Java Interview Questions and Answers: JDK, JRE, JVM, OOP Concepts, and More

Q1. Explain JDK, JRE and JVM?

JDK

JRE

JVM

Java Development Kit.

Java Runtime Environment.

Java Virtual Machine.

Tools required to compile, document and package Java programs.

Runtime environment that can execute Java bytecode.

An abstract machine that provides a runtime environment for executing Java bytecode.

Contains JRE + development tools.

Actual implementation of the JVM.

JVM has three representations: specification, implementation, and runtime instance.

Q2. Explain public static void main(String[] args) in Java.

The main() method is the entry point of any Java program and is always declared as public static void main(String[] args) .

public : access modifier; any class can invoke the method.

static : class‑level keyword; the method can be called without creating an instance.

void : return type; the method returns no value.

main : the name searched for by the JVM as the program start.

String[] args : array of arguments passed from the command line.

Q3. Why is Java platform‑independent?

Java bytecode can run on any operating system because the JVM abstracts away the underlying platform.

Q4. Why is Java not 100% object‑oriented?

Java defines eight primitive data types (boolean, byte, char, int, float, double, long, short) that are not objects.

Q5. What are wrapper classes in Java?

Wrapper classes convert primitive types to reference types. Each primitive has a corresponding wrapper (e.g., Integer for int ).

Q6. What is a constructor in Java?

A constructor initializes an object; it has the same name as the class, no return type, and is invoked automatically when an object is created.

Two types of constructors:

Default constructor : a no‑argument constructor automatically provided if no other constructors are defined.

Parameterized constructor : a constructor that accepts arguments to initialize instance variables.

Q7. What is a Singleton class and how to make a class a singleton?

A Singleton class allows only one instance per JVM. Making the constructor private enforces the singleton property.

Q8. Difference between ArrayList and Vector.

ArrayList

Vector

Not synchronized (faster).

Synchronized (thread‑safe, slower).

Increases size by ~50% when needed.

Doubles its size by default.

No defined increment size.

Has a defined increment size.

Iterated using

Iterator

only.

Can be iterated using

Enumeration

or

Iterator

.

Q9. Difference between equals() and == in Java.

equals() is a method defined in Object that can be overridden to compare logical equality of objects.

== is a binary operator that compares primitive values or object references for identity.

Q10. Difference between heap and stack memory.

Feature

Stack

Heap

Memory

Used by a single execution thread.

Shared by all parts of the application.

Access

Not accessible by other threads.

Globally accessible.

Memory management

LIFO release.

Managed per object lifecycle.

Lifetime

Exists until the thread finishes.

Exists from program start to termination.

Usage

Stores local primitives and reference variables.

Stores all objects.

Q11. What is a Java package and its advantages?

A package groups related classes and interfaces. Advantages include name conflict avoidance, easier access control, encapsulation of hidden classes, and a logical hierarchy for easier navigation.

Q12. Why does Java not use pointers?

Pointers are considered unsafe and would increase code complexity. The JVM handles memory allocation, so direct memory access is unnecessary.

Q13. What is the JIT compiler in Java?

The Just‑In‑Time (JIT) compiler translates bytecode into native machine instructions at runtime, improving performance by compiling frequently executed methods.

Q14. What are access modifiers in Java?

Java provides four access modifiers: default , private , protected , and public . Their visibility is summarized in the table below.

Modifier

Default

Private

Protected

Public

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

Q15. Define a Java class.

A class is a blueprint that contains fields (variables) and methods describing object behavior.

class Abc {
    // member variables
    // class body methods
}

Q16. What is an object in Java and how to create one?

An object represents a real‑world entity with state, behavior, and identity. It is created using the new keyword, e.g., ClassName obj = new ClassName(); .

Q17. What is Object‑Oriented Programming (OOP)?

OOP organizes programs around objects rather than functions, making it suitable for large, complex, and maintainable codebases.

Q18. Main OOP concepts in Java.

Inheritance : a class acquires properties of another class.

Encapsulation : bundling data and methods together.

Abstraction : exposing only essential features.

Polymorphism : ability of a variable, function, or object to take multiple forms.

Q19. Difference between local variables and instance variables.

Local variables are declared inside methods, constructors, or blocks and have limited scope. Instance variables are declared at class level (outside methods) and belong to each object instance.

if (x > 100) {
    String test = "Edureka";
}

class Test {
    public String empName;
    public int empAge;
}

Q20. Distinguish between methods and constructors.

Method

Constructor

Represents object behavior.

Initializes object state.

Must have a return type.

No return type.

Called explicitly.

Called implicitly.

Compiler does not provide a default.

Compiler provides a default if none is defined.

Name can differ from class name.

Name must match class name.

Q21. What does the final keyword mean in Java?

final can be applied to variables, methods, and classes:

Final variable : value cannot be changed after assignment.

Final method : cannot be overridden by subclasses.

Final class : cannot be subclassed.

Q22. Difference between break and continue .

break

continue

Can be used in switch and loops.

Can be used only in loops.

Terminates the innermost switch or loop.

Skips the current iteration and proceeds to the next.

Exits the block immediately.

Continues with the next iteration of the loop.

for (int i = 0; i < 5; i++) {
    if (i == 3) {
        break;
    }
    System.out.println(i);
}

for (int i = 0; i < 5; i++) {
    if (i == 2) {
        continue;
    }
    System.out.println(i);
}

Q23. What is an infinite loop in Java? Example.

An infinite loop runs indefinitely until externally terminated.

public class InfiniteForLoopDemo {
    public static void main(String[] args) {
        for (;;) {
            System.out.println("Welcome to Edureka!");
        }
    }
}

Q24. Difference between this() and super() .

this()

super()

Refers to the current instance of the class.

Refers to the current instance of the parent class.

Calls the class's own constructor.

Calls the parent class's constructor.

Used to access methods of the current class.

Used to access methods of the superclass.

Must be the first statement in a constructor block.

Must be the first statement in a constructor block.

Q25. What is the Java String pool?

The String pool stores string literals in heap memory; if a string already exists, the same reference is reused, improving memory usage and performance.

Q26. Distinguish static and non‑static methods.

Static method

Non‑static method

Declared with the

static

keyword.

No

static

keyword required.

Called using the class name (ClassName.methodName).

Called on an instance of the class.

Cannot access instance variables or methods directly.

Can access both static and instance members.

Q27. What is constructor chaining in Java?

Constructor chaining is the process where one constructor calls another constructor of the same class (using this() ) or the superclass (using super() ).

Q28. Difference between String, StringBuilder, and StringBuffer.

Aspect

String

StringBuilder

StringBuffer

Storage

String pool (constant area)

Heap

Heap

Mutability

Immutable

Mutable

Mutable

Thread safety

Thread‑safe (because immutable)

Not thread‑safe

Thread‑safe

Performance

Fast for read‑only operations

More efficient for frequent modifications

Less efficient due to synchronization

Q29. What is a class loader in Java?

A class loader loads class files into the JVM. Java provides three built‑in class loaders: Bootstrap ClassLoader, Extension ClassLoader, and System/Application ClassLoader.

Q30. Why are Java strings immutable?

Immutability allows strings to be safely shared, cached in the String pool, and improves security, synchronization, and performance.

Q31. Difference between arrays and ArrayLists.

Array

ArrayList

Can hold only one data type (including primitives).

Can hold objects of any type (generics allow type safety).

Size fixed at declaration.

Size dynamic; can grow/shrink.

Requires explicit index to add elements.

No need to specify index for addition.

Not parameterized.

Parameterized (e.g.,

ArrayList<String>

).

Can store primitive types and objects.

Can store only objects (primitives must be boxed).

Q32. What is a Map in Java?

A Map is an interface in java.util that maps unique keys to values. Keys are not duplicated, and each key maps to at most one value.

Q33. What are Java collection classes? Overview of methods and interfaces.

The Java Collections Framework provides interfaces (e.g., List , Set , Map ) and concrete classes (e.g., ArrayList , HashSet , HashMap ) for storing and manipulating groups of objects.

If you have any questions about these Java interview topics, feel free to comment below.