Java Inheritance

A class can extend another to reuse and specialise its behaviour.

public class Product {
    protected String name;
    protected double price;

    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String describe() {
        return name + " — NPR " + price;
    }
}

public class Book extends Product {
    private String author;

    public Book(String name, double price, String author) {
        super(name, price);      // must be the first statement
        this.author = author;
    }

    @Override
    public String describe() {
        return super.describe() + " by " + author;
    }
}

Book b = new Book("Palpasa Cafe", 480, "Narayan Wagle");
System.out.println(b.describe());
// Palpasa Cafe — NPR 480 by Narayan Wagle

super

• super(...) calls the parent constructor — required, and must come first.
• super.method() calls the parent's version of an overridden method.

@Override

Always write it. It is optional to the compiler but it catches typos — if the method doesn't actually override anything (wrong name, wrong parameters), you get a compile error instead of a silently-unused method.

Polymorphism

A variable of the parent type can hold any subclass, and the subclass version of the method runs:

Product p = new Book("Palpasa Cafe", 480, "Narayan Wagle");
System.out.println(p.describe());   // runs Book's version

This is what makes it possible to write code against Product that works for every kind of product, present and future.

Access modifiers

Single inheritance

A Java class extends exactly one class. To share behaviour across unrelated hierarchies, use interfaces (next chapter).

final

public final class Config { }       // cannot be extended
public final void save() { }         // cannot be overridden

Composition over inheritance

Inheritance couples classes tightly. Before extending, ask whether the relationship is really "is-a" — if it's "has-a", hold the other object as a field instead. A Car has an Engine; it is not an Engine.