Day 13: Understanding Inheritance in Java

What is Inheritance?

Inheritance is a mechanism in Java where one class acquires the properties and behaviors of another class.

• The class whose properties are inherited is called the Parent / Super class
• The class that inherits the properties is called the Child / Sub class

Inheritance represents an IS-A relationship.

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Why Use Inheritance?

Inheritance is used to:

• Reuse existing code
• Avoid code duplication
• Improve maintainability
• Create a parent-child relationship between classes

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Syntax of Inheritance

class Parent {
    // variables and methods
}

class Child extends Parent {
    // child class code
}

The keyword extends is used to achieve inheritance in Java.

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Types of Inheritance in Java

Java supports five types of inheritance conceptually, but only three are supported using classes.

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1. Single Inheritance

What is Single Inheritance?

A single child class inherits from one parent class.

Structure:

Parent → Child

Example:

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

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

Here, Dog inherits the eat() method from Animal.

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2. Multilevel Inheritance

What is Multilevel Inheritance?

A class inherits from another class, which itself inherits from another class.

Structure:

Grandparent → Parent → Child

Example:

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

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

class Puppy extends Dog {
    void weep() {
        System.out.println("Puppy weeps");
    }
}

Here, Puppy can access methods from both Dog and Animal.

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3. Hierarchical Inheritance

What is Hierarchical Inheritance?

Multiple child classes inherit from one parent class.

Structure:

        Parent
       /      \
   Child1   Child2

Example:

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

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

class Cat extends Animal {
    void meow() {
        System.out.println("Cat meows");
    }
}

Here, both Dog and Cat inherit from Animal.

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4. Multiple Inheritance (Not Supported with Classes)

What is Multiple Inheritance?

A single class inherits from more than one parent class.

Example (Not Allowed):

class A { }
class B { }
class C extends A, B { } // ❌ Invalid

Java does not support multiple inheritance using classes to avoid ambiguity (Diamond Problem).

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5. Hybrid Inheritance (Not Supported with Classes)

Hybrid inheritance is a combination of multiple inheritance types.

Java does not support hybrid inheritance using classes, but it can be achieved using interfaces.

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Summary of Inheritance Support in Java

          | Type         | Supported Using Classes |
          | ------------ | ----------------------- |
          | Single       | Yes                     |
          | Multilevel   | Yes                     |
          | Hierarchical | Yes                     |
          | Multiple     | No                      |
          | Hybrid       | No                      |

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Important Points to Remember

• Inheritance uses the extends keyword
• Java supports only single inheritance with classes
• Parent class members are accessible in the child class
• Constructors are not inherited
• Inheritance promotes code reusability

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When to Use Inheritance?

Use inheritance when:

• There is a clear IS-A relationship
• Child class needs parent class behavior
• Code reuse is required

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Key Points Summary

• Inheritance allows one class to reuse another class’s code
• It creates a parent-child relationship
• Java supports single, multilevel, and hierarchical inheritance
• Multiple inheritance is not supported using classes