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

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8.3 Abstract classes — abstract and concrete methods

Lesson

A partly finished class

An abstract class sits between a normal class and an interface. It can hold fields and finished (concrete) methods like any class, but it can also declare abstract methods — a signature with no body, marked abstract, which every subclass must implement. Because it is unfinished, an abstract class cannot be instantiated: new Shape(...) is a compile error.

Worked example

abstract class Shape {
    private String name;
    public String getName() { return name; }    // concrete: shared code
    public abstract double area();               // abstract: each subclass decides
}

class Circle extends Shape {
    public double area() { return 3.14 * r * r; }   // must implement
}

Shape s = new Shape("X");   // ERROR: Shape is abstract; cannot be instantiated

Circle inherits getName() for free and supplies its own area().

▶ Try it: run this — Circle provides area() and inherits getName() from the abstract class.

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) {
        Circle c = new Circle(2);
        System.out.println(c.area());
        System.out.println(c.getName());
    }
}

An abstract method is...

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What happens with new Shape("X") when Shape is abstract?

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Type exactly what this program prints.

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

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Unlike an interface, an abstract class can have...

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True or false?

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