Mostly Asked Java OOP Interview Questions

Q1. What are the four main principles of OOP?What are the four main principles of OOP?

Encapsulation: Wrapping data and methods into a single unit (class), restricting direct access to some components.
Inheritance: Mechanism where one class acquires the properties and behaviors of another class.
Polymorphism: Ability of an object to take many forms, typically via method overriding or overloading.
Abstraction: Hiding internal details and showing only functionalities.

Q2. What is encapsulation in Java?What is encapsulation in Java?

Encapsulation is the technique of wrapping data (variables) and code (methods) together as a single unit (class). It protects data by making variables private and providing public getter/setter methods to access and update the data.

public class Employee {
  private String name;
  private int id;

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }
}

Q3. Explain inheritance with example.Explain inheritance with example.

Inheritance allows a class (child/subclass) to inherit fields and methods from another class (parent/superclass), promoting code reuse.

class Employee {
  String name;
  void display() {
    System.out.println("Name: " + name);
  }
}
class Manager extends Employee {
  int deptId;
  void displayDept() {
    System.out.println("Dept ID: " + deptId);
  }
}

Q4. What is method overloading and overriding?What is method overloading and overriding?

Method Overloading: Same method name with different parameters in the same class.
Method Overriding: Subclass provides a specific implementation of a method already defined in its superclass.

public class Employee {
  void setDetails(String name) {}
  void setDetails(String name, int id) {}
}

class Manager extends Employee {
  @Override
  void setDetails(String name) {
    // overridden method
  }
}

Q5. What is polymorphism?What is polymorphism?

Polymorphism allows objects to be treated as instances of their parent class rather than their actual class. The two types:
1. Compile-time polymorphism (method overloading)
2. Runtime polymorphism (method overriding)

Employee e = new Manager();
e.display();  // calls Manager's display if overridden

Q6. What is abstraction?What is abstraction?

Abstraction is hiding the complex implementation details and showing only the necessary features of an object. Achieved using abstract classes and interfaces.

abstract class Employee {
  abstract void work();
}

class Manager extends Employee {
  void work() {
    System.out.println("Managing tasks");
  }
}

Q7. Difference between abstract class and interface?Difference between abstract class and interface?

Abstract class can have both abstract and concrete methods; interfaces (Java 7 and before) only abstract methods. Since Java 8, interfaces can have default and static methods.
Class can extend only one abstract class but implement multiple interfaces.

abstract class Employee {
  abstract void work();
}
interface Payable {
  void calculateSalary();
}
class Manager extends Employee implements Payable {
  void work() {}
  public void calculateSalary() {}
}

Q8. What is the use of the 'this' keyword?What is the use of the 'this' keyword?

'this' refers to the current object instance. It is used to resolve naming conflicts between instance variables and parameters or to pass the current object as a parameter.

public class Employee {
  String name;
  Employee(String name) {
    this.name = name;
  }
}

Q9. Explain final keyword in Java.Explain final keyword in Java.

'final' keyword is used to declare constants, prevent method overriding, and inheritance of classes.

final variable: value cannot be changed
final method: cannot be overridden
final class: cannot be subclassed

final variable: वैल्यू बदली नहीं जा सकती
final method: ओवरराइड नहीं हो सकती
final class: सबक्लास नहीं बनाई जा सकती

final class Employee {
  final int id = 100;
  final void display() {
    System.out.println("ID: " + id);
  }
}