Java Inheritance

What is Inheritance?

Inheritance allows a class (child/subclass) to inherit fields and methods from another class (parent/superclass). It promotes code reuse and establishes an "is-a" relationship.

java

// Parent class
public class Animal {
    String name;

    public void eat() {
        System.out.println(name + " is eating.");
    }
}

// Child class
public class Dog extends Animal {
    public void bark() {
        System.out.println(name + " is barking!");
    }
}

// Usage
Dog d = new Dog();
d.name = "Buddy";
d.eat();   // Inherited from Animal
d.bark();  // Defined in Dog

The `extends` Keyword

Use extends to create a subclass:

java

public class Vehicle {
    String brand;
    int maxSpeed;

    public void displayInfo() {
        System.out.println(brand + " | Max Speed: " + maxSpeed + " km/h");
    }
}

public class Car extends Vehicle {
    int numDoors;

    public void honk() {
        System.out.println(brand + ": Beep beep!");
    }
}

Method Overriding

A subclass can override a parent method to provide its own implementation:

java

public class Shape {
    public double area() {
        return 0;
    }
}

public class Circle extends Shape {
    double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

public class Rectangle extends Shape {
    double width, height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        return width * height;
    }
}

The `super` Keyword

super refers to the parent class:

java

public class Employee {
    String name;
    double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public void display() {
        System.out.printf("%s earns ₹%.2f%n", name, salary);
    }
}

public class Manager extends Employee {
    String department;

    public Manager(String name, double salary, String department) {
        super(name, salary);       // Call parent constructor
        this.department = department;
    }

    @Override
    public void display() {
        super.display();           // Call parent method
        System.out.println("Department: " + department);
    }
}

Types of Inheritance in Java

Type	Description	Supported?
Single	A extends B	✓ Yes
Multilevel	C extends B extends A	✓ Yes
Hierarchical	B, C both extend A	✓ Yes
Multiple	A extends B, C (two parents)	✗ No (use interfaces)

Java does not support multiple inheritance with classes to avoid the "Diamond Problem." Use interfaces instead.

Practical Example

java

public class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void introduce() {
        System.out.printf("Hi, I'm %s, %d years old.%n", name, age);
    }
}

public class Student extends Person {
    String course;
    double gpa;

    public Student(String name, int age, String course, double gpa) {
        super(name, age);
        this.course = course;
        this.gpa = gpa;
    }

    @Override
    public void introduce() {
        super.introduce();
        System.out.printf("I'm studying %s with GPA %.1f%n", course, gpa);
    }
}

Summary

Inheritance lets a child class reuse parent class code with extends
Use @Override to override parent methods in the child class
The super keyword accesses the parent's constructor and methods
Java supports single, multilevel, and hierarchical inheritance
Java does not support multiple class inheritance — use interfaces instead
Inheritance models an "is-a" relationship: a Dog is an Animal

What is Inheritance?

The extends Keyword

Method Overriding

The super Keyword

Types of Inheritance in Java

Practical Example

Summary

The `extends` Keyword

The `super` Keyword