java - Java 中的装饰器设计模式 - 为什么它不添加新成分到列表中？

Question

我尝试用 Java 实现一个简单的装饰器模式。主要思想是具体装饰器必须向基本列表中添加一些内容。但是，我的实现无法正常工作，我不知道为什么。

输出如下所示:

ING -1,ING 0,ING 1.

但应该是:

ING -1,ING 0,ING 1, ING 2.

这是我的代码:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package newpackage;

import java.util.ArrayList;
import java.util.List;

abstract class Tester {

    protected List<String> ingridients = new ArrayList();
    protected String description;

    public String getDescription() {
        description = "";
        for (String i : ingridients) {
            description += i;
            description += ",";
        }
        description = description.substring(0, description.length() - 1);
        description += ".";
        return description;
    }
}

abstract class Decorator extends Tester {

    @Override
    public abstract String getDescription();
}

class Test1 extends Tester {

    public Test1() {
        this.ingridients.add("ING -1");
        this.ingridients.add("ING 0");
    }
}

class Ing1 extends Decorator {

    private Tester t;

    public Ing1(Tester t) {
        this.t = t;
    }

    @Override
    public String getDescription() {
        this.t.ingridients.add("ING 1");
        return this.t.getDescription();
    }
}

class Ing2 extends Decorator {

    private Tester t;

    public Ing2(Tester t) {
        this.t = t;
    }

    @Override
    public String getDescription() {
        this.t.ingridients.add("ING 2");
        return this.t.getDescription();
    }
}

public class Test {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Tester t = new Test1();
        t = new Ing1(t);
        t = new Ing2(t);

        System.out.println(t.getDescription());
    }
}

编辑后的代码:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package newpackage;

import java.util.ArrayList;
import java.util.List;

interface Tester {

    List<String> ingridients = new ArrayList();
    public String getDescription();
}

abstract class Decorator implements Tester {

    @Override
    public abstract String getDescription();
}


class Test1 implements Tester {

    public Test1() {
        ingridients.add("ING -1");
        ingridients.add("ING 0");
    }

    @Override
    public String getDescription() {
        String description = "";
        for (String i : ingridients) {
            description += i;
            description += ",";
        }
        description = description.substring(0, description.length() - 1);
        description += ".";
        return description;
    }
}

class Ing1 extends Decorator {

    private Tester t;

    public Ing1(Tester t) {
        this.t = t;
    }

    @Override
    public String getDescription() {
        this.t.ingridients.add("ING 1");
        return this.t.getDescription();
    }
}

class Ing2 extends Decorator {

    private Tester t;

    public Ing2(Tester t) {
        this.t = t;
    }

    @Override
    public String getDescription() {
        this.t.ingridients.add("ING 2");
        return this.t.getDescription();
    }
}

public class Test {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Tester t = new Test1();
        t = new Ing1(t);
        t = new Ing2(t);

        System.out.println(t.getDescription());
    }
}

Answer 1

我在调试器中运行它，我可以看到你的装饰器不仅仅是装饰器，因为它们有自己的状态。让您的 Tester 成为一个接口(interface)，并让装饰器仅包装具体实例，而没有自己的状态。

// adds to the list in t1
this.t.ingridients.add("ING 2");

// add to the list in t
this.t.ingridients.add("ING 1");

// returns the contents of t.
return this.t.getDescription();

最后 t.ingredients 有 3 个元素，t1.ingredients 有 1 个元素，t2.ingredients 没有。

<小时/>

你可以这样写

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;


public class Test {
    public static void main(String[] ignored) {
        Tester t012 = new Ing2(new Ing1(new Ing0(new None())));
        System.out.println(t012.getDescription());

        Tester t210 = new Ing0(new Ing1(new Ing2(new None())));
        System.out.println(t210.getDescription());
    }
}

abstract class Tester {
    public List<String> getIngredients() {
        return Collections.emptyList();
    }

    public String getDescription() {
        StringBuilder sb = new StringBuilder();
        String sep = "";
        for (String s : getIngredients()) {
            sb.append(sep).append(s);
            sep=", ";
        }
        sb.append(".");
        return sb.toString();
    }
}

class None extends Tester {
}

class Ing0 extends Tester {
    private final Tester wrapped;
    Ing0(Tester wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public List<String> getIngredients() {
        List<String> list = new ArrayList<>(wrapped.getIngredients());
        list.add("ING -1");
        list.add("ING 0");
        return Collections.unmodifiableList(list);
    }
}

class Ing1 extends Tester {
    private final Tester wrapped;
    Ing1(Tester wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public List<String> getIngredients() {
        List<String> list = new ArrayList<>(wrapped.getIngredients());
        list.add("ING 1");
        return Collections.unmodifiableList(list);
    }
}

class Ing2 extends Tester {
    private final Tester wrapped;
    Ing2(Tester wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public List<String> getIngredients() {
        List<String> list = new ArrayList<>(wrapped.getIngredients());
        list.add("ING 2");
        return Collections.unmodifiableList(list);
    }
}

打印

ING -1, ING 0, ING 1, ING 2.
ING 2, ING 1, ING -1, ING 0.