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3.3 依赖倒转原则(DIP)

依赖倒转原则：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;// cpuprivate 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、cpuKingstonMemory k = new KingstonMemory();//表示内存InterCpu  i = new InterCpu();// 表示 cpuXiJieHardDisk 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{@Overridepublic void runCpu() {System.out.println("InterCpu正在运行...");}
}
==========================================================// 新增Amd类型的cpu 
public class AmdCpu implements Cpu{@Overridepublic void runCpu() {System.out.println("AMD Cpu正在运行...");}
}
==========================================================// KingstonMemory类实现Memory接口
public class KingstonMemory implements Memory{@Overridepublic 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{@Overridepublic void saveDate(String date) {System.out.println("从硬盘获取的数据是:"+date);}@Overridepublic 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。