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

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8.4 Choosing — combining both

Lesson

Inheritance for code, interfaces for capability

Real designs use both. A class extends an abstract class to inherit shared state and code, and implements interfaces to advertise capabilities. Then an interface-typed array can hold objects from completely unrelated class hierarchies — something a superclass array could never do.

Worked example

class Rect extends Shape implements Printable { ... }   // a shape, and printable
class Tag implements Printable, Sized { ... }           // not a shape, still printable

Printable[] items = { new Rect(2, 3), new Tag("hi") };
for (int i = 0; i < items.length; i++) {
    System.out.println(items[i].label());
}
// [Rect]
// <hi>

Rect and Tag share no superclass — only the Printable contract — yet one loop handles both.

▶ Try it: run this — a Printable[] mixes a Shape subclass with a totally unrelated class.

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) {
        Printable[] items = { new Rect(2, 3), new Tag("hi") };
        for (int i = 0; i < items.length; i++) {
            System.out.println(items[i].label());
        }
    }
}

A Printable[] can hold...

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Why can an interface array mix unrelated classes, but a superclass array cannot?

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Type exactly what this program prints.

interface Printable { String label(); }
class A implements Printable { public String label(){ return "A"; } }
class B implements Printable { public String label(){ return "B"; } }
public class Main { public static void main(String[] x){
    Printable[] arr = { new A(), new B() };
    for (int i = 0; i < arr.length; i++) System.out.println(arr[i].label());
}}

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Match each need to the better tool.

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True or false?

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