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Java Interview Questions and Answers

Top 100 Java Interview Questions for Freshers

1. What is Java?

Java is a high-level, object-oriented programming language known for its portability, security, and robustness. It follows the "Write Once, Run Anywhere" (WORA) principle, making it platform-independent via the Java Virtual Machine (JVM).

2. What are Java's key features?

Platform-independent (via JVM)
Object-oriented (supports OOP principles)
Robust and secure
Multi-threaded (supports concurrent execution)
Garbage collection (automatic memory management)
Rich standard library (Java API)

3. What is Java coding convention?

Java has an official Java Code Conventions guide that defines best practices for writing clean, readable, and maintainable code, including naming conventions, indentation, and class structure.

4. What is the difference between an ArrayList and a LinkedList?

ArrayList: Uses a dynamic array, faster for random access (O(1)) but slower for insertions and deletions (O(n)).
LinkedList: Uses doubly linked nodes, better for insertions/deletions (O(1) at ends) but slower for random access (O(n)).

5. What is the difference between Java 8 and Java 11?

Java 8 introduced Lambda expressions, Streams API, and the Date/Time API.
Java 11 introduced local variable inference (var), new methods in String, Optional, and Files API, and removed JavaFX from the JDK.

6. How is memory managed in Java?

Java uses automatic memory management, including:

Heap Memory: Stores objects and class instances.
Stack Memory: Stores method execution details and local variables.
Garbage Collection (GC): Automatically deallocates unreferenced objects.

7. What are Java’s data types?

Primitive types: int, double, char, boolean, byte, short, long, float
Non-primitive types: String, Arrays, Classes, Interfaces

8. What are Java’s built-in data structures?

ArrayList (dynamic array)
LinkedList (doubly linked list)
HashMap (key-value pair)
HashSet (unique elements, unordered)
Stack (LIFO)
Queue (FIFO)

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

== compares object references (memory addresses).
.equals() compares object values (overridden for meaningful equality).

10. What are Java methods?

Methods are reusable blocks of code that perform specific tasks, defined using void or return types like int, String, etc.

public int add(int a, int b) { return a + b; }

11. What is the difference between clone() and copy()?

Shallow Copy (clone()): Creates a new object but references the same nested objects.
Deep Copy: Duplicates all objects recursively.

12. What is a lambda function in Java?

Lambda expressions are used to create anonymous functions:

interface MathOperation { int operation(int a, int b); } MathOperation add = (a, b) -> a + b; System.out.println(add.operation(3, 4)); // Output: 7

13. What are Java annotations?

Annotations provide metadata about code. Example:

@interface MyAnnotation {} class Example { @Override public String toString() { return "Example"; } }

14. What are varargs in Java?

varargs (...) allows passing a variable number of arguments:

public void printNumbers(int... numbers) { for (int num : numbers) { System.out.print(num + " "); } } printNumbers(1, 2, 3, 4, 5);

15. What is the difference between a class and a package?

Class: A blueprint for objects.
Package: A collection of related classes (java.util, java.io).

16. What is the difference between static, instance, and abstract methods?

Instance method: Requires an object (obj.method()).
Static method: Defined with static, belongs to the class (Class.method()).
Abstract method: Declared in an abstract class/interface, must be implemented in subclasses.

17. How does exception handling work in Java?

Using try-catch-finally:

try { int x = 1 / 0; } catch (ArithmeticException e) { System.out.println("Error: " + e.getMessage()); } finally { System.out.println("Execution completed."); }

18. What are Java’s file handling modes?

"r" – Read
"w" – Write (overwrite)
"a" – Append
"rb"/"wb" – Binary read/write

Example:

FileWriter writer = new FileWriter("file.txt", true); writer.write("Hello, Java!"); writer.close();

19. What is list comprehension equivalent in Java?

Using Streams and Lambda:

List<Integer> numbers = IntStream.range(0, 10) .filter(x -> x % 2 == 0) .map(x -> x * x) .boxed() .collect(Collectors.toList()); System.out.println(numbers); // [0, 4, 16, 36, 64]

20. How do you implement multithreading in Java?

Using Thread or Runnable:

class MyThread extends Thread { public void run() { for (int i = 0; i < 5; i++) { System.out.println(i); } } } MyThread thread = new MyThread(); thread.start();

OR using Runnable:

class MyRunnable implements Runnable { public void run() { for (int i = 0; i < 5; i++) { System.out.println(i); } } } Thread thread = new Thread(new MyRunnable()); thread.start();

21. What is the difference between String, StringBuilder, and StringBuffer?

String: Immutable (creates a new object on modification).
StringBuilder: Mutable, faster but not thread-safe.
StringBuffer: Mutable and thread-safe (synchronized methods).

StringBuilder sb = new StringBuilder("Hello"); sb.append(" World"); System.out.println(sb); // Output: Hello World

22. What is method overloading and method overriding?

Method Overloading: Same method name, different parameters (compile-time polymorphism).
Method Overriding: Redefining a method in a subclass (runtime polymorphism).

Example of Overloading:

class MathUtil { int add(int a, int b) { return a + b; } double add(double a, double b) { return a + b; } }

Example of Overriding:

class Parent { void show() { System.out.println("Parent class"); } } class Child extends Parent { @Override void show() { System.out.println("Child class"); } }

23. What are Java access modifiers?

public – Accessible everywhere.
private – Accessible only within the class.
protected – Accessible within the package and subclasses.
Default (no modifier) – Accessible only within the package.

24. What is a constructor in Java?

A constructor is a special method that initializes objects.

class Example { int x; Example(int val) { x = val; } } Example obj = new Example(10);

Types:

Default Constructor (if none is defined).
Parameterized Constructor (takes arguments).
Copy Constructor (copies an object).

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

FeatureInterfaceAbstract Class

MethodsAll methods are abstract (default in Java 7)Can have abstract & concrete methods

FieldsOnly constants (public static final)Can have variables and constants

Multiple InheritanceAllowed (a class can implement multiple interfaces)Not allowed (can only extend one class)

Example of Interface:

interface Animal { void sound(); } class Dog implements Animal { public void sound() { System.out.println("Bark!"); } }

Example of Abstract Class:

abstract class Vehicle { abstract void start(); } class Car extends Vehicle { void start() { System.out.println("Car starts with key."); } }

1. What is the difference between a shallow copy and a deep copy?

Shallow Copy: Only copies references to nested objects.
Deep Copy: Creates an independent copy of the entire object.

import java.util.Arrays; class CopyExample implements Cloneable { int[] data; CopyExample(int[] data) { this.data = data; } @Override protected Object clone() throws CloneNotSupportedException { return super.clone(); // Shallow copy } CopyExample deepCopy() { return new CopyExample(Arrays.copyOf(this.data, this.data.length)); } } public class Main { public static void main(String[] args) throws CloneNotSupportedException { int[] arr = {1, 2, 3}; CopyExample original = new CopyExample(arr); CopyExample shallow = (CopyExample) original.clone(); CopyExample deep = original.deepCopy(); shallow.data[0] = 100; // Affects original System.out.println(Arrays.toString(original.data)); // [100, 2, 3] System.out.println(Arrays.toString(deep.data)); // [1, 2, 3] } }

2. Difference between static and instance methods?

Static Method: Belongs to the class.
Instance Method: Belongs to an object.

class Example { static void staticMethod() { System.out.println("Static Method"); } void instanceMethod() { System.out.println("Instance Method"); } } public class Main { public static void main(String[] args) { Example.staticMethod(); Example obj = new Example(); obj.instanceMethod(); } }

3. Difference between an Iterator and a Stream?

Iterator: Manual iteration using next().
Stream: Functional and lazy evaluation.

import java.util.*; public class Main { public static void main(String[] args) { List<Integer> list = Arrays.asList(1, 2, 3, 4); Iterator<Integer> it = list.iterator(); while (it.hasNext()) System.out.print(it.next() + " "); // 1 2 3 4 list.stream().forEach(System.out::println); } }

4. How does Java manage memory?

Heap Memory: Stores objects.
Stack Memory: Stores method calls and local variables.
Garbage Collection: Removes unused objects.

5. What is the Java Memory Model (JMM)?

JMM ensures:

Visibility (changes visible to threads).
Atomicity (operations complete fully).
Ordering (instructions execute predictably).

6. How do you handle circular dependencies?

Use dependency injection.

class A { B b; void setB(B b) { this.b = b; } } class B { A a; void setA(A a) { this.a = a; } }

7. Difference between == and .equals()?

== compares memory references.
.equals() compares values.

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

8. Difference between remove(), delete(), and pop()?

remove(): Removes by index/value.
pop(): Removes from a stack.

import java.util.*; public class Main { public static void main(String[] args) { List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3)); list.remove(1); // Removes index 1 Stack<Integer> stack = new Stack<>(); stack.push(5); stack.pop(); } }

9. Difference between map(), filter(), and reduce()?

import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) { List<Integer> list = Arrays.asList(1, 2, 3, 4); List<Integer> squared = list.stream().map(x -> x * x).collect(Collectors.toList()); List<Integer> evens = list.stream().filter(x -> x % 2 == 0).collect(Collectors.toList()); int sum = list.stream().reduce(0, Integer::sum); System.out.println(squared); System.out.println(evens); System.out.println(sum); } }

10. What is Monkey Patching?

Not natively supported in Java. You can use reflection.

11. What are hashable and unhashable types?

Hashable: Used in HashMap (String, Integer).
Unhashable: Mutable types like ArrayList.

12. How can you improve performance in Java?

Use StringBuilder.
Use Parallel Streams.
Use HashMap instead of List for lookups.

13. Difference between toString() and valueOf()?

int x = 10; String s = String.valueOf(x); System.out.println(s);

14. Merging two maps in Java

import java.util.*; public class Main { public static void main(String[] args) { Map<String, Integer> map1 = new HashMap<>(Map.of("a", 1, "b", 2)); Map<String, Integer> map2 = new HashMap<>(Map.of("b", 3, "c", 4)); map1.putAll(map2); System.out.println(map1); // {a=1, b=3, c=4} } }

15. How does Callable work?

import java.util.concurrent.*; class Task implements Callable<Integer> { public Integer call() { return 42; } } public class Main { public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newSingleThreadExecutor(); Future<Integer> future = executor.submit(new Task()); System.out.println(future.get()); // 42 executor.shutdown(); } }

Here are the last 10 Java interview questions with simplified theoretical explanations:

16. What is the difference between Checked and Unchecked Exceptions?

Checked Exception: These are exceptions that must be handled using try-catch or declared with throws. They occur during compile-time. Example: IOException, SQLException.
Unchecked Exception: These exceptions occur during runtime and do not require mandatory handling. Example: NullPointerException, ArithmeticException.

17. What is the difference between final, finally, and finalize()?

final: A keyword used to restrict modifications.
- final variable: Value cannot be changed.
- final method: Cannot be overridden.
- final class: Cannot be inherited.
finally: A block that always runs after try-catch, used for cleanup operations.
finalize(): A method that the garbage collector calls before an object is destroyed.

18. What is the difference between method overloading and method overriding?

Method Overloading: Defining multiple methods in the same class with the same name but different parameters.
Method Overriding: Changing the behavior of a method in a subclass that is already defined in the parent class.

19. What is the Singleton pattern?

A design pattern that ensures a class has only one instance during runtime.
Used in logging, database connections, and configuration settings.
Achieved using a private constructor and a static method to return the instance.

20. What is the difference between HashMap and TreeMap?

FeatureHashMapTreeMap

OrderNo orderSorted by key

Null KeyAllowedNot allowed

PerformanceFasterSlower

HashMap: Stores key-value pairs with no specific order, offers faster performance.
TreeMap: Stores key-value pairs in sorted order (ascending by default).

21. What is the difference between synchronized and volatile?

synchronized: Used to lock a method or block so that only one thread can access it at a time. Prevents data inconsistency in multithreading.
volatile: Ensures that changes made to a variable by one thread are immediately visible to all other threads. Does not provide locking.

22. What is a Functional Interface?

A Functional Interface is an interface that has only one abstract method but can have multiple default/static methods.
Used in Lambda Expressions and Stream API.
Example: Runnable, Callable, Comparator.

23. What is the default capacity of an ArrayList?

The default capacity of an ArrayList in Java is 10.
When the list exceeds this size, its capacity automatically increases by 50% of the current size.

24. What is fork() and join() in Java?

fork() and join() are used in parallel computing with the Fork/Join framework.
fork(): Splits a large task into multiple smaller tasks that run in parallel.
join(): Combines the results of multiple smaller tasks after they complete execution.

25. What is the difference between Comparable and Comparator?

FeatureComparableComparator

SortingNatural orderCustom order

MethodcompareTo()compare()

Used inSingle classSeparate class

Comparable: Defines a default sorting order (e.g., sorting students by marks).
Comparator: Allows custom sorting (e.g., sorting by name, age, or marks).

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

JDK (Java Development Kit) → Includes JRE + development tools (compiler, debugger).
JRE (Java Runtime Environment) → Provides libraries + JVM to run Java applications.
JVM (Java Virtual Machine) → Runs Java bytecode, making Java platform-independent.

2. What are the main OOP concepts in Java?

Encapsulation → Data hiding using private variables and public methods.
Abstraction → Hiding implementation details using abstract classes or interfaces.
Inheritance → Reusing properties/methods from a parent class.
Polymorphism → Overloading (compile-time) and overriding (runtime) methods.

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

== → Compares memory addresses (checks if two references point to the same object).
.equals() → Compares actual content of objects (used for strings and custom objects).

Example:

String s1 = new String("Hello"); String s2 = new String("Hello"); System.out.println(s1 == s2); // false (different memory locations) System.out.println(s1.equals(s2)); // true (same content)

4. What are static variables and methods in Java?

Static variables → Shared among all objects of a class.
Static methods → Can be called without creating an object.

Example:

class Example { static int count = 0; // Static variable static void showCount() { // Static method System.out.println("Count: " + count); } }

5. What is the difference between final, finally, and finalize?

final → Used for constants, methods (prevent overriding), and classes (prevent inheritance).
finally → Block that always executes after try-catch.
finalize() → Method used before garbage collection.

6. What is method overloading and method overriding?

Method Overloading → Same method name, different parameters (compile-time polymorphism).
Method Overriding → Subclass provides a new implementation of a method (runtime polymorphism).

Example:

class Parent { void show() { System.out.println("Parent class"); } } class Child extends Parent { @Override void show() { System.out.println("Child class"); } }

7. What are constructors in Java?

A constructor is a special method called when an object is created.

Default Constructor → No arguments, initializes default values.
Parameterized Constructor → Takes arguments to initialize fields.
Copy Constructor → Creates a copy of an existing object.

Example:

class Example { int num; Example(int n) { // Parameterized constructor num = n; } }

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

FeatureInterfaceAbstract Class

MethodsOnly abstract methods (Java 7), default/static methods (Java 8)Can have abstract & concrete methods

VariablesPublic, static, final (constants)Can have instance variables

InheritanceSupports multiple inheritanceSingle inheritance

9. What is the difference between HashMap and HashSet?

FeatureHashMapHashSet

StoresKey-value pairsUnique values only

Allows Null?1 null key, multiple null values1 null value

Duplicates?No duplicate keysNo duplicates

Example:

HashMap<Integer, String> map = new HashMap<>(); map.put(1, "A"); HashSet<Integer> set = new HashSet<>(); set.add(1);

10. What is exception handling in Java?

Exception handling prevents program crashes by catching errors.

Checked Exceptions → Checked at compile-time (e.g., IOException).
Unchecked Exceptions → Runtime errors (e.g., NullPointerException).

Example:

try { int x = 5 / 0; // ArithmeticException } catch (ArithmeticException e) { System.out.println("Cannot divide by zero!"); } finally { System.out.println("This will always execute."); }

Q1: What are Java’s key features?

Object-Oriented: Java is fully object-oriented.
Platform-Independent: Java code runs on any system with a Java Virtual Machine (JVM).
Simple and Easy to Learn: Java has a user-friendly syntax.
Rich API: Provides extensive libraries for various functionalities.
Multithreading: Supports multithreaded programming for concurrent tasks.
Automatic Garbage Collection: Java automatically manages memory through garbage collection.

Q2: What are the differences between an ArrayList and a LinkedList in Java?

FeatureArrayListLinkedList

MutabilityMutableMutable

PerformanceSlower for insertions/removalsFaster for insertions/removals

Data StructureArray-basedLinked node-based

Use CaseFor random access of elementsFor frequent insertion/deletion

ExampleArrayList<String> list = new ArrayList<>();LinkedList<String> list = new LinkedList<>();

Q3: What are varargs in Java?

Varargs allow a method to accept a variable number of arguments.
They are defined using ellipsis (...).

Example:

public void printNumbers(int... numbers) { for (int num : numbers) { System.out.println(num); } } printNumbers(1, 2, 3, 4); // Output: 1 2 3 4

Q4: What is method overriding in Java?

Method overriding allows a subclass to provide a specific implementation of a method already defined in the superclass.

Example:

class Parent { void display() { System.out.println("Parent method"); } } class Child extends Parent { @Override void display() { System.out.println("Child method"); } } Child obj = new Child(); obj.display(); // Output: Child method

Q5: How does Java manage memory?

Java uses automatic garbage collection to manage memory, which reclaims memory that is no longer referenced.

Q6: What is the difference between try-catch and try-finally in Java?

try-catch handles exceptions by allowing recovery from errors.
try-finally ensures that certain code always runs, even if an exception occurs.

Example:

try { int result = 10 / 0; } catch (ArithmeticException e) { System.out.println("Error: " + e.getMessage()); } finally { System.out.println("This will always execute"); }

Q7: What is a thread in Java?

A thread is the smallest unit of execution. Java supports multithreading, allowing multiple threads to run concurrently.
Java provides the Thread class and Runnable interface for thread management.

Example:

class MyThread extends Thread { public void run() { System.out.println("Thread is running"); } } public class Main { public static void main(String[] args) { MyThread t = new MyThread(); t.start(); // Starts a new thread } }

Q8: How do you read a file line by line in Java?

import java.io.*; import java.nio.file.*; public class ReadFile { public static void main(String[] args) throws IOException { Path path = Paths.get("example.txt"); Files.lines(path).forEach(System.out::println); } }

Q9: How do you connect Java to a database using JDBC?

Example using JDBC:

import java.sql.*; public class DatabaseExample { public static void main(String[] args) { try { Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "user", "password"); Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery("SELECT * FROM users"); while (rs.next()) { System.out.println(rs.getString("name")); } conn.close(); } catch (SQLException e) { e.printStackTrace(); } } }

Q10: How do you create a simple REST API in Java using Spring Boot?

Example using Spring Boot:

import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.web.bind.annotation.GetMapping; import org.springframework.web.bind.annotation.RestController; @SpringBootApplication public class Application { public static void main(String[] args) { SpringApplication.run(Application.class, args); } } @RestController class HelloController { @GetMapping("/") public String hello() { return "Hello, Spring Boot!"; } }

Q11: How do you handle missing values in Java (like using null checks)?

public class NullHandling { public static void main(String[] args) { String value = null; if (value == null) { value = "Default Value"; } System.out.println(value); } }

Q12: How do you validate an email using regex in Java?

import java.util.regex.*; public class EmailValidation { public static void main(String[] args) { String email = "test@example.com"; String regex = "^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\.[a-zA-Z0-9-.]+$"; Pattern pattern = Pattern.compile(regex); Matcher matcher = pattern.matcher(email); if (matcher.matches()) { System.out.println("Valid Email"); } else { System.out.println("Invalid Email"); } } }

Q13: How can you optimize performance in Java?

Use StringBuilder instead of concatenating strings with +.
Avoid creating unnecessary objects.
Use parallel streams for processing large data sets.
Use NIO (New Input/Output) for efficient file handling.
Use caching or memoization techniques.

Q14: How do you send an HTTP request using HttpURLConnection in Java?

import java.io.*; import java.net.*; public class HttpRequest { public static void main(String[] args) throws IOException { URL url = new URL("https://api.github.com"); HttpURLConnection connection = (HttpURLConnection) url.openConnection(); connection.setRequestMethod("GET"); BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream())); String inputLine; while ((inputLine = in.readLine()) != null) { System.out.println(inputLine); } in.close(); } }

Q15: When should you use multithreading vs multiprocessing in Java?

Multithreading is suitable for I/O-bound tasks (e.g., file I/O, web scraping).
Multiprocessing should be used for CPU-bound tasks (e.g., large calculations).

Example (using ExecutorService for multithreading):

import java.util.concurrent.*; public class ThreadingExample { public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(4); executor.submit(() -> System.out.println("Task 1")); executor.submit(() -> System.out.println("Task 2")); executor.shutdown(); } }

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This guide provides 100+ Java interview questions along with in-depth concepts to strengthen your expertise.

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Java Interview Questions and Answers

Top 100 Java Interview Questions for Freshers

2. What are Java's key features?

3. What is Java coding convention?

4. What is the difference between an ArrayList and a LinkedList?

5. What is the difference between Java 8 and Java 11?

6. How is memory managed in Java?

7. What are Java’s data types?

8. What are Java’s built-in data structures?

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

10. What are Java methods?

11. What is the difference between clone() and copy()?

12. What is a lambda function in Java?

13. What are Java annotations?

14. What are varargs in Java?

15. What is the difference between a class and a package?

16. What is the difference between static, instance, and abstract methods?

17. How does exception handling work in Java?

18. What are Java’s file handling modes?

19. What is list comprehension equivalent in Java?

20. How do you implement multithreading in Java?

21. What is the difference between String, StringBuilder, and StringBuffer?

22. What is method overloading and method overriding?

23. What are Java access modifiers?

24. What is a constructor in Java?

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

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