.99%	+0.14%
19	12	R	0.93%	-0.20%
20	20	Rust	0.91%	+0.16%

Source : https://www.tiobe.com/tiobe-index/

List of Exercises with Solutions :

HTML CSS Exercises, Practice, Solution
JavaScript Exercises, Practice, Solution
jQuery Exercises, Practice, Solution
jQuery-UI Exercises, Practice, Solution
CoffeeScript Exercises, Practice, Solution
Twitter Bootstrap Exercises, Practice, Solution
C Programming Exercises, Practice, Solution
C# Sharp Programming Exercises, Practice, Solution
PHP Exercises, Practice, Solution
Python Exercises, Practice, Solution
R Programming Exercises, Practice, Solution
Java Exercises, Practice, Solution
SQL Exercises, Practice, Solution
MySQL Exercises, Practice, Solution
PostgreSQL Exercises, Practice, Solution
SQLite Exercises, Practice, Solution
MongoDB Exercises, Practice, Solution

[ Want to contribute to Java exercises? Send your code (attached with a .zip file) to us at w3resource[at]yahoo[dot]com. Please avoid copyrighted materials.]

Do not submit any solution of the above exercises at here, if you want to contribute go to the appropriate exercise page.

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Basic Java Interview Questions

1. what is java.

Java is an object-oriented, high-level, general-purpose programming language originally designed by James Gosling and further developed by the Oracle Corporation. It is one of the most popular programming languages in the world .

2. What is the Java Virtual Machine?

JVM is a program that interprets the intermediate Java byte code and generates the desired output. It is because of bytecode and JVM that programs written in Java are highly portable.

3. What are the features of Java?

The following are the various features of the Java programming language:

High Performance: Using a JIT (Just-In-Time) compiler allows high performance in Java. The JIT compiler converts the Java bytecode into machine language code, which then gets executed by the JVM.
Multi-threading: A thread is a flow of execution. The JVM creates a thread which is called the main thread. Java allows the creation of several threads using either extending the thread class or implementing the Runnable interface.
OOPs Concepts: Java follows various OOPs concepts , namely abstraction, encapsulation, inheritance, object-oriented, and polymorphism
Platform Independency: Java makes use of the Java Virtual Machine or JVM, which allows a single Java program to operate on multiple platforms without any modifications.

You may want to check out detailed explanations of Java features here .

4. How does Java enable high performance?

In Just-in-Time compilation, the required code is executed at run time. Typically, it involves translating bytecode into machine code and then executing it directly. It allows for high performance. The JIT compiler is enabled by default in Java and gets activated as soon as a method is called.

It then compiles the bytecode of the Java method into native machine code.

After that, the JVM calls the compiled code directly instead of interpreting it.

5. What are the differences between JVM, JRE, and JDK?


Java Virtual Machine	Java Runtime Environment	Java Development Kit
It provides a runtime environment to execute Java bytecode.	It is a set of software tools used for developing Java applications.	It is a software development environment used to develop Java applications.
It is a runtime instance created when we run a Java class.	It exists physically.	It exists physically.
Its implementation is known as JRE	It is the implementation of JVM	It is an implementation of any one of the below given Java Platforms released by Oracle Corporation: Standard Edition Java Platform Enterprise Edition Java Platform Micro Edition Java Platform

6. What is the JIT compiler?

JIT compiler runs after the program is executed and compiles the code into a faster form, hosting the CPU's native instructing set. JIT can access dynamic runtime information, whereas a standard compiler doesn't and can make better optimizations like inlining functions that are used frequently.

7. What are Java IDEs?

A Java IDE is a software that allows Java developers to easily write as well as debug Java programs. It is basically a collection of various programming tools, accessible via a single interface, and several helpful features, such as code completion and syntax highlighting.

Codenvy, Eclipse, and NetBeans are some of the most popular Java IDEs .

8. Java is a platform-independent language. Why?

Java does not depend on any particular hardware or software because it is compiled by the compiler and then converted into byte code. Byte code is platform-independent and can run on multiple systems. The only requirement is that Java needs a runtime environment, i.e., JRE, which is a set of tools used for developing Java applications.

9. Explain Typecasting.

The concept of assigning a variable of one data type to a variable of another data type. This is not possible for the boolean data type. There are two types: implicit and explicit.

10. What are the different types of typecasting?

The different types of typecasting are:

Implicit: Storing values from a smaller data type to the larger data type. It is automatically done by the compiler.
Explicit: Storing the value of a larger data type into a smaller data type. This results in information loss:
Truncation: While converting a value from a larger data type to a smaller data type, the extra data will be truncated. This code example explains it :

After execution, the variable i will contain only 3 and not the decimal portion.

Out of Range: Typecasting does not allow assigning value more than its range; if that happens then the data is lost in such cases. This code example explains it:

long l = 123456789;

byte b = ( byte ) l; // byte is of not the same range as long so there will be loss of data.

11. Explain access modifiers in Java.

Access modifiers are predefined keywords in Java that are used to restrict the access of a class, method, constructor, and data member in another class. Java supports four access modifiers:


Same class	yes	yes	yes	yes
Same package subclass	yes	no	yes	yes
Same package non-subclass	yes	no	yes	yes
Different package subclass	no	no	yes	yes
Different package non-subclass	no	no	no	yes

12. What are the default values for local variables?

The local variables are not initialized to any default value, neither primitives nor object references.

OOPs Java Interview Questions

13. what is object-oriented programming .

OOPs is a programming paradigm centered around objects rather than functions. It is not a tool or a programming language, it is a paradigm that was designed to overcome the flaws of procedural programming .

There are many languages that follow OOPs concepts — some popular ones are Java, Python, and Ruby. Some frameworks also follow OOPs concepts, such as Angular.

14. Explain the OOPs concepts.

The following are the various OOPS Concepts:

Abstraction: Representing essential features without the need to give out background details. The technique is used for creating a new suitable data type for some specific application.
Aggregation: All objects have their separate lifecycle, but ownership is present. No child object can belong to some other object except for the parent object.
Association: The relationship between two objects, where each object has its separate lifecycle. There is no ownership.
Class: A group of similar entities.
Composition: Also called the death relationship, it is a specialized form of aggregation. Child objects don't have a lifecycle. As such, they automatically get deleted if the associated parent object is deleted.
Encapsulation: Refers to the wrapping up of data and code into a single entity. This allows the variables of a class to be only accessible by the parent class and no other classes.
Inheritance: When an object acquires the properties of some other object, it is called inheritance. It results in the formation of a parent-child relationship amongst classes involved. This offers a robust and natural mechanism of organizing and structuring software.
Object: Denotes an instance of a class. Any class can have multiple instances. An object contains the data as well as the method that will operate on the data
Polymorphism: Refers to the ability of a method, object, or variable to assume several forms.

Decision Making Java Interview Questions

15. Differentiate between break and continue.


Used with both loop and switch statement.	Used with only loop statements.
It terminates the loop or switch block.	It does not terminate but skips to the next iteration.

Classes, Objects, and Methods Java Interview Questions

16. what is an object.

An instance of a Java class is known as an object. Two important properties of a Java object are behavior and state . An object is created as soon as the JVM comes across the new keyword.

17. Define classes in Java.

A class is a collection of objects of similar data types. Classes are user-defined data types and behave like built-in types of a programming language.

Syntax of a class:

Example of Class:

18. What are static methods and variables?

A class has two sections: one declares variables, and the other declares methods. These are called instance variables and instance methods, respectively. They are termed so because every time a class is instantiated, a new copy of each of them is created.

Variables and methods can be created that are common to all objects and accessed without using a particular object by declaring them static. Static members are also available to be used by other classes and methods.

19. What do you mean by Constructor?

A constructor is a method that has the same name as that of the class to which it belongs. As soon as a new object is created, a constructor corresponding to the class gets invoked. Although the user can explicitly create a constructor, it is created on its own as soon as a class is created. This is known as the default constructor. Constructors can be overloaded.

If an explicitly-created constructor has a parameter, then it is necessary to create another constructor without a parameter.

20. What are local variables and instance variables?

Variables that are only accessible to the method or code block in which they are declared are known as local variables. Instance variables, on the other hand, are accessible to all methods in a class.

While local variables are declared inside a method or a code block, instance variables are declared inside a class but outside a method. Even when not assigned, instance variables have a value that can be null, 0, 0.0, or false. This isn't the case with local variables that need to be assigned a value, where failing to assign a value will yield an error. Local variables are automatically created when a method is called and destroyed as soon as the method exits. For creating instance variables, the new keyword must be used.

21. What is Method Overriding?

Method overriding in Java allows a subclass to offer a specific implementation of a method that has already been provided by its parent or superclass. Method overriding happens if the subclass method and the Superclass method have:

The same name
The same argument
The same return type

22. What is Overloading?

Overloading is the phenomenon when two or more different methods (method overloading) or operators (operator overloading) have the same representation. For example, the + operator adds two integer values but concatenates two strings. Similarly, an overloaded function called Add can be used for two purposes

To add two integers
To concatenate two strings

Unlike method overriding, method overloading requires two overloaded methods to have the same name but different arguments. The overloaded functions may or may not have different return types.

23. What role does the final keyword play in Java? What impact does it have on a variable, method, and class?

The final keyword in Java is a non-access modifier that applies only to a class, method, or variable. It serves a different purpose based on the context where it is used.

With a class: When a class is declared as final, then it is disabled from being subclassed i.e., no class can extend the final class.
With a method: Any method accompanying the final keyword is restricted from being overridden by the subclass.
With a variable: A variable followed by the final keyword is not able to change the value that it holds during the program execution. So, it behaves like a constant.

Arrays, Strings and Vectors Java Interview Questions

24. draw a comparison between array and arraylist..

An array necessitates stating the size during the time of declaration, while an array list doesn't necessarily require size as it changes size dynamically. To put an object into an array, there is the need to specify the index. However, no such requirement is in place for an array list. While an array list is parameterized, an array is not parameterized.

25. What are the differences between String, Stringbuilder, and Stringbuffer?

String variables are stored in a constant string pool. With the change in the string reference, it becomes impossible to delete the old value. For example, if a string has stored a value "Old," then adding the new value "New" will not delete the old value. It will still be there, however, in a dormant state. In a Stringbuffer, values are stored in a stack. With the change in the string reference, the new value replaces the older value. The Stringbuffer is synchronized (and therefore, thread-safe) and offers slower performance than the StringBuilder, which is also a Stringbuffer but is not synchronized. Hence, performance is faster in Stringbuilder than the Stringbuffer.

26. What is the String Pool?

The string pool is a collection of strings stored in the heap memory refers to. Whenever a new object is created, it is checked if it is already present in the string pool. If it is already present, then the same reference is returned to the variable, otherwise a new object is created in the string pool, and the respective reference is returned.

Advanced Java Interview Questions

Interfaces and Abstract Classes Java Interview Questions

27. what do you know about interfaces.

A Java interface is a template that has only method declarations and not method implementations. It is a workaround for achieving multiple inheritances in Java. Some worth remembering important points regarding Java interfaces are:

A class that implements the interface must provide an implementation for all methods declared in the interface.
All methods in an interface are internally public abstract void.
All variables in an interface are internally public static final.
Classes do not extend but implement interfaces.

28. How is an Abstract class different from an Interface?

There are several differences between an Abstract class and an Interface in Java, summed up as follows:

Constituents: An abstract class contains instance variables, whereas an interface can contain only constants.
Constructor and Instantiation: While an interface has neither a constructor nor it can be instantiated, an abstract class can have a default constructor that is called whenever the concrete subclass is instantiated.
Implementation of Methods – All classes that implement the interface need to provide an implementation for all the methods contained by it. A class that extends the abstract class, however, doesn't require implementing all the methods contained in it. Only abstract methods need to be implemented in the concrete subclass.
Type of Methods: Any abstract class has both abstract as well as non-abstract methods. Interface, on the other hand, has only a single abstract method.

29. Please explain Abstract class and Abstract method.

An abstract class in Java is a class that can't be instantiated. Such a class is typically used for providing a base for subclasses to extend as well as implementing the abstract methods and overriding or using the implemented methods defined in the abstract class.

To create an abstract class, it needs to be followed by the abstract keyword. Any abstract class can have both abstract as well as non-abstract methods. A method in Java that only has the declaration and not implementation is known as an abstract method. Also, an abstract method name is followed by the abstract keyword. Any concrete subclass that extends the abstract class must provide an implementation for abstract methods.

30. What is multiple inheritance? Does Java support multiple inheritance? If not, how can it be achieved?

If a subclass or child class has two parent classes, that means it inherits the properties from two base classes; it has multiple inheritances. Java does not have multiple inheritances as in case the parent classes have the same method names. Then at runtime, it becomes ambiguous, and the compiler is unable to decide which method to execute from the child class.

Packages Java Interview Questions

31. what are packages in java state some advantages..

Packages are Java's way of grouping a variety of classes and/or interfaces together. The functionality of the objects decides how they are grouped. Packagers act as "containers" for classes.

Enlisted below are the advantages of Packages:

Classes of other programs can be reused.
Two classes with the same can exist in two different packages.
Packages can hide classes, thus denying access to certain programs and classes meant for internal use only.
They also separate design from coding.

Multithreading Java Interview Questions

32. how do you make a thread in java give examples..

To make a thread in Java, there are two options:

Extend the Thread Class : The thread is available in the java.lang.Thread class. To make a thread, you need to extend a thread class and override the run method. For example,

A disadvantage of using the thread class is that it becomes impossible to extend any other classes.

Nonetheless, it is possible to overload the run() method in the class

Implement Runnable Interface: Another way of making a thread in Java is by implementing a runnable interface. For doing so, there is the need to provide the implementation for the run() method that is defined as follows:

33. Why do we use the yield() method?

The yield() method belongs to the thread class. It transfers the currently running thread to a runnable state and also allows the other threads to execute. In other words, it gives equal priority threads a chance to run. Because yield() is a static method, it does not release any lock.

34. Explain the thread lifecycle in Java.

The thread lifecycle has the following states and follows the following order:

New: In the very first state of the thread lifecycle, the thread instance is created, and the start() method is yet to be invoked. The thread is considered alive now.
Runnable: After invoking the start() method, but before invoking the run() method, a thread is in the runnable state. A thread can also return to the runnable state from waiting or sleeping.
Running: The thread enters the running state after the run() method is invoked. This is when the thread begins execution.
Non-Runnable: Although the thread is alive, it is not able to run. Typically, it returns to the runnable state after some time.
Terminated: The thread enters the terminated state once the run() method completes its execution. It is not alive now.

35. When is the Runnable interface preferred over thread class and vice-versa?

In Java, it is possible to extend only one class. Hence, the thread class is only extended when no other class needs to be extended. If it is required for a class to extend some other class than the thread class, then we need to use the Runnable interface.

36. Draw a comparison between notify() and notifyAll() methods.

The notify() method is used for sending a signal to wake up a single thread in the waiting pool. Contrarily, the notifyAll() method is used for sending a signal to wake up all threads in a waiting pool.

37. How will you distinguish processes from threads?

There are several fundamental differences between a process and a thread, stated as follows:

Definition: A process is an executing instance of a program whereas, a thread is a subset of a process.
Changes: A change made to the parent process doesn't affect child processes. However, a change in the main thread can yield changes in the behavior of other threads of the same process.
Communication – While processes require inter-process communication for communicating with sibling processes, threads can directly communicate with other threads belonging to the same process.
Control: Processes are controlled by the operating system and can control only child processes. On the contrary, threads are controlled by the programmer and are capable of exercising control over threads of the same process to which they belong.
Dependence: Processes are independent entities while threads are dependent entities
Memory: Threads run in shared memory spaces, but processes run in separate memory spaces.

38. What is the join() method? Give an example.

We use the join() method for joining one thread with the end of the currently running thread. It is a non-static method and has an overloaded version. Consider the example below:

The main thread starts execution in the example mentioned above. As soon as the execution reaches the code t.start(), then the thread t starts its stack for execution. The JVM switches between the main thread and the thread there. Once the execution reaches the t.join(), then the thread t alone is executed and allowed to complete its task. Afterwards, the main thread resumes execution.

39. How do you make a thread stop in Java?

There are three methods in Java to stop the execution of a thread:

Blocking: This method is used to put the thread in a blocked state. The execution resumes as soon as the condition of the blocking is met. For instance, the ServerSocket.accept() is a blocking method that listens for incoming socket connections and resumes the blocked thread only when a connection is made.
Sleeping: This method is used for delaying the execution of the thread for some time. A thread upon which the sleep() method is used is said to enter the sleep state. It enters the runnable state as soon as it wakes up i.e., the sleep state is finished. The time for which the thread needs to enter the sleep state is mentioned inside the braces of the sleep() method. It is a static method.
Waiting: Although it can be called on any Java object, the wait() method can only be called from a synchronized block.

Exception Handling Java Interview Questions

40. what are the various types of exceptions how do you handle them.

Java has provision for two types of exceptions:

Checked Exceptions: Classes that extend the Throwable class, except Runtime exception and Error, are called checked exceptions. Such exceptions are checked by the compiler during the compile time. These types of exceptions must either have appropriate try/catch blocks or be declared using the throws keyword. ClassNotFoundException is a checked exception.
Unchecked Exceptions: Such exceptions aren't checked by the compiler during the compile time. As such, the compiler doesn't necessitate handling unchecked exceptions. Arithmetic Exception and ArrayIndexOutOfBounds Exception are unchecked exceptions.

Exceptions in Java are handled in two ways:

Declaring the throws keyword: We can declare the exception using throws keyword at the end of the method. For example:

Using try/catch: Any code segment that is expected to yield an exception is surrounded by the try block. Upon the occurrence of the exception, it is caught by the catch block that follows the try block. For example:

41. Draw the Java Exception Hierarchy.

42. Is it possible to write multiple catch blocks under a single try block?

Yes, it is possible to write several catch blocks under a single try block. However, the approach needs to be from specific to general. The following example demonstrates it:

43. How does the throw keyword differ from the throws keyword?

While the throws keyword allows declaring an exception, the throw keyword is used to explicitly throw an exception.

Checked exceptions can't be propagated with throw only, but throws allow doing so without the need for anything else.

The throws keyword is followed by a class, whereas the throw keyword is followed by an instance. The throw keyword is used within the method, but the throws keyword is used with the method signature.

Furthermore, it is not possible to throw multiple exceptions, but it is possible to declare multiple exceptions.

44. Explain various exceptions handling keywords in Java.

There are two crucial exception handling keywords in Java, followed by the third keyword final, which may or may not be used after handling exceptions.

If and when a code segment has chances of having an abnormality or an error, it is placed within a try block. When the exception is raised, it is handled and caught by the catch block.

The try block must have a catch() or a final() or both blocks after it.

When an exception is raised in the try block, it is handled in the catch block.

This block is executed regardless of the exception. It can be placed either after try{} or catch {} block.

45. Explain exception propagation.

The method at the top of the stack throws an exception if it is not caught. It moves to the next method and goes on until caught.

The stack of the above code is:

If an exception occurred in the add() method is not caught, then it moves to the method addition(). It is then moved to the main() method, where the flow of execution stops. It is called Exception Propagation.

File Handling Java Interview Questions

46. is an empty file name with .java extension a valid file name.

Yes, Java permits us to save our java file by .java only. It is compiled by javac and run by the java class name. Here’s an example:

To compile: javac.java

To run: Java Any

Collections Java Interview Questions

47. what are collections what are their constituents.

A group of objects in Java is known as collections. Java.util package contains, along with date and time facilities, internationalization, legacy collection classes, etc., the various classes and interfaces for collecting. Alternatively, collections can be considered as a framework designed for storing the objects and manipulating the design in which the objects are stored. You can use collections to perform the following operations on objects:

Manipulation

Following are the various constituents of the collections framework:

Classes: Array List, Linked List, Lists, and Vector
Interfaces: Collection, List, Map, Queue, Set, Sorted Map, and Sorted Set
Maps: HashMap, HashTable, LinkedHashMap, and TreeMap
Queues: Priority Queue
Sets: Hash Set, Linked Hash Set, and Tree Set

48. How do you differentiate HashMap and HashTable?

HashMap in Java is a map-based collection class, used for storing key & value pairs. It is denoted as HashMap<Key, Value> or HashMap<K, V> HashTable is an array of a list, where each list is called a bucket.

Values contained in a HashTable are unique and depend on the key. Methods are not synchronized in HashMap, while key methods are synchronized in HashTable.

However, HashMap doesn't have thread safety, while HashTable has the same.

For iterating values, HashMap uses an iterator and HashTable uses an enumerator. HashTable doesn't allow anything that is null, while HashMap allows one null key and several null values.

In terms of performance, HashTable is slow. Comparatively, HashMap is faster.

49. What is a Map and what are the types?

A Java Map is an object that maps keys to values. It can't contain duplicate keys, and each key can map to only one value. In order to determine whether two keys are the same or distinct, Map makes use of the equals() method. There are 4 types of Map in Java, described as follows:

HashMap: It is an unordered and unsorted map and hence, is a good choice when there is no emphasis on the order. A HashMap allows one null key and multiple null values and doesn't maintain any insertion order.
HashTable: Doesn't allow anything null and has methods that are synchronized. As it allows for thread safety, the performance is slow.
LinkedHashMap: Slower than a HashMap but maintains insertion order and has a faster iteration.
TreeMap: A sorted Map providing support for constructing a sort order using a constructor.

50. What is a Priority Queue?

A priority queue, like a regular queue, is an abstract data type, but it has a priority associated with each element contained by it.

The element with the high priority is served before the element with low priority in a priority queue. Elements in a priority queue are ordered either according to the comparator or naturally. The order of the elements in a priority queue represents their relative priority.

51. What is a Set? Explain the types in Java Collections.

In Java, a set is a collection of unique objects. It uses the equals() method to determine whether two objects are the same or not. The various types of set in Java Collections are:

Hash Set: An unordered and unsorted set that uses the hash code of the object for adding values. Used when the order of the collection isn't important
Linked Hash Set: This is an ordered version of the hash set that maintains a doubly-linked list of all the elements. Used when iteration order is mandatory. Insertion order is the same as that of how elements are added to the Set.
Tree Set: One of the two sorted collections in Java, it uses Read-Black tree structure and ensures that the elements are present in the ascending order.

52. What is ordered and sorted when it comes to collections?

Ordered: Values are stored in a collection in a specific order, but the order is independent of the value. Example: List
Sorted: The collection has an order which is dependent on the value of an element. Example: SortedSet

Miscellaneous Java Interview Questions

53. what are the various types of garbage collectors in java.

The Java programming language has four types of garbage collectors:

Serial Garbage Collector: Using only a single thread for garbage collection, the serial garbage collector works by holding all the application threads. It is designed especially for single-threaded environments. Because serial garbage collector freezes all application threads while performing garbage collection, it is most suitable for command-line programs only. For using the serial garbage collector, one needs to turn on the -XX:+UseSerialGC JVM argument.
Parallel Garbage Collector: Also known as the throughput collector, the parallel garbage collector is the default garbage collector of the JVM. It uses multiple threads for garbage collection, and like a serial garbage collector freezes all application threads during garbage collection.
CMS Garbage Collector: Short for Concurrent Mark Sweep, CMS garbage collector uses multiple threads for scanning the heap memory for marking instances for eviction, followed by sweeping the marked instances. There are only two scenarios when the CMS garbage collector holds all the application threads:
When marking the referenced objects in the tenured generation space.
If there is a change in the heap memory while performing the garbage collection, CMS garbage collector ensures better application throughput over parallel garbage collectors by using more CPU resources. For using the CMS garbage collector, the XX:+USeParNewGC JVM argument needs to be turned on.
G1 Garbage Collector: Used for large heap memory areas, G1 garbage collector works by separating the heap memory into multiple regions and then executing garbage collection in them in parallel. Unlike the CMS garbage collector that compacts the memory on STW (Stop The World) situations , G1 garbage collector compacts the free heap space right after reclaiming the memory. Also, the G1 garbage collector prioritizes the region with the most garbage. Turning on the –XX:+UseG1GC JVM argument is required for using the G1 garbage collector.

54. What do you understand by synchronization? What is its most significant disadvantage?

If several threads try to access a single block of code, then there is an increased chance of producing inaccurate results. Synchronization is used to prevent this. Using the synchronization keyword makes a thread need a key to access the synchronized code. Simply, synchronization allows only one thread to access a block of code at a time. Each Java object has a lock, and every lock has only one key. A thread can access a synchronized method only if it can get the key to the lock of the object. The following example demonstrates synchronization:

Note : It is recommended to avoid implementing synchronization for all methods. This is because when only one thread can access the synchronized code, the next thread needs to wait. Consequently, it results in slower performance of the program.

55. What is the difference between execute(), executeQuery(), and executeUpdate()?

execute(): Used for executing an SQL query. It returns TRUE if the result is a ResultSet, like running Select queries, and FALSE if the result is not a ResultSet, such as running an Insert or an Update query.
executeQuery(): Used for executing Select queries. It returns the ResultSet, which is not null, even if no records are matching the query. The executeQuery() method must be used when executing select queries so that it throws the java.sql.SQLException with the 'executeQuery method cannot be used for update' message when someone tries to execute an Insert or Update statement.
executeUpdate(): Used for executing Delete/Insert/Update statements or DDL statements that return nothing. The output varies depending on whether the statements are Data Manipulation Language (DML) statements or Data Definition Language (DDL) statements. The output is an integer and equals the total row count for the former case, and 0 for the latter case.

Note : The execute() method needs to be used only in a scenario when there is no certainty about the type of statement. In all other cases, either use executeQuery() or executeUpdate() method.

56. Provide an example of the Hibernate architecture.

57. Could you demonstrate how to delete a cookie in JSP with a code example?

The following code demonstrates deleting a cookie in JSP:

58. Write suitable code examples to demonstrate the use of final, final, and finalize.

Final: The final keyword is used for restricting a class, method, and variable. A final class can't be inherited, a final method is disabled from overriding, and a final variable becomes a constant i.e., its value can't be changed.

Finally: Any code inside the final block will be executed, irrespective of whether an exception is handled or not.

Finalize: The finalize method performs the clean up just before the object is garbage collected.

59. What purpose does the Volatile variable serve?

The value stored in a volatile variable is not read from the thread's cache memory but from the main memory. Volatile variables are primarily used during synchronization.

60. Please compare serialization and deserialization.

Serialization is the process by which Java objects are converted into the byte stream.

Deserialization is the exact opposite process of serialization where Java objects are retrieved from the byte stream.

A Java object is serialized by writing it to an ObjectOutputStream and deserialized by reading it from an ObjectInputStream.

61. What is OutOfMemoryError?

Typically, the OutOfMemoryError exception is thrown when the JVM is not able to allocate an object due to running out of memory. In such a situation, no memory could be reclaimed by the garbage collector.

There can be several reasons that result in the OutOfMemoryError exception, of which the most notable ones are:

Holding objects for too long
Trying to process too much data at the same time
Using a third-party library that caches strings
Using an application server that doesn't perform a memory cleanup post the deployment
When a native allocation can't be satisfied

62. Explain public static void main(String args[ ]) in Java

The execution Java program starts with public static void main(String args[ ]), also called the main() method.

public: It is an access modifier defining the accessibility of the class or method. Any class can access the main() method defined public in the program.
static: The keyword indicates the variable, or the method is a class method. The method main() is made static so that it can be accessed without creating the instance of the class. When the method main() is not made static, the compiler throws an error because the main() is called by the JVM before any objects are made, and only static methods can be directly invoked via the class.
void: It is the return type of the method. Void defines the method that does not return any type of value.
main: JVM searches this method when starting the execution of any program, with the particular signature only.
String args[]: The parameter passed to the main method.

Java Programming Masterclass Updated to Java 17

63. What are wrapper classes in Java?

Wrapper classes are responsible for converting the Java primitives into the reference types (objects). A class is dedicated to every primitive data type. They are known as wrapper classes because they wrap the primitive data type into an object of that class. It is present in the Java.lang package. The table below displays the different primitive types and wrapper classes.


boolean	Boolean
char	Character
double	Double
float	Float
int	Integer
long	Long

64. Explain the concept of boxing, unboxing, autoboxing, and auto unboxing.

Boxing: The concept of putting a primitive value inside an object is called boxing.
Unboxing: Getting the primitive value from the object.
Autoboxing: Assigning a value directly to an integer object.
Auto unboxing: Getting the primitive value directly into the integer object.

65. Define the Singleton class. How can a class be made Singleton?

A Singleton class allows only one instance of the class to be created. A class can be made singleton with the following steps:

Creating a static instance of the class with the class.
By not allowing the user to create an instance with default constructor by defining a private constructor.
Create a static method to return the object of an instance of A.

66. What if the public static void is replaced by static public void, will the program still run?

Yes, the program would compile and run without any errors as the order of the specifiers doesn't matter.

67. What is the difference between == and equals()?


It is a method of String class	It is an operator
Content comparison	Address comparison

68. Why don't we use pointers in Java?

Pointers are considered to be unsafe, and increase the complexity of the program, adding the concept of pointers can be contradicting. Also, JVM is responsible for implicit memory allocation; thus, to avoid direct access to memory by the user, pointers are discouraged in Java.

69. What is the difference between this() and super()?


Represents the current instance of the class	Represents the current instance of the parent/base class
It is used to call the default constructor of the same class	It is used to call the default constructor of the parent/base class.
Accesses method of the current class	Accesses method of the base class
Points current class instance	Points to the superclass instance.
Must be the first line of the block	It must be the first line of the block.

Java Coding Interview Questions

Apart from having good knowledge about concepts of Java programming, you are also tested for your skills in coding in Java programming language. The following are Java coding interview questions that are relevant for freshers and are quite popular amongst Java programming interviews.

70. Take a look at the two code snippets below. What is the important difference between the two?

Code snippet i. is an example of method overloading while the code snippet ii. demonstrates method overriding.

71. Write a program for string reversal without using an inbuilt function.

72. write a program to delete duplicates from an array., 73. write a program to reverse a number., 74. write a program that implements binary search., 75. write a program to check if a number is prime., 76. write a program to print fibonacci series., 77. write a program to check if the given string is a palindrome., 78. write a program to print the following pattern., 79. write a program to swap two numbers., 80. write a program to check if the given number is an armstrong number..

These core Java Interview Questions and Java Programming Interview Questions are a great way to prepare you for the interview. Review the answers, including the coding examples, and put your best foot forward.

Please note that we also provided a PDF for your preparation so that you can download and learn and prepare on the go:

Download Java Interview Questions PDF

Prefer on-demand videos? Check out this course for further reading and preparation for a Java-based interview: Java Interview Guides: 200+ Interview Question and Answer

Otherwise, we recommend this book to help you succeed in your future Java interviews: Elements of Programming Interviews in Java: The insider guide second edition

Frequently Asked Questions

1. What are the basic Java questions asked in an interview?

There are several basic Java interview questions that can appear in an interview. Look at the ones we’ve listed above to get a sense of them.

2. How should I prepare for a Java interview?

You should prepare for a Java interview by learning both the theory and practicing coding. There are several related questions above.

3. What are the advanced Java interview questions?

Advanced Java interview questions can be of many kinds, including both theory and coding-based questions. Check out the list of Java programming questions above to see what they are like.

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Java interview questions

In this blog, we cover 20 Java language-specific interview questions on a variety of topics. We’re also including answers to these questions to help you prepare for them. For more interview questions, check out the links at the end of this blog.

Question	Description	Sample Input	Sample Output
FizzBuzz	Print numbers from 1 to 100, replacing multiples of three with “Fizz”, multiples of five with “Buzz”, and multiples of both three and five with “FizzBuzz”.	N/A	1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, FizzBuzz, …
Palindrome Check	Determine if a given string is a palindrome.	“racecar”	true
Fibonacci Series	Generate the Fibonacci series up to a given number of terms.	5	0, 1, 1, 2, 3
Reverse a String	Reverse a given string.	“world”	“dlrow”
Find the Missing Number	Find the missing number in an array of integers from 1 to n.	[1, 2, 4, 5]	3

Advantages	Disadvantages
Enables code reusability	Java does not allow multiple inheritance
Defines common methods and variables for subclasses
Provides a consistent structure across related classes
Allows for future extension through subclassing

Skills/Knowledge	Entry-Level Java Interviews	Experienced Java Interviews
Core Java	✔️	✔️
Object-Oriented Programming	✔️	✔️
Design Patterns		✔️
Concurrency		✔️
Performance Optimization		✔️
Architectural Principles		✔️

ArrayList	LinkedList
Implements the List interface.	Implements the List and Deque interfaces.
Uses a dynamic array to store elements.	Uses a linked list to store elements.
Provides efficient random access and modification of elements.	Provides efficient insertion and deletion of elements.
Slower performance for insertion and deletion of elements.	Fast performance for insertion and deletion of elements.
Requires more memory when compared to LinkedList.	Requires less memory when compared to ArrayList.

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COMMENTS

Top 50 Java Programming Interview Questions
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