/ Design Patterns / Structural patterns / Adapter

Adapter Design Pattern in Java: Before and after

Before

Because the interface between Line and Rectangle objects is incompatible, the user has to recover the type of each shape and manually supply the correct arguments. 

class Line {
    public void draw(int x1, int y1, int x2, int y2) {
        System.out.println("Line from point A(" + x1 + ";" + y1 + "), to point B(" + x2 + ";" + y2 + ")");
    }
}

class Rectangle {
    public void draw(int x, int y, int width, int height) {
        System.out.println("Rectangle with coordinate left-down point (" + x + ";" + y + "), width: " + width
                + ", height: " + height);
    }
}

public class AdapterDemo {
    public static void main(String[] args) {
        Object[] shapes = {new Line(), new Rectangle()};
        int x1 = 10, y1 = 20;
        int x2 = 30, y2 = 60;
        int width = 40, height = 40;
        for (Object shape : shapes) {
            if (shape.getClass().getSimpleName().equals("Line")) {
                ((Line)shape).draw(x1, y1, x2, y2);
            } else if (shape.getClass().getSimpleName().equals("Rectangle")) {
                ((Rectangle)shape).draw(x2, y2, width, height);
            }
        }
    }
}

Output

Line from point A(10;20), to point B(30;60)
Rectangle with coordinate left-down point (30;60), width: 40, height: 40


After

The Adapter's "extra level of indirection" takes care of mapping a user-friendly common interface to legacy-specific peculiar interfaces. 

interface Shape {
    void draw(int x, int y, int z, int j);
}

class Line {
    public void draw(int x1, int y1, int x2, int y2) {
        System.out.println("Line from point A(" + x1 + ";" + y1 + "), to point B(" + x2 + ";" + y2 + ")");
    }
}

class Rectangle {
    public void draw(int x, int y, int width, int height) {
        System.out.println("Rectangle with coordinate left-down point (" + x + ";" + y + "), width: " + width
                + ", height: " + height);
    }
}

class LineAdapter implements Shape {
    private Line adaptee;

    public LineAdapter(Line line) {
        this.adaptee = line;
    }

    @Override
    public void draw(int x1, int y1, int x2, int y2) {
        adaptee.draw(x1, y1, x2, y2);
    }
}

class RectangleAdapter implements Shape {
    private Rectangle adaptee;

    public RectangleAdapter(Rectangle rectangle) {
        this.adaptee = rectangle;
    }

    @Override
    public void draw(int x1, int y1, int x2, int y2) {
        int x = Math.min(x1, x2);
        int y = Math.min(y1, y2);
        int width = Math.abs(x2 - x1);
        int height = Math.abs(y2 - y1);
        adaptee.draw(x, y, width, height);
    }
}

public class AdapterDemo {
    public static void main(String[] args) {
        Shape[] shapes = {new RectangleAdapter(new Rectangle()),
                          new LineAdapter(new Line())};
        int x1 = 10, y1 = 20;
        int x2 = 30, y2 = 60;
        for (Shape shape : shapes) {
            shape.draw(x1, y1, x2, y2);
        }
    }
}

Output

Rectangle with coordinate left-down point (10;20), width: 20, height: 40
Line from point A(10;20), to point B(30;60)

Code examples

Java Adapter in Java: Before and after Adapter in Java
C++ Adapter in C++ Adapter in C++: External Polymorphism
PHP Adapter in PHP
Delphi Adapter in Delphi
Python Adapter in Python