java 装饰者设计模式

装饰器的引入：

BufferedReader 对FileReader拓展了一个功能， readLine.

*** 需求1 ：***编写一个类对BufferedReader的功能进行增强， 增强其readLine方法，返回数据带有行号。

*** 需求2 ：***编写一个类对BufferedReader的功能进行增强， 增强其readLine方法，返回数据带有分号。

package com.wxhl.jq0803;

import java.io.*;

//带行号的缓冲类。
class BufferedLineNum extends BufferedReader {
    int count = 1;

    public BufferedLineNum(Reader in) {
        super(in); // 该句话只是为了让编译器编译不报错而已。
    }

    @Override
    public String readLine() throws IOException {
        String line = super.readLine(); // 调用其父类的readLine方法， null
        if (line == null) {
            return null;
        }
        line = count + ":" + line;
        count++;
        return line;
    }
}

// 带有冒号的缓冲类
class BufferedSemi extends BufferedReader {

    public BufferedSemi(Reader in) {
        super(in);
    }

    @Override
    public String readLine() throws IOException {
        String line = super.readLine();
        if (line == null) {
            return null;
        }
        line = line + ":";
        return line;
    }
}

// 带有行号+分号的缓冲类
class LineNumAndSemi extends BufferedLineNum {
    public LineNumAndSemi(Reader in) {
        super(in);
    }

    @Override
    public String readLine() throws IOException {
        String line = super.readLine();
        if (line == null) {
            return null;
        }
        line = line + ";";
        return line;
    }
}

class Demo01 {
    public static void main(String[] args) throws IOException {
        File file = new File(System.getProperty("user.home")+"/Desktop/user.txt");
        FileReader fileReader = new FileReader (file);
        LineNumAndSemi lineSemi = new LineNumAndSemi(fileReader);
        String line = null;
        while((line = lineSemi.readLine())!=null){
            System.out.println(line);
        }
        lineSemi.close();
    }
}

分析:

从上可知,想拓展readLine()方法,返回带有行号的数据时,BufferedLineNum类需要继承自BufferedReader类,且重写readLine()方法,然后在父类的readLine()基础上拓展出"有行号"功能

当又想拓展"有行号"功能,加上"有分号"功能时,LineNumAndSemi类又需要继承自BufferedLineNum类,再在其父类readLine()功能基础上进行拓展

这样,如果再需要拓展出其他功能,又需要继承自LineNumAndSemi类,是不是好一个庞大的继承体系呀,好臃肿有没有

改进(装饰者设计模式):

import java.io.*;

//返回带有行号的缓冲类
class BufferedLineNum2 extends BufferedReader{
    
    BufferedReader bufferedReader;    // BufferedReader bufferedReader = new BufferedSemi();
    
    int count = 1; 
    
    public BufferedLineNum2(BufferedReader bufferedReader){  // BufferedReader bufferedReader = new BufferedSemi();
        super(bufferedReader);
        this.bufferedReader = bufferedReader;
    }
    
    public String readLine() throws IOException {
        String line = bufferedReader.readLine();  // 解决方案：如果这里的readLine方法是BufferedSemi的readline方法那么该问题解决了。
        if(line==null){
            return null;
        }
        line = count+":"+line;
        count++;
        return line;
    }   
}

//返回带有很好的缓冲类
class BufferedSemi2 extends BufferedReader{
    
    BufferedReader bufferedReader;    // BufferedReader bufferedReader = new BufferedSemi();
    
    public BufferedSemi2(BufferedReader bufferedReader){  // BufferedReader bufferedReader = new BufferedSemi();
        super(bufferedReader);
        this.bufferedReader = bufferedReader;
    }
    
    public String readLine() throws IOException {
        String line = bufferedReader.readLine();  // 解决方案：如果这里的readLine方法是BufferedSemi的readline方法那么该问题解决了。
        if(line==null){
            return null;
        }
        line = line+";";
        return line;
    }   
}

public class Demo02{
    public static void main(String[] args) throws IOException {
        File file = new File(System.getProperty("user.home")+"/Desktop/user.txt");
        FileReader fileReader = new FileReader (file);
        BufferedReader bufReader = new BufferedReader(fileReader);
        LineNumAndSemi lineSemi = new LineNumAndSemi(bufReader);
        String line = null;
        while((line = lineSemi.readLine())!=null){
            System.out.println(line);
        }
        lineSemi.close();
    }
}

从上可以看出,虽然只是实现了相同的功能,但是装饰者模式明显搭配灵活,需要用到哪个类的拓展功能,直接传入其对象即可
而且还避免了不断继承的臃肿

另外的模拟

package cn.itcast.add;

/*
需求：    一家三口都要工作， 儿子工作负责绘画, 妈妈可以在儿子的工作上进行增强---》上涂料， 爸爸的工作就是在妈妈的基础上增强--->上画框 
*/

interface Work{
    
    public  void work();
}


class Son implements Work{

    @Override
    public void work() {
        System.out.println("在画画...");
    }
}


class  Mother implements Work{

    //在内部维护一个 需要被增强的类
    Work w;  // work w = new Son();
    
    public Mother(Work w){
        this.w = w;
    }
    
    @Override
    public void work() {
        w.work();
        System.out.println("妈妈上涂料...");
    }
}


//
class Father implements Work{
    
    Work w;
    
    public Father(Work w){
        this.w = w;
    }

    @Override
    public void work() {
        w.work();
        System.out.println("爸爸上画框...");
    }
    
    
}
public class Demo3 {
    
    public static void main(String[] args) {
        Son s = new Son();
        Mother m = new Mother(s);
        Father father  = new Father(m);
        father.work();
    }
}