Java Inheritance

Creating new classes based on existing ones

🧬 What is Inheritance?

Inheritance allows a class to inherit properties and methods from another class. The child class gets all features of the parent class and can add new ones. 


// Parent class
class Animal {
    public void eat() {
        System.out.println("Animal is eating");
    }
}

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

Output:

Dog can both eat() and bark()

Key Inheritance Concepts

👨‍👦

Parent Class

The class being inherited from 

class Vehicle {
    String brand;
}
👶

Child Class

The class that inherits 

class Car extends Vehicle {
    int doors;
}
🔄

Method Override

Child class changes parent method 

@Override
public void start() {
    // New implementation
}
🔗

extends Keyword

Creates inheritance relationship 

class Student extends Person {
    // Student inherits from Person
}

🔹 Complete Inheritance Example

Here's a practical inheritance example: 

// Parent class
class Person {
    protected String name;
    protected int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public void introduce() {
        System.out.println("Hi, I'm " + name + ", age " + age);
    }
    
    public void sleep() {
        System.out.println(name + " is sleeping");
    }
}

// Child class
class Student extends Person {
    private String school;
    private String major;
    
    public Student(String name, int age, String school, String major) {
        super(name, age);  // Call parent constructor
        this.school = school;
        this.major = major;
    }
    
    // Override parent method
    @Override
    public void introduce() {
        System.out.println("Hi, I'm " + name + ", a " + major + 
                          " student at " + school);
    }
    
    // New method specific to Student
    public void study() {
        System.out.println(name + " is studying " + major);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        Student student = new Student("Alice", 20, "MIT", "Computer Science");
        student.introduce();  // Overridden method
        student.study();      // Student-specific method
        student.sleep();      // Inherited method
    }
}

Output:

Hi, I'm Alice, a Computer Science student at MIT
Alice is studying Computer Science
Alice is sleeping

🔹 Types of Inheritance

Java supports different inheritance patterns:

🔸 Single Inheritance 

class Animal { }
class Dog extends Animal { }  // Dog inherits from Animal

🔸 Multilevel Inheritance 

class Animal { }
class Mammal extends Animal { }
class Dog extends Mammal { }  // Dog → Mammal → Animal

🔸 Hierarchical Inheritance 

class Animal { }
class Dog extends Animal { }
class Cat extends Animal { }  // Both inherit from Animal

Note: Java doesn't support multiple inheritance (one class extending multiple classes) to avoid complexity and conflicts.

🔹 Method Overriding Rules

  • @Override annotation: Recommended for clarity
  • Same method signature: Name, parameters, return type
  • Access level: Cannot be more restrictive than parent
  • final methods: Cannot be overridden 
class Shape {
    public void draw() {
        System.out.println("Drawing a shape");
    }
}

class Circle extends Shape {
    @Override
    public void draw() {  // Overriding parent method
        System.out.println("Drawing a circle");
    }
    
    public void calculateArea() {  // New method
        System.out.println("Calculating circle area");
    }
}

🧠 Test Your Knowledge

Which keyword is used to inherit from a parent class in Java?