Practice›Java Fundamentals›Module 8. Interfaces & Abstract Classes›8.4 Choosing

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8.4 Choosing — interface or abstract class?

Lesson

Two ways to share a type

Both let different classes be treated through one type, but they answer different questions. An abstract class says what a thing is and can carry state (fields), constructors, and shared code. An interface says what a thing can do — a contract with no state. Rule of thumb: use an abstract class when the classes are naturally related and share code; use an interface when unrelated classes must share a capability.

The decisive rule

A class can extends only one class, but can implements as many interfaces as it likes — and it can do both at once:

class Rect extends Shape implements Printable { ... }   // one class, one interface
class C extends A, B { }   // ERROR: cannot extend two classes

So inheritance is your single "is-a" slot; interfaces are unlimited "can-do" labels.

▶ Try it: run this — Rect inherits from Shape and also fulfills Printable.

interface Printable {
    String label();
}
interface Sized {
    int size();
}
abstract class Shape {
    private String name;
    public Shape(String name) { this.name = name; }
    public String getName() { return name; }
    public abstract double area();
}
class Rect extends Shape implements Printable {
    private double w, h;
    public Rect(double w, double h) { super("Rect"); this.w = w; this.h = h; }
    public double area() { return w * h; }
    public String label() { return "[" + getName() + "]"; }
}
class Tag implements Printable, Sized {
    private String text;
    public Tag(String text) { this.text = text; }
    public String label() { return "<" + text + ">"; }
    public int size() { return text.length(); }
}
public class Main {
    public static void main(String[] args) {
        Rect r = new Rect(2, 3);
        System.out.println(r.getName());
        System.out.println(r.area());
        System.out.println(r.label());
    }
}

How many classes can a Java class extend?

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How many interfaces can a class implement?

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Unrelated classes must share a capability but no code. Use...

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Related classes share fields and finished methods. Use...

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True or false?

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Type exactly what this program prints.

interface Printable {
    String label();
}

abstract class Shape {
    public abstract double area();
}

class Box extends Shape implements Printable {
    private double s;

    public Box(double s) {
        this.s = s;
    }

    public double area() {
        return s * s;
    }

    public String label() {
        return "[box]";
    }
}

public class Main {
    public static void main(String[] args) {
        Box b = new Box(3);
        System.out.println(b.area());
        System.out.println(b.label());
    }
}

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