【設計模式】裝飾器模式

使用頻率：★★★☆☆

一、什么是裝飾模式

通過關聯機制給類增加行為，其行為的擴展由修飾對象來決定；

如JAVA IO流里的以下形式，BufferedReader為裝飾類，其關聯了一個具體對象（new FileReader(new File("test.txt"))），並對其進行裝飾，裝飾后擁有readLine行為（方法）：

new BufferedReader(new FileReader(new File("test.txt")));

二、補充說明

與繼承相似，不同點在於繼承是在編譯期間擴展父類，而裝飾器模式在運行期間動態擴展原有對象；

或者說，繼承是對類進行擴展，裝飾模式是對對象進行擴展；

三、角色

抽象構件

具體構件

抽象裝飾類

具體裝飾類

說明：具體構件、抽象裝飾類、具體裝飾類的共同父類是抽象構件，具體裝飾類繼承抽象裝飾類並在運行期間裝飾具體構件；

四、例子

例子說明：

畫家接口Painter，為抽象構件，有兩個方法，獲取畫家描述信息及繪畫；

PaintBeginner實現Painter接口，為具體構件；

PainterDecorator實現Painter接口，為抽象裝飾類，其內部關聯一個Painter對象，通過構造函數獲取；

HillPainterDecorator、RiverPainterDecorator、TreePainterDecorator為具體裝飾類，表明被裝飾的畫家能夠繪畫Hill、River、Tree；

類圖：

代碼實現：

Painter.java

package com.pichen.dp.decorator;

public interface Painter {


    public abstract String getDescription();
    
    public abstract String painting();
    
}

PaintBeginner.java

package com.pichen.dp.decorator;

public class PaintBeginner implements Painter{

    @Override
    public String getDescription() {

        return "";
    }

    @Override
    public String painting() {
        /* do nothing */
        return "";
    }

}

PainterDecorator.java

package com.pichen.dp.decorator;

public abstract class PainterDecorator implements Painter{

    private Painter decoratedPainter;
    public PainterDecorator(Painter decoratedPainter) {
        this.decoratedPainter = decoratedPainter;
    }
    
    public Painter getPainter(){
        return this.decoratedPainter;
    }
}

HillPainterDecorator.java

package com.pichen.dp.decorator;

public class HillPainterDecorator extends PainterDecorator{

    public HillPainterDecorator(Painter paper) {
        super(paper);
    }

    
    
    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint hill, ";
    }

    @Override
    public String painting() {
        /* painting the hill */
        return this.getPainter().painting() + paintingHill();
    }
    
    public String paintingHill(){
        return "Hill, ";
    }

}

RiverPainterDecorator.java

package com.pichen.dp.decorator;

public class RiverPainterDecorator extends PainterDecorator{

    public RiverPainterDecorator(Painter paper) {
        super(paper);
    }

    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint river, ";
    }

    @Override
    public String painting() {

        /* painting the river */
        return this.getPainter().painting() + paintingRiver();
    }
    
    public String paintingRiver(){
        return "River, ";
    }

}

TreePainterDecorator.java

package com.pichen.dp.decorator;

public class TreePainterDecorator extends PainterDecorator{

    public TreePainterDecorator(Painter paper) {
        super(paper);
    }

    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint tree, ";
    }

    @Override
    public String painting() {
        /* painting the tree */
        return this.getPainter().painting() + paintingTree();
    }
    
    public String paintingTree(){
        return "Tree, ";
    }

}

Main.java

package com.pichen.dp.decorator;

public class Main {

    public static void main(String[] args) {
        
        Painter p0 = new PaintBeginner();
        System.out.println("Painter description:" + p0.getDescription());
        System.out.println("Painting:" + p0.painting() + "\n");

        HillPainterDecorator p2 = new HillPainterDecorator(new PaintBeginner());
        System.out.println("Painter description:" + p2.getDescription());
        System.out.println("Painting:" + p2.painting());
        System.out.println("Painting:" + p2.paintingHill() + "\n"); //新增的行為
        
        RiverPainterDecorator p3 = new RiverPainterDecorator(new PaintBeginner());
        System.out.println("Painter description:" + p3.getDescription());
        System.out.println("Painting:" + p3.painting());
        System.out.println("Painting:" + p3.paintingRiver() + "\n"); //新增的行為
        
        HillPainterDecorator p4 = new HillPainterDecorator(new RiverPainterDecorator(new TreePainterDecorator(new PaintBeginner())));
        System.out.println("Painter description:" + p4.getDescription());
        System.out.println("Painting:" + p4.painting());
        System.out.println("Painting:" + p4.paintingHill() + "\n"); //新增的行為

        
        

    }

}

執行結果如下，PaintBeginner類的對象未裝飾前，無行為；在被裝飾器裝飾后，行為可以改變：

五、JAVA IO流與裝飾模式

這里簡單的以Reader、BufferedReader、FileReader舉個例子，如下代碼：

        BufferedReader br = new BufferedReader(new FileReader(new File("test.txt")));
        br.readLine();

說明：

其中BufferedReader與FileReader有一個共同抽象父類Reader，Reader為抽象構件；

new FileReader(new File("test.txt"))為具體構件，運行期間被修飾的對象；

BufferedReader為具體修飾類，運行期間修飾具體構件；

裝飾后，被修飾的對象新增的行為是擁有readLine方法；

ps：查看源碼，沒發現BufferedReader對應的抽象裝飾類，個人覺得沒有抽象裝飾類，裝飾模式也是可以正常工作的，抽象構件（Reader）可以由具體修飾類關聯；

另外，具體修飾類也可以作為基類，被其它類繼承的，繼承后的類同樣也是具體修飾類，如LineNumberReader就是繼承BufferedReader；

所以，上面語句還可以這樣寫（ps：只是舉例，其實沒必要用BufferedReader修飾，直接LineNumberReader裝飾下就可以）：

        BufferedReader br = new LineNumberReader(new BufferedReader(new FileReader(new File("test.txt"))));
        br.readLine();

links：

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