Basic Java Interview Questions

1. What is Java?

Java is an object-oriented, high-level programming language created by Sun Microsystems, which is currently owned by Oracle. With a Java Virtual Machine (JVM), Java applications can run on any platform by adhering to the "Write Once, Run Anywhere" (WORA) principle.

2. What are the key features of Java?

Java has several features that make it popular:

Platform Independence – Java programs run on any OS with a JVM.
Object-Oriented – Everything in Java is based on objects and classes.
Automatic Memory Management – Java uses garbage collection to handle memory.
Multi-threading – Java supports concurrent execution of multiple threads.
Security – Java provides runtime security with its robust API.

3. What is the difference between JDK, JRE, and JVM?

JDK (Java Development Kit) – Includes tools for developing Java applications, such as a compiler and debugger.
JRE (Java Runtime Environment) – Contains libraries and JVM to run Java programs.
JVM (Java Virtual Machine) – Converts Java bytecode into machine-specific code for execution.

4. What distinguishes an interpreted language from a compiled language?While an interpreted language transforms code into machine code line by line while it is being executed, a compiled language transforms the complete source code into machine code prior to execution. Java employs both strategies: it converts source code into bytecode, which the JVM interprets.

5. What is bytecode in Java?

Java source code is represented by bytecode, which is run by the JVM instead of the CPU directly. It enables platform independence for Java apps.

Object-Oriented Programming (OOP) Concepts in Java

6. What are the four main OOP principles?

The four main principles of object-oriented programming in Java are:

Encapsulation – Restricts direct access to object data using access modifiers (private, protected).
Inheritance – Allows one class to inherit properties and behavior from another class.
Polymorphism – Enables a single method or function to take different forms (overloading and overriding).
Abstraction – Hides implementation details and exposes only relevant functionalities.

7. What is the difference between an interface and an abstract class?

Abstract Class – Can have both abstract and concrete methods. It can have constructors and instance variables.
Interface – Contains only abstract methods (before Java 8) and constants. Multiple interfaces can be implemented in a class.

8. What is method overloading and method overriding?

Method Overloading – Occurs when multiple methods in a class have the same name but different parameter lists.
Method Overriding – Occurs when a subclass provides a specific implementation of a method already defined in its parent class.

9. What is the difference between static and non-static methods?

Static Methods – Belong to the class and can be called without creating an object.
Non-Static Methods – Require an instance of the class to be invoked.

10. What is the significance of the ‘super’ keyword?

In order to access the parent class's methods, constructors, and attributes, the super keyword is utilized.

Exception Handling in Java

11. What is an exception in Java?

An occurrence that interferes with regular program execution is called an exception. Java comes with a built-in exception-handling system that uses throw, catch, try, and finally.

12. What is the difference between checked and unchecked exceptions?

Checked Exceptions – Must be handled using try-catch or declared using throws. Examples: IOException, SQLException.
Unchecked Exceptions – Occur at runtime and do not require handling. Examples: NullPointerException, ArithmeticException.

13. What is the difference between ‘throw’ and ‘throws’?

throw – Used to explicitly throw an exception within a method.
throws – Declares exceptions that a method might throw.

14. What is the finally block?

Regardless of whether an exception is raised, the code in the finally block runs after the try block. Database connections and file streams are among the resources that are released via it.

15. How do you create a custom exception in Java?

Implementing a constructor and extending the Exception class results in a custom exception.

Java Collections Framework

16. What is the Java Collections Framework?

A collection of classes and interfaces for effectively storing and managing collections of objects are offered by the Java Collections Framework (JCF).

17. What are the main interfaces in the Collections Framework?

List – Ordered collection (ArrayList, LinkedList).
Set – Unique elements (HashSet, TreeSet).
Queue – FIFO structure (PriorityQueue).
Map – Key-value pairs (HashMap, TreeMap).

18. What is the difference between ArrayList and LinkedList?

ArrayList – Fast for retrieval but slow for insertions/deletions.
LinkedList – Efficient for insertions/deletions but slower for access.

19. What is the difference between HashSet and TreeSet?

HashSet – Unordered and allows null elements.
TreeSet – Ordered and does not allow null elements.

20. What is the difference between HashMap and TreeMap?

HashMap – Unordered, faster operations.
TreeMap – Sorted, slower operations.

Multi-threading in Java

21. What is multi-threading?

Multi-threading allows multiple tasks to execute concurrently to improve performance.

22. What are the two ways to create a thread in Java?

Extending the Thread class.
Implementing the Runnable interface.

23. What is the difference between process and thread?

Process – An independent executing instance.
Thread – A lightweight process that shares resources.

24. What is synchronization in Java?

Synchronization ensures that only one thread accesses a shared resource at a time, preventing data inconsistencies.

25. What is the difference between wait() and sleep()?

wait() – Releases the lock and waits for a signal.
sleep() – Pauses execution without releasing the lock.

Java Memory Management & Garbage Collection

26. What is memory management in Java?

Java manages memory automatically using Heap and Stack memory. The JVM allocates memory for objects and reclaims unused memory using Garbage Collection.

27. What is the difference between Stack and Heap memory?

Feature	Stack Memory	Heap Memory
Usage	Stores method calls and local variables.	Stores objects and class instances.
Access	Fast access and automatically managed.	Slower but allows dynamic allocation.
Scope	Exists only during method execution.	Exists as long as the object is referenced.

28. What is Garbage Collection in Java?

Garbage Collection (GC) is a process in Java where the JVM automatically removes unused objects to free up memory.

29. What are the different types of Garbage Collectors in Java?

Serial GC – Best for small applications with a single-threaded environment.
Parallel GC – Uses multiple threads for better performance.
CMS (Concurrent Mark-Sweep) GC – Reduces application pauses.
G1 (Garbage First) GC – Designed for large heap memory.

30. How can you request Garbage Collection in Java?

By calling System.gc() or using the Runtime.getRuntime().gc(). However, GC is managed by the JVM, so calling these methods does not guarantee immediate execution.

Java 8 Features & Collections Framework

31. What are the key features introduced in Java 8?

Lambda Expressions – Enables functional programming.
Stream API – Helps process data in a declarative way.
Functional Interfaces – Interfaces with a single abstract method (@FunctionalInterface).
Default Methods in Interfaces – Allow method implementation inside interfaces.
Optional Class – Helps avoid NullPointerException.

32. What is a Lambda Expression in Java?

Lambda Expressions provide a concise way to write anonymous functions in Java. Example:

// Without Lambda

Runnable r = new Runnable() {

public void run() {

System.out.println("Running...");

}

};

// With Lambda

Runnable r = () -> System.out.println("Running...");

33. What is the Java Collections Framework?

The Java Collections Framework (JCF) is a set of classes and interfaces that provide data structures like List, Set, Queue, and Map for managing and manipulating collections of objects.

34. What is the difference between List, Set, and Map?

Feature	List	Set	Map
Duplicates	Allowed	Not allowed	Keys unique, values can be duplicate
Ordering	Maintains insertion order	No specific order	Key-value pairs
Examples	ArrayList, LinkedList	HashSet, TreeSet	HashMap, TreeMap

35. What is the difference between ArrayList and LinkedList?

Feature	ArrayList	LinkedList
Data Structure	Dynamic array	Doubly linked list
Access Speed	Fast (O(1) for get)	Slow (O(n) for get)
Insertion/Deletion	Slow (O(n))	Fast (O(1) at head/tail)

36. What is the difference between HashMap and TreeMap?

Feature	HashMap	TreeMap
Ordering	Unordered	Sorted in natural order
Performance	O(1) for get/put	O(log n) for get/put

37. What is the difference between fail-fast and fail-safe iterators?

Fail-fast iterators throw ConcurrentModificationException if modified during iteration (e.g., ArrayList).
Fail-safe iterators allow modifications without exceptions (e.g., CopyOnWriteArrayList).

Java Threads and Concurrency

39. What is thread safety in Java?

Thread safety ensures that shared resources are accessed synchronously to avoid race conditions and data inconsistency.

40. What is the difference between synchronized method and synchronized block?

Feature	Synchronized Method	Synchronized Block
Scope	Entire method	Specific part of the code
Performance	Slower	Faster

41. What is a volatile keyword?

The volatile keyword ensures that a variable’s value is always read from main memory instead of CPU cache, preventing inconsistent data.

42. What is the difference between Executor and ExecutorService?

Executor – Interface for launching tasks in a thread pool.
ExecutorService – Extends Executor, providing more control like task termination.

43. What are design patterns in Java?

Design patterns are reusable solutions to common software design problems. They provide best practices for object-oriented programming.

44. What are the three types of design patterns?

Creational Patterns – Focus on object creation (e.g., Singleton, Factory).
Structural Patterns – Define relationships between objects (e.g., Adapter, Decorator).
Behavioral Patterns – Manage object interactions (e.g., Observer, Strategy).

45. What is the Singleton pattern?

The Singleton pattern ensures a class has only one instance throughout the application.

46. What is the Factory pattern?

The Factory pattern creates objects without exposing the instantiation logic to the client.

47. What is the Observer pattern?

The Observer pattern allows an object to notify multiple observers when a state change occurs.

48. What is serialization in Java?

Serialization converts an object into a byte stream for saving or transferring over a network.

49. What is the difference between transient and static variables in serialization?

transient – Not serialized, meaning it won't be saved.
static – Not part of object state, so it’s not serialized.

50. What is the serialVersionUID?

serialVersionUID is a unique identifier that ensures object compatibility during deserialization.

Conclusion

Java is a powerful, versatile language, and mastering these interview questions will help you excel in Java programming roles.

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Top 50 Java Interview Questions and Answers