Java interview

1) Reason primitive data-type usage.

Primitive Type Description    Size Example
int       Integer numbers     4 bytes       int a = 10;
double        Decimal numbers     8 bytes       double pi = 3.14;
char       Single character     2 bytes       char c = 'A';
boolean       True/False     1 bit*       boolean flag = true;

Wrapper classes?

Primitive Type Wrapper Class
byte Byte
short Short
int Integer
long Long
float Float
double Double
char Character
boolean Boolean

JVM - Java Virtual Machine
JRE - Java Runtime Environment.



2) Example for static & dynamic memory allocation?

1. Static Memory Allocation

  • Memory is allocated at compile time

  • Used for primitive types and static variables


public class StaticExample {
    static int x = 10;       // Static variable (Method Area)
    
    public static void main(String[] args) {
        int a = 5;           // Local variable (Stack)
        System.out.println("a = " + a);
        System.out.println("x = " + x);
    }
}

Memory Allocated:

  • x → Stored in Method Area

  • a → Stored in Stack

2. Dynamic Memory Allocation

  • Memory is allocated at runtime

  • Used when creating objects or arrays

public class DynamicExample {

    public static void main(String[] args) {

        String name = new String("Java");  // Object in Heap

        int[] nums = new int[3];   💗💗💗 // Array in Heap

        nums[0] = 10;

        System.out.println("Name = " + name);

        System.out.println("First number = " + nums[0]);

    }

}

Memory Allocated:

  • name and nums[] → Stored in Heap

  • Reference variables → Stored in Stack

3) Array?

int[] numbers = new int[5];  // Declaration + memory for 5 integers

Operators?
 
1. Arithmetic Operators
Operator Meaning Example Result
+ Addition 10 + 5 15
- Subtraction 10 - 5 5
* Multiplication 10 * 5 50
/ Division 10 / 5 2
% Modulus 10 % 3 1

2. Relational / Comparison Operators
Operator Meaning Example Result
== Equal to a == b false
!= Not equal to a != b true
> Greater than a > b false
< Less than a < b true
>= Greater or Equal a >= b false
<= Less or Equal a <= b true

3. Assignment Operators
Operator Meaning Example Result
= Assign a = 10 a = 10
+= Add and assign a += 5 a = a + 5
-= Subtract and assign a -= 3 a = a - 3
*= Multiply and assign a *= 2 a = a * 2
/= Divide and assign a /= 2 a = a / 2


4) What type of conditional statement is this?
if(10>=20) { "wrong" : " Ture"}

Java Ternary operator is used as one line replacement for if-then-else statement

Syntax:

condition ? if true : if false;

 Example 1:

public class OperatorExample{

public static void main(String args[]){

int a=2;

int b=5;

int min=(a<b)? a : b; System.out.println(min);

}

}


5) Access Modifier?

1. private

Only accessible within the same class.

class Test {
    private int data = 100;

    private void show() {
        System.out.println("Private method");
    }
}

2. default (No keyword)

Accessible only within the same package.

class Demo {
    int number = 50; // default
    void display() {
        System.out.println("Default access");
    }
}

3. protected

Accessible in the same package + subclasses (even in other packages).

// File: Animal.java
package animals;

public class Animal {
    protected void sound() {
        System.out.println("Animal makes sound");
    }
}

// File: Dog.java
package animals;

public class Dog extends Animal {
    void makeSound() {
        sound(); // ✅ Accessible due to protected
    }
}


4. public

Accessible everywhere.

// File: Message.java
package messages;

public class Message {
    public void display() {
        System.out.println("Hello World");
    }
}

// File: Main.java
package test;

import messages.Message;

public class Main {
    public static void main(String[] args) {
        Message m = new Message();
        m.display(); // ✅ Works everywhere
    }
}


6) Constructor?
A constructor is a special method in Java that is used to initialize objects when they are created

class Student {
    String name;
    int age;

    Student(String n, int a) {
        name = n;
        age = a;
    }
}

7) OOP concepts?

1. Encapsulation
Wrapping data (variables) and methods into a single unit (class) and restricting direct access

class Student {
    private String name; // hidden from outside

    public void setName(String n) {
        name = n;
    }

    public String getName() {
        return name;
    }
}

2. Inheritance
One class inherits properties and behaviors from another using extends

class Animal {
    void sound() {
        System.out.println("Animal makes sound");
    }
}

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

3. Polymorphism

One method behaves differently based on the object or context.

➤ Compile-time Polymorphism (Method Overloading)

class Add {
    int sum(int a, int b) {
        return a + b;
    }
    double sum(double a, double b) {
        return a + b;
    }
}

➤ Run-time Polymorphism (Method Overriding)

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Cat extends Animal {
    void sound() {
        System.out.println("Meow");
    }
}

4. Abstraction
Hiding internal logic and showing only what is necessary.

abstract class Shape {
    abstract void draw();
}

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

8) Difference between class and object?

A class is like a blueprint or design

1. Class 

class Car {
    String color;
    void drive() {
        System.out.println("Car is driving");
    }
}

This class defines:

  • Properties: color

  • Behavior: drive()


2. Object

An object is an instance of a class. It’s like a real item based on the blueprint.

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();    // Object created
        myCar.color = "Red";      // Assigning value
        myCar.drive();            // Calling method
    }
}


9) Can we create more than 1 object for a same class?
Yes, we can create multiple objects more than 1

class Car {
    String color;

    void start() {
        System.out.println(color + " car is starting");
    }
}

public class Main {
    public static void main(String[] args) {
        Car car1 = new Car();  // Object 1
        car1.color = "Red";

        Car car2 = new Car();  // Object 2
        car2.color = "Blue";

        car1.start();  // Output: Red car is starting
        car2.start();  // Output: Blue car is starting
    }
}


10) Types of inheritance?

1. Single Inheritance - One child inherits one parent.

2. Multilevel Inheritance - Grandchild inherits parent, parent inherits grandparent

3. Hierarchical Inheritance - Many children inherit one parent.

4. Multiple Inheritance - One class inherits from two interfaces.
5. Hybrid Inheritance - Combination of types, using interfaces


11) Polymorphism? Types of Polymorphism?
Compile time, Runtime

Abstract class? Abstract method?

abstract class Animal {
    abstract void sound();  // only the name of the method
}

12) Difference between normal class and abstract class?        

Normal class    -  Full class – you can use it directly.
Abstract Class  -  Half class – needs to be completed


13) for (int i = 0; i < 6; i++) {
    int y = i;
    System.out.println(y);
}

output  - 
0
1
2
3
4
5


14) for (int i = 0; i < 5; i++) {
    int y = i++;
    System.out.println(y); // Assuming "soul(y)" means "System.out.println(y);"
}

output:
0
2
4

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