How to implement decoration mode of Java structural design pattern

Introduction

Intent: Dynamically add some additional responsibilities to an object. In terms of adding functionality, the decoration mode is more flexible than generating subclasses.

Main solution: We often use inheritance to extend a class. Since inheritance introduces static features to the class, and as the extension functions increase, the subclass will become more and more expanded.

How to use: Extend without adding many subclasses.

How to solve: Divide specific functional responsibilities and inherit the decorator pattern.

Key code:

1. The Component class plays an abstract role and should not be implemented concretely.

2. The modified class references and inherits the Component class, and the specific extension class overrides the parent class method.

Usage scenarios:

1. Extend the functionality of a class.

2. Dynamically add functions and dynamically cancel them.

Implementation

Create a Shape interface and an entity class that implements the Shape interface. Then create an abstract decoration class ShapeDecorator that implements the Shape interface, and use the Shape object as its instance variable. RedShapeDecorator is an entity class that implements ShapeDecorator. The DecoratorPatternDemo class uses RedShapeDecorator to decorate Shape objects.

Step 1:

public interface Shape{
    void draw();
}

Step 2:

public class Rectangle implements Shape{
    @Override
    public ovid draw(){
        System.out.println("draw Rectangle");
    }
}

public class Cricle implements Shape{
    @Override
    public ovid draw(){
        System.out.println("draw Circle");
    }
}

Step 3:

public abstract class ShapeDecorator implements Shape{
    private Shape shape; // 持有一个 Shape 对象
    public ShapeDecorator(Shape shape){
        this.shape = shape;
    }
    public void draw(){
        shape.draw();// TODO 根据传进来的具体 Shape 对象，调用对应的 draw 方法
    }
}

Step 4:

public class RedShapeDecorator extends ShapeDecorator{
    public RedShapeDecorator(Shape shape){
        super(shape);
    }
    @Override
    public void draw(){
        shape.draw();
        setRedBorder(shape);
    }
    public void setRedBorder(Shape shape){
        System.out.println("Border Color: Red");
    }
}

Step 5

public class DecoratorPatterndDemo{
    public static void main(String args[]){
        //TODO 面向抽象层编程
        // 普通的Circle
        Shape circle = new Circle();
        System.out.println("Circle with normal Border");        
        circle.darw();
        // 红色边界的 Circle
        ShapeDecorator redCircle = new RedShapeDecorator(new Circle); 
        System.out.println("Circle of red Border");
        redCircle.draw();
        // 红色边界的 Rectangle
        ShapeDecorator redRectangle = new RedShapeDecorator(new Rectangel):
        System.out.println("Rectangle of red Border");
        redRectangle.draw();
    }
}

Advantages and Disadvantages

Advantages: Decorating classes and decorated classes can develop independently and will not be coupled to each other. Decoration mode is an alternative mode to inheritance. Decoration mode The functionality of an implementation class can be dynamically extended.

Disadvantages: Multi-layer decoration is more complicated.

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