装饰器模式（Decorator Pattern）允许向一个现有的对象添加新的功能，同时又不改变其结构。这种类型的设计模式属于结构型模式，它是作为现有的类的一个包装。

这种模式创建了一个装饰类，用来包装原有的类，并在保持类方法签名完整性的前提下，提供了额外的功能。

坦克：

  • 想加一个外壳显示
  • 想加一个血条
  • 想加一条尾巴

子弹：

  • 想加一条尾巴
  • 想加一个外壳

如果用继承，那样耦合度太高，不够灵活，装饰器模式就是用聚合来代替继承。

alt

下面看一些代码（核心是 装饰器 和 被装饰者 都实现同一接口）：

public interface Shape {
    void draw();
}

public class Rectangle implements Shape {

    @Override
    public void draw() {
        System.out.println("Shape: Rectangle");
    }
}

public class Circle implements Shape {

    @Override
    public void draw() {
        System.out.println("Shape: Circle");
    }
}

public abstract class ShapeDecorator implements Shape {

    protected Shape decoratedShape;

    public ShapeDecorator(Shape decoratedShape){
        this.decoratedShape = decoratedShape;
    }

    public void draw(){
        decoratedShape.draw();
    }
}

public class RedShapeDecorator extends ShapeDecorator {

    public RedShapeDecorator(Shape decoratedShape) {
        super(decoratedShape);
    }

    @Override
    public void draw() {
        decoratedShape.draw();
        setRedBorder(decoratedShape);
    }

    private void setRedBorder(Shape decoratedShape){
        System.out.println("Border Color: Red");
    }
}

下面是client调用的代码：

public static void main(String[] args) {

        Shape circle = new Circle();
        ShapeDecorator redCircle = new RedShapeDecorator(new Circle());
        ShapeDecorator redRectangle = new RedShapeDecorator(new Rectangle());
        //Shape redCircle = new RedShapeDecorator(new Circle());
        //Shape redRectangle = new RedShapeDecorator(new Rectangle());
        System.out.println("Circle with normal border");
        circle.draw();

        System.out.println("\nCircle of red border");
        redCircle.draw();

        System.out.println("\nRectangle of red border");
        redRectangle.draw();
    }

运行截图： alt