30+ Java Basic Interview Questions with Answers, Explanations & Use Cases

 

🟢 30 Basic Java Interview Questions for Freshers (with Examples & Use Cases)

If you are preparing for interviews in service-based companies like TCS, Infosys, Accenture, Wipro, Tech Mahindra, these are the most commonly asked Java interview questions for freshers (<1 year experience).

Each question has:
✅ Simple explanation
✅ Real-life use case
✅ Small code example

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1. What are the main features of Java?

Answer:

• Object-Oriented

• Platform Independent

• Simple, Secure, Robust

• Multithreaded & Portable

👉 Use Case: Run same .class file on Windows & Linux.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Java is platform independent!");
    }
}

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2. What is JVM, JRE, and JDK?

Answer:

• JVM: Runs Java bytecode.

• JRE: Libraries + JVM.

• JDK: JRE + tools (javac, debugger).

// javac HelloWorld.java  → JDK compiles to bytecode
// java HelloWorld        → JRE runs program inside JVM

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3. Why is Java platform independent?

Because Java code is compiled into bytecode, which runs on any JVM.

class Demo {
    public static void main(String[] args) {
        System.out.println("Runs anywhere with JVM!");
    }
}

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4. Difference between == and equals()?

• == → compares memory reference.

• equals() → compares values.

String s1 = new String("Java");
String s2 = new String("Java");
System.out.println(s1 == s2);      // false
System.out.println(s1.equals(s2)); // true

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5. Difference between Primitive and Wrapper?

• Primitive → int, char (not objects).

• Wrapper → Integer, Character (objects with utilities).

int a = 5;                 // primitive
Integer b = Integer.valueOf(5); // wrapper
System.out.println(b.toString()); // wrapper provides methods

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6. What are Constructors?

Special method used to initialize objects.

class Student {
    String name;
    Student(String n) { name = n; }
}
public class Test {
    public static void main(String[] args) {
        Student s = new Student("Dip");
        System.out.println(s.name);
    }
}

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7. Difference between Overloading and Overriding?

• Overloading: Same method, different parameters.

• Overriding: Subclass changes parent method.

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

// Overriding
class Animal { void sound() { System.out.println("Animal sound"); } }
class Dog extends Animal { void sound() { System.out.println("Bark"); } }

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8. What is Inheritance?

One class acquires properties of another.

class Animal { void eat() { System.out.println("eating"); } }
class Dog extends Animal { void bark() { System.out.println("barking"); } }

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9. What is Polymorphism?

One method behaves differently depending on context.

class Animal { void sound() { System.out.println("Generic sound"); } }
class Cat extends Animal { void sound() { System.out.println("Meow"); } }
Animal a = new Cat();
a.sound(); // "Meow"

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10. What is Abstraction?

Hiding implementation, showing only behavior.

abstract class Shape {
    abstract void draw();
}
class Circle extends Shape {
    void draw() { System.out.println("Drawing Circle"); }
}

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11. What is Encapsulation?

Binding variables & methods, restricting direct access.

class Bank {
    private int balance = 1000;
    public int getBalance() { return balance; }
    public void deposit(int amt) { balance += amt; }
}

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12. Difference between Abstract Class & Interface?

• Abstract: Can have abstract + concrete methods.

• Interface: Only abstract (till Java 7), can have default/static (Java 8+).

interface Vehicle { void drive(); }
class Car implements Vehicle { public void drive() { System.out.println("Driving"); } }

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13. What are Access Modifiers?

• public, private, protected, default.

class Example {
    private int a = 5; // only inside class
    public int b = 10; // accessible everywhere
}

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14. Difference between final, finally, finalize()?

• final: Constant/prevent inheritance.

• finally: Executes after try-catch.

• finalize(): Cleanup before GC.

final int x = 10; // cannot be changed
try { int a=5/0; } catch(Exception e) { } finally { System.out.println("Always runs"); }

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15. Difference between String, StringBuffer, StringBuilder?

• String → Immutable.

• StringBuffer → Mutable + thread-safe.

• StringBuilder → Mutable + fast.

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

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16. What are Static Variables & Methods?

Shared by all objects, belongs to class.

class Counter {
    static int count=0;
    Counter() { count++; }
}

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17. Use of this keyword?

Refers to current object.

class Student {
    String name;
    Student(String name) { this.name = name; }
}

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18. Checked vs Unchecked Exceptions?

• Checked: Must be handled (IOException).

• Unchecked: Runtime (NullPointerException).

try {
    FileReader f = new FileReader("abc.txt"); // checked
} catch (IOException e) { e.printStackTrace(); }

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19. What is Garbage Collection?

Automatic memory management.

public class GCExample {
    public void finalize() { System.out.println("Object destroyed"); }
    public static void main(String[] args) {
        GCExample obj = new GCExample();
        obj = null;
        System.gc();
    }
}

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20. What are Threads?

Lightweight processes for multitasking.

class MyThread extends Thread {
    public void run() { System.out.println("Running..."); }
}
new MyThread().start();

---

21. What is Synchronization?

Ensures one thread at a time.

class Table {
    synchronized void printTable(int n) {
        for(int i=1;i<=5;i++) System.out.println(n*i);
    }
}

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22. Difference between Array & ArrayList?

• Array: Fixed size.

• ArrayList: Dynamic.

int[] arr = {1,2,3};
ArrayList<Integer> list = new ArrayList<>();
list.add(1); list.add(2);

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23. Difference between HashMap & Hashtable?

• HashMap → Not synchronized, allows null.

• Hashtable → Synchronized, no null keys.

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

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24. Difference between Iterator & ListIterator?

• Iterator → one direction.

• ListIterator → both directions.

List<String> list = new ArrayList<>();
list.add("A"); list.add("B");
ListIterator<String> it = list.listIterator();
while(it.hasNext()) System.out.println(it.next());

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25. Difference between throw & throws?

• throw → used to throw exception.

• throws → declares exceptions in method.

void myMethod() throws IOException {
    throw new IOException("File error");
}

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26. Compile-time vs Runtime Polymorphism?

• Compile-time: Overloading.

• Runtime: Overriding.

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

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27. What are Packages?

Collection of related classes.

package mypack;
public class Demo { }

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28. Why public static void main(String[] args)?

• public: accessible

• static: no object needed

• void: no return

• main: entry point

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29. Heap vs Stack Memory?

• Stack: stores method calls & local vars.

• Heap: stores objects.

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30. Process vs Thread?

• Process: Independent, own memory.

• Thread: Part of process, shared memory.

class Task extends Thread {
    public void run() { System.out.println("Thread running"); }
}

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