Module 6: Inheritance and Polymorphism

Inheritance Basics with `extends`

Create a subclass that reuses fields and methods from a parent class when there is a true “is-a” relationship.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-16

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, edited by hand, and checked against current Java 25 documentation and runnable examples.

Learning goals

Understand the parent-child class relationship
Use `extends` to build a simple subclass
Recognize when inheritance fits a real “is-a” idea

Inheritance lets one class reuse another class’s structure: A child class can inherit fields and methods from a parent class.

Use inheritance for true “is-a” relationships: A Dog is an Animal, and a SavingsAccount is an Account.

Why this matters: Shared behavior can live once in the parent class instead of being copied into every child class.

Keep the relationship honest: If the child is not really a more specific version of the parent, inheritance is probably the wrong tool.

Runnable examples

A subclass inherits from a parent class

class Animal {
    String name;

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

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

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.name = "Buddy";
        dog.eat();
        dog.bark();
    }
}

Expected output

Buddy is eating.
Woof

Common mistakes

Using inheritance for a “has-a” relationship

If one class contains or uses another, composition is usually better than inheritance.

Mini exercise

Create a `Vehicle` class and a `Car` class that extends it.

Summary

Inheritance models a true “is-a” relationship.
`extends` creates the parent-child connection.
Child classes inherit useful members from the parent.

Next step

Next, override parent methods so subclasses can provide their own behavior.

Sources used

Inheritance Basics with `extends`

A subclass inherits from a parent class

When is inheritance usually appropriate?