Core Java Interview Questions and Answers

1. What is Java?

**Java** is a high-level, object-oriented, and class-based programming language developed by **Sun Microsystems** (now owned by Oracle). It is designed to have as few implementation dependencies as possible, making it a **write once, run anywhere** (WORA) language. Java applications are compiled into bytecode that can run on any Java Virtual Machine (JVM), making them platform-independent.

2. Explain the Features of Java

Important features of Java include:

Object-Oriented: Everything in Java is treated as an object.
Platform-Independent: Java code runs on any system that has the JVM installed.
Secure: Java has runtime checking and bytecode verification for enhanced security.
Multithreaded: Java supports multithreaded programming.
Robust: It handles memory management and exceptions effectively.
High Performance: Bytecode is highly optimized, enabling faster execution.

3. What is JVM, JRE, and JDK?

JVM (Java Virtual Machine): Executes Java bytecode. It's platform-dependent.

JRE (Java Runtime Environment): Includes JVM and libraries required to run Java applications.

JDK (Java Development Kit): Contains JRE and development tools like the compiler and debugger, used for developing Java programs.

4. What is the Difference Between == and .equals() Method?

The == operator compares the **reference** (memory address), whereas the .equals() method compares the **content** of objects. For example:

 String a = new String("Java"); String b = new String("Java"); System.out.println(a == b); // false System.out.println(a.equals(b)); // true

5. What is a Constructor in Java?

A **constructor** is a special method used to initialize objects. It has the same name as the class and does not have a return type. Java supports two types of constructors:

Default Constructor – Provided by Java if no constructor is defined.
Parameterized Constructor – Accepts arguments for initialization.

6. Difference Between Method Overloading and Method Overriding

Method Overloading occurs when multiple methods in the same class have the same name but different parameters. It is **compile-time polymorphism**.

Method Overriding occurs when a subclass provides a specific implementation for a method already defined in its parent class. It is **runtime polymorphism**.

7. What is Inheritance in Java?

**Inheritance** allows a class to inherit properties and methods of another class using the extends keyword. It promotes code reusability. Java supports single, multilevel, and hierarchical inheritance but does not support multiple inheritance using classes to avoid ambiguity.

8. What is Abstraction?

**Abstraction** is the process of hiding the implementation details and showing only the functionality. In Java, abstraction is achieved using:

Abstract Classes: Declared using the abstract keyword.
Interfaces: Declared using the interface keyword. Java 8+ allows default and static methods in interfaces.

9. What is Encapsulation in Java?

**Encapsulation** is the technique of wrapping data and methods into a single unit, typically using a class. It’s achieved by making fields private and providing public getter/setter methods. This ensures controlled access and data protection.

10. What is the Difference Between Array and ArrayList?

Array: Fixed in size, can hold both primitives and objects.

ArrayList: Part of the Collection Framework, resizable, holds only objects, and offers methods like add(), remove(), etc.

11. What is Exception Handling in Java?

**Exception Handling** is a mechanism to handle runtime errors. Java provides five keywords:

try – Code that might throw an exception.
catch – Handles the exception.
finally – Code that always executes.
throw – Used to explicitly throw an exception.
throws – Declares exceptions.

12. Explain the Lifecycle of a Thread in Java

The **thread lifecycle** consists of the following states:

New: Thread object created.
Runnable: Thread is ready to run.
Running: Thread is executing.
Blocked/Waiting: Thread is waiting for a resource.
Terminated: Thread has completed execution.

13. What is the Difference Between final, finally, and finalize?

final: A keyword used with variables, methods, and classes to restrict modification.

finally: A block that always executes after try-catch, used for cleanup code.

finalize(): A method called by garbage collector before object destruction (deprecated in newer Java versions).

14. What is the Difference Between HashMap and Hashtable?

HashMap: Not synchronized, permits one null key and multiple null values.

Hashtable: Synchronized, does not allow null keys or null values. HashMap is preferred for non-thread-safe environments due to better performance.

15. What are Wrapper Classes in Java?

Wrapper classes convert primitive types into objects. For example:

int → Integer
char → Character
double → Double

They are useful in collections, as collections can only store objects.

16. What is Garbage Collection in Java?

**Garbage Collection** is the process of automatically deleting unused objects to free up memory. Java provides automatic garbage collection via the JVM. The System.gc() method suggests the JVM to perform garbage collection.

17. Explain Static Keyword in Java

The static keyword is used for memory management. It can be applied to variables, methods, blocks, and nested classes. Static members belong to the class, not the instance.

18. What is the Difference Between Abstract Class and Interface?

Abstract Class: Can have both abstract and concrete methods, supports constructors, and can maintain state.

Interface: Cannot have constructors, only allows static and default methods (from Java 8), and is fully abstract by default.

19. What are Access Modifiers in Java?

Access modifiers define the scope of class members:

private: Accessible within the class.
default: Accessible within the same package.
protected: Accessible in the same package or subclasses.
public: Accessible everywhere.

20. What is the Use of the 'super' Keyword?

The super keyword refers to the immediate parent class object. It can be used to:

Call the parent class constructor.
Access parent class methods and variables.

Conclusion

Mastering these **Core Java interview questions and answers** can significantly boost your confidence and improve your chances of success in technical interviews. Consistent practice, along with understanding the underlying concepts, will help you tackle real-world Java problems with ease.