Java Interview Questions and Answers for 2 to 3 years Experienced: Top 50

If you have been working as a Java programmer for 2 to 3 years, you would be considered as a beginner or at the intermediate level.

Don’t worry—you are not expected to perform tasks such as tuning garbage collection, profiling, or creating complex Java designs. However, having a solid understanding of Java basics would be best.

For Java developers with 2-3 years of experience, interviews typically cover core Java concepts, object-oriented programming (OOP), multithreading, exception handling, collections, and more. Knowing OOPS concepts like Abstraction, Inheritance, Encapsulation, Composition, and Polymorphism. You should also be familiar with Java collection API, such as different types of collections like Set, List, and Map, as well as popular classes like HashMap, Vector, and ArrayList.

During Java interviews, expect questions about Java threading API, wait-notify mechanism, and handling exceptions. Additionally, you might be asked to solve simple programming exercises like reversing a String in Java.

List of Common Java Interview Questions for 2 to 3 years Experienced

Some of the Java interview questions for professionals with 2 to 3 years of experience  are discussed below-

1. Tell me the difference between Method Overloading and Method Overriding in Java.

This is one of the core Java interview questions for mid-experienced professionals.
Method Overloading is used to increase the program’s readability. It has the same method name but different parameters.
Method Overriding provides the specific implementation of the method. It has the same method and same parameters.

2. What do you mean by the class loader in Java? 

In the Java Virtual Machine (JVM), the class loader is a type of subsystem. It is used for loading class files. When you run the program, it will be first loaded by the class loader. The built-in class loaders are of three types in Java-

• The Bootstrap Classloader is the first-class loader in JVM. It is the superclass of the Extension class loader. It loads the rt.jar file that constitutes all the class files of Java Standard Edition. The files being java.lang package classes, java.net package classes, java.util packages classes, java.io packages classes, java.sql packages classes, etc.
• The Extension Classloader is the class loader of Bootstrap and the parent class loader of the system classloader. It generally loads the jar files located inside the $JAVA_HOME_/jre/lib/ext directory.
• The System Application Classloader is the type child class loader of the Extension class loader. From the cross path, the System Application Classloader loads the class files. It is by default; the classpath is set to the current directory. Therefore, it is also called the Application class loader.

3. What do you mean by inheritance in Java?

To this Java interview question, you can answer it by saying; In Java, inheritance is a mechanism through which an object acquires all the properties and behavior of another class. It is usually used for Method Overriding and Code Reusability.

The concept of inheritance in Java is based on the fact that it creates new classes that are built upon the existing classes. Therefore, these methods and fields of the parent class can be reused if we inherit from an existing class. And also, we can add new methods and fields to our current class.

In Java, the inheritance is of five types-

• Hybrid Inheritance
• Hierarchical Inheritance
• Single-level Inheritance
• Multi-level Inheritance
• Multiple Inheritance

4. Why can we not override the static method in Java?

This is one of the core Java programming interview questions for experienced professionals in the Java interview. 

The reasons why we cannot override the static method in Java are :

(a) The static method does not belong to the object level. Instead, it belongs to the class level. The object decides which method can be called in the method overriding.

(b) In the static method, which is the class-level method, the type reference decides which method to call without referring to the object. It concludes that the called method is determined at the compile time.

If any child class defines the static method with the same signature as the parent class, then the method in the child class hides the method in the parent class.

5. Can you tell us what the Dynamic Method Dispatch is in Java? 

The Dynamic method dispatch is a process through which a call towards an overridden method is solved at a run time. It is the object that is being referred to, not the type reference variable. It decides which version of the overridden method needs to be executed.

6. What is a Java ClassPath?

A Java ClassPath is an environment variable that the Java Virtual Machine (JVM) uses to collect all classes used by the program.

7. What can be stated as a volatile keyword in Java?

If a variable is marked as volatile, it can be read from the main memory instead of the cache memory.

8. Where does the final block not execute in Java?

To this Java interview question, you can answer it by saying; There is only one case where the final block does not execute in Java. The final block does not execute when you run System.exit(0) in the try or catch block in the Java programs.

9. What are the major points of distinction between StringBuffer and StringBuilder in Java?

A StringBuffer is thread-safe. Therefore, simultaneously two threads cannot call the methods of StringBuffer. On the other hand, in comparison to StringBuffer, a StringBuilder is not known to be thread-safe. Hence, it means that two threads can call the methods of StringBuilder at the same time.

On the basis of performance StringBuffer’s performance is less efficient as it is thread-safe. Whereas StringBuilder’s performance is more efficient as it is not thread-safe.

10. Can you tell the difference between the Vector and ArrayList in Java?

You can answer this Java interview question by listing the differences between the vector and the ArrayList.

• (a) The common difference between the vector and the ArrayList in Java is that the vector is synchronized and thread-safe while the ArrayList is not.
• (b) Since vector is synchronized, it is slow, but since the ArrayList is not synchronized, it is comparatively faster

11. Why is String immutable in Java?

You can answer this Java interview question by saying there are several reasons why String is immutable or unchangeable in Java.

String pool– if you assign a value to String using the double quotes (” “), it gets stored in the string literal pool area, and a single String can be referenced by several reference variables, which will make it affect all reference variables if the String becomes mutable.

Classloading- String is used for the mechanism of class loading. If String becomes mutable, then it will become a security threat because anyone can hack it.

Cache hash value– when you use String as a key in HashMap or any other collection, you can cache its hash value. You do not need to calculate each time as it will always be constant because the string is immutable.

12. Can you tell the difference between the HashMap and HashSet in Java?

This is one of the advanced Java Interview questions for 2 to 3 years of experience candidates. The interviewer will expect you to know the answer to this Java interview question.

To answer this Java interview question, HashMap implements the Map interface, which maps keys to value. It is not synchronized, and it is not thread safe. Null keys and values are allowed, whereas duplicate keys are not allowed.

HashMap<Integer,String> studentHashMap=new HashMap<Integer,String>();

studentHashMap.put(1, “Anushka”);

studentHashMap.put(2. “Akansha”);

In the case of HashSet, it implements a Set interface that does not allow duplicate values. As a result, it is neither synchronized nor thread-safe.

HashSet studentSet=new HashSet();

studentSet.add(“Anushka”);

studentSet.add(“Akansha”);

studentSet.add(“Anjana”);

13. What is the Producer-Consumer Problem? What are the advantages of the Producer-Consumer Pattern?

This is one of the advanced Java interview questions. The Producer-Consumer Pattern can be implemented so that the Producer should wait if the bucket is full. The Customer needs to wait if the bucket is empty. The classical way of solving the Producer-Consumer Problem can be implemented by using the wait and notify method. It is to communicate between Consumer and Producer threads and also blocks each of them on individual conditions such as empty queue and full queue.

The Producer-Consumer Pattern is usually while writing the concurrent code and the multithreaded code. The advantages of the Producer-Consumer Pattern are-

• The Producer does not need to know about the consumer or the number of consumers there are. The same case goes with the Producer as well.
• The Producer and the Consumer can work at separate, different speeds. Usually, if you monitor the speed of the consumer, then more consumers can be introduced for better utilization.
• If you functionally separate the Producer and Consumer, it can result in clean, readable, and manageable code.

14. How do you think we can create an immutable class in Java?

An immutable class has only one state, and the constructor carefully instantiates it.

To create an immutable class in Java, you can follow these steps-

• (a) You can make your class final so that no class will be able to extend that. Therefore, no one will be able to override methods in this class.
• (b) You can make your instance variables private, then any class will not be able to access instance variables. You can make the final so that you cannot change it.
• (c) You should not create a setter method for instance variables so that no explicit way will be there to change the state of the instance variables.
• (d) Initialize the variables in the constructor and take proper care while working with the mutable object. You must do the deep copy in the case of mutable objects.
• (e) You should return a clone of the object from the getter method so that it won’t return the original object. After that, your original object will be intact.

15. Can you tell us the difference between the fail-safe and fail-fast iterator in java?

This is an advanced Java question for experienced professionals. To answer this question, the difference between the Fail-fast iterator and fail-safe iterator in Java are-

fail-fast iterator– It fails as soon as they realize that the structure of the collection is changed since the beginning of the iteration. The structural changes here are removing, adding, or updating any element from the collection when one thread is iterating over that other collection. The fail-fast can be implemented by maintaining a modification count. When the iteration thread will realize the change in the count, it throws ConcurrentModificationException.

fail-safe iterator in Java- It does not throw an Exception when the collection is modified in a structural manner while one iteration is working over it. It works on the clone collection instead of the original collection.

16. What is an interface, and can you explain its features?

An interface in Java is a reference type similar to a class that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces cannot contain instance fields or constructors. Key features include:

• Multiple inheritance: A class can implement multiple interfaces.
  • Abstract methods: All methods in an interface are abstract by default (unless specified as default or static).
  • Default methods: Interfaces can have default methods with an implementation starting from Java 8.
  • Static methods: Interfaces can have static methods as of Java 8.

17. What are the key principles of SOLID design?

SOLID is an acronym for five design principles aimed at making software designs more understandable, flexible, and maintainable:

• Single Responsibility Principle (SRP): A class should have only one reason to change, meaning it should only have one job or responsibility.
• Open/Closed Principle (OCP): Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This promotes the use of interfaces and abstract classes.
• Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.
• Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use. This promotes smaller and more specific interfaces.
• Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules; both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions.

18. What is a Java package?

A Java package is a namespace that organizes a set of related classes and interfaces. It helps to avoid name conflicts and makes it easier to manage large software projects. Packages can be categorized into:

• Built-in packages: Such as java.util, java.lang, etc.
• User-defined packages: Created by developers to group related classes.

Packages are declared using the package keyword at the top of a Java source file.

19. What is the difference between a Set and a List in Java?

• Set: A collection that does not allow duplicate elements. It is unordered, meaning that the elements may not maintain any specific order. Common implementations include HashSet, LinkedHashSet, and TreeSet.
• List: An ordered collection that allows duplicate elements. Elements in a List can be accessed by their position (index). Common implementations include ArrayList and LinkedList.

20. Explain the concept of Java Generics.

Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. This feature allows for type safety at compile time and eliminates the need for casting. For example, using List<String> ensures that only String objects can be added to the list. Generics improve code reusability and maintainability.

21. What is the synchronized keyword?

The synchronized keyword in Java controls access to a block of code or an object by multiple threads. It prevents thread interference and ensures that only one thread can access the synchronized block or method simultaneously. It can be applied to methods or specific code blocks for thread safety.

22. What are lambda expressions in Java?

Lambda expressions, introduced in Java 8, clearly and concisely represent a functional interface (an interface with a single abstract method). They allow developers to write instances of functional interfaces in a more compact and readable way. The syntax consists of parameters, the arrow operator (->), and the body.
For example:
(x, y) -> x + y
This represents a function that takes two parameters and returns their sum.

23. What is a functional interface?

A functional interface in Java is an interface that contains exactly one abstract method. They can have multiple default or static methods. Functional interfaces can be used as the assignment target for lambda expressions or method references. An example of a functional interface is Runnable.

24. Explain the difference between throw and throws.

• throw: This keyword is used to explicitly throw an exception from a method or block of code.
  For example, throw new IllegalArgumentException("Invalid argument");.
• throws: This keyword is used in method declarations to indicate that a method may throw one or more exceptions. It informs callers of the method to handle these exceptions.

For example:
public void myMethod() throws IOException, SQLException  

25. What is the Java Map interface?

The Map interface represents a collection of key-value pairs. Each key is unique, and each key maps to exactly one value. Common implementations of the Map interface include:

• HashMap: Unordered, allows null keys and values, and offers constant-time performance for basic operations.
• LinkedHashMap: Maintains insertion order while being similar to HashMap.
• TreeMap: Implements SortedMap, storing keys in a sorted order.

26. What is the difference between HashMap and Hashtable?

• HashMap: Allows null keys and values, is not synchronized (not thread-safe), and has better performance than Hashtable.
• Hashtable: Does not allow null keys or values, is synchronized (thread-safe), and is considered legacy, as it has been largely replaced by HashMap.

27. What is the default keyword in interfaces?

The default keyword in Java interfaces allows the creation of methods with a default implementation. This feature, introduced in Java 8, enables adding new methods to interfaces without breaking existing implementations. Classes implementing the interface can choose to override the default method or use it as-is.

28. What are the types of inner classes in Java?

Java supports four types of inner classes:

1. Non-static Inner Class: Associated with an instance of the outer class and can access its members directly.
2. Static Nested Class: Similar to static methods; it does not require an instance of the outer class and can only access static members.
3. Method-local Inner Class: Defined within a method and can access local variables and parameters of the method if they are final or effectively final.
4. Anonymous Inner Class: A class defined without a name at the time of instantiation. It is often used to provide a one-time implementation of an interface or a subclass.

29. Explain the concept of method references in Java.

It is one of the important Java interview questions. Method references are a shorthand notation of a lambda expression to call a method. They provide a way to refer to methods without invoking them. There are four types of method references:

1. Static Method Reference: Refers to a static method.javaCopy codeClassName::staticMethodName
2. Instance Method Reference of a Particular Object: Refers to an instance method of a specific object.javaCopy codeinstance::instanceMethodName
3. Instance Method Reference of an Arbitrary Object of a Particular Type: Refers to an instance method of an arbitrary object of a specific type.javaCopy codeClassName::instanceMethodName
4. Constructor Reference: Refers to a constructor.javaCopy codeClassName::new

30. What is the difference between StringBuilder and StringBuffer?

• StringBuilder: Not synchronized, hence it is faster but not thread-safe. It is suitable for use in a single-threaded environment.
• StringBuffer: Synchronized, making it thread-safe but slower due to the overhead of synchronization. It is used in multi-threaded environments where shared data is modified.

31. What are the main features of Java 8?

This Java interview question can be answered in the following manner.
Key features introduced in Java 8 include:

• Lambda Expressions: Simplifying the syntax for functional interfaces.
• Streams API: Enabling functional-style operations on streams of elements.
• Default Methods in Interfaces: Allowing methods with implementations in interfaces.
• Optional Class: Providing a way to avoid null references and manage optional values.
• New Date and Time API: A more comprehensive date and time handling.

32. Explain the use of the try-with-resources statement.

The try-with-resources statement, introduced in Java 7, is used to manage resources that need to be closed after their use, such as files or sockets. Resources that implement the AutoCloseable interface are declared within the parentheses of the try statement. This ensures that each resource is closed automatically at the end of the statement, preventing resource leaks.
For example:
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) catch (IOException e)

33. What is the difference between Comparable and Comparator?

• Comparable: An interface that defines a natural ordering of objects. It requires implementing the compareTo method within the class itself. It is used when you want to sort objects of the same class.
• Comparator: An interface that defines an external ordering of objects. It requires implementing the compare method in a separate class or lambda expression. It allows sorting of objects from different classes or by different criteria.

34. What is the significance of the instanceof operator?

The instanceof operator in Java is used to test whether an object is an instance of a specific class or subclass. It returns true if the object is an instance of the specified type, and false otherwise. This operator is particularly useful for checking object types before casting, helping to avoid ClassCastException.

35. Explain the concept of super keyword.

The super keyword in Java is used to refer to the immediate parent class of the current object. It can be used to:

• Access parent class methods and attributes.
• Invoke a parent class constructor.
  For example, super() can be called within a subclass constructor to invoke the parent constructor.

36. What is the finalize() method?

The finalize() method is a method of the Object class that is called by the garbage collector just before an object is reclaimed. It allows an object to perform cleanup operations, such as releasing resources. However, its use is discouraged because there is no guarantee when or if it will be called, leading to unpredictability in resource management.

37. What are the Java Collections Framework interfaces?

The Java Collections Framework provides several key interfaces for working with collections:

• Collection: The root interface for all collections.
• List: An ordered collection that allows duplicates.
• Set: A collection that does not allow duplicates.
• Map: A collection of key-value pairs.
• Queue: A collection designed for holding elements prior to processing.

38. What is a Thread in Java?

A Thread in Java is a lightweight subprocess, the smallest unit of processing. It allows concurrent execution of two or more parts of a program. Java provides the Thread class and the Runnable interface to create and manage threads. Multithreading helps to perform multiple tasks simultaneously, improving application performance and responsiveness.

39. How do you create a thread in Java?

There are two main ways to create a thread in Java:

1. Extending the Thread Class:
   class MyThread extends Thread } MyThread thread = new MyThread(); thread.start();
2. Implementing the Runnable Interface:
   class MyRunnable implements Runnable } Thread thread = new Thread(new MyRunnable()); thread.start();

40. What is a deadlock?

A deadlock is a situation in multithreading where two or more threads are blocked forever, each waiting for the other to release a resource. For example, if Thread A holds Resource 1 and waits for Resource 2 while Thread B holds Resource 2 and waits for Resource 1, neither can proceed. To avoid deadlocks, techniques such as lock ordering, timeout, or using higher-level abstractions like java.util.concurrent can be employed.

41. What is the purpose of the volatile keyword in Java?

The volatile keyword indicates that a variable’s value will be modified by different threads. Declaring a variable as volatile ensures that changes to the variable are visible to all threads. It provides a lighter form of synchronization compared to synchronized, making it suitable for scenarios where thread safety is needed without locking overhead.

42. What is the purpose of the clone() method?

The clone() method in Java is used to create a copy of an object. The Object class provides a clone() method, but it must be overridden in classes that implement cloning. The class should implement the Cloneable interface, indicating that it allows for object duplication. By default, clone() performs a shallow copy, but it can be modified to perform a deep copy if necessary.

43. What is a Queue in Java?

Answer: A Queue in Java is a collection designed for holding elements prior to processing. It follows the First-In-First-Out (FIFO) principle, where elements are added to the end of the queue and removed from the front. The Queue interface provides various implementations, such as LinkedList, PriorityQueue, and ArrayDeque, allowing for different behaviors and performance characteristics.

44. Explain the concept of synchronization in Java.

Synchronization in Java is a mechanism that controls access to shared resources by multiple threads to prevent data inconsistency. It ensures that only one thread can access a resource at a time. Synchronization can be achieved using synchronized methods, synchronized blocks, or higher-level constructs like Lock and ReadWriteLock. Proper synchronization is crucial for maintaining data integrity in multithreaded environments.

45. What is the difference between shallow copy and deep copy?

• Shallow Copy: A shallow copy creates a new object, but it does not create copies of the objects contained in the original object. Instead, it copies references to the original objects. Changes to the original objects affect the copied object.
• Deep Copy: A deep copy creates a new object and recursively copies all objects contained in the original object. This results in a completely independent copy, meaning changes to the original object do not affect the deep copied object.

46. What are Java annotations used for?

Java annotations are a form of metadata that provide information about the code but do not affect its execution. They can be used for:

• Compile-time checks: Annotations like @Override help the compiler ensure correctness.
• Runtime processing: Annotations can be read by frameworks or libraries to alter their behavior, as seen with Spring or Hibernate.
• Documentation: Annotations can be used for generating documentation, such as with Javadoc.

47. What is the Java Memory Model?

The Java Memory Model (JMM) defines how threads interact through memory and how changes made by one thread become visible to others. It specifies rules about visibility, atomicity, and ordering of operations to ensure consistent behavior in multithreaded environments. Understanding the JMM is crucial for writing correct and efficient concurrent programs.

48. Explain the concept of reflection in Java.

Answer: Reflection in Java is a powerful feature that allows the inspection and manipulation of classes, methods, fields, and other components at runtime. Using the java.lang.reflect package, developers can access and modify the properties of classes dynamically. Reflection is useful for tasks such as object serialization, debugging, and developing frameworks but should be used judiciously due to performance overhead and security concerns.

49. What is the purpose of the join() method in threading?

The join() method is used to pause the execution of the current thread until the thread on which join() is called has completed. This is useful for ensuring that one thread waits for another to finish before continuing.
For example:
Thread thread = new Thread(new MyRunnable());
thread.start();
thread.join(); // Wait for the thread to finish

50. What are the benefits of using the Java Stream API?

The Java Stream API provides several benefits:

• Conciseness: It allows for writing more readable and concise code using a functional style.
• Lazy Evaluation: Streams use lazy evaluation, processing elements only when necessary, which can improve performance.
• Parallel Processing: Streams can be processed in parallel with minimal effort, improving performance on multicore processors.
• Declarative Approach: Streams encourage a declarative approach to programming, focusing on what to do rather than how to do it.

51. What is the difference between == and equals() when comparing two objects?

• == checks if two references point to the same object in memory. It compares the memory addresses.
• equals() checks if two objects are logically equivalent. This method can be overridden in custom classes to define what equality means for those objects.

52. What are default methods in interfaces?

Answer: Default methods in interfaces, introduced in Java 8, allow developers to add new methods to interfaces without breaking existing implementations. Default methods provide a method body

Key Takeaways from Java Interview Questions for 2 to 3 years experienced

If you are a Java programmer with 2 to 3 years of experience and looking for a job in this field, then you must know what is specifically expected from an experienced Java programmer in an interview.
Some of the Java interview questions that are discussed above are to be expected from a Java-experienced programmer. You need to have a basic knowledge of most of the above to qualify for the interview, as the world is always evolving, and someone will always be ahead of you in the race. As they say, “Knowledge is power. ” You can ace the Java interview by knowing things and hoping for the best.

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