Practice›Java Fundamentals›Module 8. Interfaces & Abstract Classes›8.3 Abstract classes

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8.3 Abstract classes — sharing code with subclasses

Lesson

Write once, use in every subclass

This is the real payoff: a concrete method in the abstract class can call an abstract method. The shared method is written once, in the superclass, but its behavior changes per subclass — because the abstract call resolves to the actual object's implementation.

Worked example

abstract class Shape {
    public abstract double area();
    public String describe() {
        return getName() + " has area " + area();   // calls the subclass's area()
    }
}

Shape c = new Circle(1);
Shape r = new Rect(2, 3);
System.out.println(c.describe());   // Circle has area 3.14
System.out.println(r.describe());   // Rect has area 6.0

describe() is written once, yet each shape describes itself correctly.

▶ Try it: run this — the inherited describe() plugs in each subclass's own area().

abstract class Shape {
    private String name;
    public Shape(String name) { this.name = name; }
    public String getName() { return name; }
    public abstract double area();
    public String describe() { return name + " has area " + area(); }
}
class Circle extends Shape {
    private double r;
    public Circle(double r) { super("Circle"); this.r = r; }
    public double area() { return 3.14 * r * r; }
}
class Rect extends Shape {
    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 class Main {
    public static void main(String[] args) {
        Shape c = new Circle(1);
        Shape r = new Rect(2, 3);
        System.out.println(c.describe());
        System.out.println(r.describe());
    }
}

A concrete method in an abstract class can...

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The shared describe() gives different results because...

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Type exactly what this program prints.

abstract class Shape {
    public abstract double area();
    public String describe(){ return "area " + area(); }
}
class Sq extends Shape { public double area(){ return 4.0; } }
class Tri extends Shape { public double area(){ return 6.0; } }
public class Main { public static void main(String[] a){
    System.out.println(new Sq().describe());
    System.out.println(new Tri().describe());
}}

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Putting describe() in the abstract class means...

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True or false?

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