3.3 依赖倒转原则(DIP)

Dependency Inversion Principle，DIP

3.3.1概述

高层模块不应该依赖低层模块，两者都应该依赖其抽象；抽象不应该依赖细节，细节应该依赖抽象。简单的说就是要求对抽象进行编程，不要对实现进行编程，这样就降低了客户与实现模块间的耦合。

3.3.2 案例

下面看一个例子来理解依赖倒转原则

【例】组装电脑

现要组装一台电脑，需要配件cpu，硬盘，内存条。只有这些配置都有了，计算机才能正常的运行。选择cpu有很多选择，如Intel，AMD等，硬盘可以选择希捷，西数等，内存条可以选择金士顿，海盗船等。

public class InterCpu {
    public void runCpu(){
        System.out.println("Inter的cpu正在运行...");
    }
}
====================================================
  public class KingstonMemory {
    public void saveMemory(){
        System.out.println("使用金士顿作为内存条...");
    }
}
====================================================
  public class XiJieHardDisk {
    public void saveDisk(String data){
        System.out.println("从硬盘获取的数据是:"+data);
    }
    public String  getData(){
        System.out.println("获取硬盘数据操作...");
        return "获取硬盘数据完成。";
    }
}
====================================================
  public class Computer {
    private XiJieHardDisk hardDisk;//硬盘
    private InterCpu cpu;// cpu
    private KingstonMemory memory;//内存

    public Computer() {}

    public Computer(XiJieHardDisk hardDisk, InterCpu cpu, KingstonMemory memory) {
        this.hardDisk = hardDisk;
        this.cpu = cpu;
        this.memory = memory;
    }

    public XiJieHardDisk getHardDisk() {return hardDisk;}

    public void setHardDisk(XiJieHardDisk hardDisk) {
        this.hardDisk = hardDisk;
    }

    public InterCpu getCpu() {return cpu;}

    public void setCpu(InterCpu cpu) {this.cpu = cpu;}

    public KingstonMemory getMemory() {return memory;}

    public void setMemory(KingstonMemory memory) {
        this.memory = memory;
    }

    public void runComputer(){
        System.out.println("计算机正在工作...");
        cpu.runCpu();
        memory.saveMemory();
        hardDisk.saveDisk("你好Java。");
        String data = hardDisk.getData();
        System.out.println(data);
    }
}
=========================================================
  //测试类
  public class ComputerTest {
    public static void main(String[] args) {

        // 创建计算机组件内存、硬盘、cpu
        KingstonMemory k = new KingstonMemory();//表示内存
        InterCpu  i = new InterCpu();// 表示 cpu
        XiJieHardDisk x = new XiJieHardDisk();// 表示硬盘

        // 创建计算机对象，并组装(即，传入组件参数)
        Computer computer = new Computer(x,i,k);
        // 运行计算机
        computer.runComputer();
    }
}

上面代码可以看到已经组装了一台电脑，但是似乎组装的电脑的cpu只能是Intel的，内存条只能是金士顿的，硬盘只能是希捷的，这对用户肯定是不友好的，用户有了机箱肯定是想按照自己的喜好，选择自己喜欢的配件。如果想换成不是Intel的cpu需要修改Computer类，这就违背了开闭原则。

根据依赖倒转原则进行改进：

// Cpu接口
public interface Cpu {
    public abstract void runCpu();
}
===========================================================
  // 硬盘接口
public interface HardDisk {
    public abstract void saveDate(String date);

    public abstract String getDate();
}
==========================================================
  // 内存接口
public interface Memory {
    public abstract void saveMemory();
}
==========================================================
  //IntelCpu类实现Cpu接口
  public class IntelCpu implements Cpu{
    @Override
    public void runCpu() {
        System.out.println("InterCpu正在运行...");
    }
}
==========================================================
  // 新增Amd类型的cpu 
public class AmdCpu implements Cpu{
    @Override
    public void runCpu() {
        System.out.println("AMD Cpu正在运行...");
    }
}
==========================================================
  // KingstonMemory类实现Memory接口
public class KingstonMemory implements Memory{
    @Override
    public void saveMemory() {
        System.out.println("使用金士顿作为内存条...");
    }
}
==========================================================
  // XiJieHardDisk类实现HardDisk接口
public class XiJieHardDisk implements HardDisk{
    @Override
    public void saveDate(String date) {
        System.out.println("从硬盘获取的数据是:"+date);
    }

    @Override
    public String getDate() {
        System.out.println("获取硬盘数据操作...");
        return "获取硬盘数据完成。";
    }
}
============================================================
  //聚合各种组件
  public class Computer {
    private HardDisk hardDisk;
    private Cpu cpu;
    private Memory memory;

    public Computer() {}

    public Computer(HardDisk hardDisk, Cpu cpu, Memory memory) {
        this.hardDisk = hardDisk;
        this.cpu = cpu;
        this.memory = memory;
    }

    public HardDisk getHardDisk() {return hardDisk;}

    public void setHardDisk(HardDisk hardDisk) {
        this.hardDisk = hardDisk;
    }

    public Cpu getCpu() { return cpu;}

    public void setCpu(Cpu cpu) { this.cpu = cpu;}

    public Memory getMemory() { return memory;}

    public void setMemory(Memory memory) {
        this.memory = memory;
    }
    //运行计算机方法
    public void runComputer(){
        System.out.println("计算机正在工作...");
        cpu.runCpu();
        memory.saveMemory();
        hardDisk.saveDate("你好Java。");
        String data = hardDisk.getDate();
        System.out.println(data);
    }
}
=======================================================
  //测试类
  public class ComputerTest {
    public static void main(String[] args) {

        // 创建计算机的组件：内存、cpu、硬盘
        // Cpu cpu = new IntelCpu();
        Cpu cpu = new AmdCpu();
        HardDisk hardDisk = new XiJieHardDisk();
        Memory memory = new KingstonMemory();

      // 创建计算机
     Computer computer = new Computer(hardDisk,cpu,memory);

      // 运行计算机
      computer.runComputer();
    }
}

上述代码根据依赖倒转原则改进后扩展性比较好，如想换AMD类型的Cpu,只需子新增一个AmdCpu类去实现Cpu接口，重写Cpu里的抽象方法，再在测试类中去用Cpu接口去接AmdCpu的对象即可，这样就不用修改Computer类里面的代码了。