目录

• 一、单一职责
• 二、接口隔离
• 三、依赖倒置（倒转）
• 四、里氏替换
• 五、迪米特法则（Law of Demeter）
• 六、开闭
• 七、合成复用

一、单一职责

• 类（或方法）功能的专一性。一个类（或方法）不应该承担太多功能，一个类（或方法）最好只承担 一种类型 的功能。

public class SingleResponsibility {public static void main(String[] args) {Vehicle vehicle = new Vehicle();vehicle.run("汽车");vehicle.run("火车");vehicle.run("自行车");vehicle.run("飞机"); // 有问题vehicle.run("轮船"); // 有问题}
}class Vehicle {public void run(String vehicleName) {System.out.println(vehicleName + "在公路上行驶");}
}

Vehicle 类既处理陆地上的交通工具，又处理天空中的交通工具；它的作用太广泛了，不单一。

public class SingleResponsibility {public static void main(String[] args) {RoadVehicle roadVehicle = new RoadVehicle();roadVehicle.run("汽车");roadVehicle.run("火车");roadVehicle.run("自行车");SkyVehicle skyVehicle = new SkyVehicle();skyVehicle.run("飞机");WaterVehicle waterVehicle = new WaterVehicle();waterVehicle.run("轮船");}
}class RoadVehicle {public void run(String vehicleName) {System.out.println(vehicleName + "在公路上行驶");}
}class WaterVehicle {public void run(String vehicleName) {System.out.println(vehicleName + "在水中行驶");}
}class SkyVehicle {public void run(String vehicleName) {System.out.println(vehicleName + "在天空中行驶");}
}

在类级别遵循了单一职责原则

当业务功能比较简单的时候也没有必要将其拆分为多个类（如下所示）

public class SingleResponsibility {public static void main(String[] args) {Vehicle vehicle = new Vehicle();vehicle.run("汽车", VehicleType.ROAD);vehicle.run("火车", VehicleType.ROAD);vehicle.run("自行车", VehicleType.ROAD);vehicle.run("飞机", VehicleType.SKY);vehicle.run("轮船", VehicleType.WATER);}
}enum VehicleType {ROAD, WATER, SKY;
}class Vehicle {public void run(String name, VehicleType type) {String sentence = "";if (type == VehicleType.ROAD) {sentence = "在公路上行驶";} else if (type == VehicleType.WATER) {sentence = "在水中行驶";} else if (type == VehicleType.SKY) {sentence = "在天空中行驶";}System.out.println(name + sentence);}// public void roadRun(String name) {}// public void skyRun(String name) {}// public void waterRun(String name) {}
}

Vehicle 类不符合单一职责原则，但其功能简单 。
当功能非常非常简单的时候，不一定必须遵循单一职责原则。（杀鸡别用宰牛刀）

• 降低了类的功能的复杂度
• 提高了代码的维护性
• 代码修改导致连带错误几率降低

二、接口隔离

① Clients should not be forced to depend on methods they do not use. 客户端不应该被迫依赖它不使用的方法。
② The dependency of one class to another one should depend on the smallest possible interface. 一个类对另一个类的依赖应该建立在最小的接口上。

public class InterfaceSegregation {public static void main(String[] args) {InOutAbleImpl1 impl1 = new InOutAbleImpl1();InOutAbleImpl2 impl2 = new InOutAbleImpl2();Cat cat = new Cat();cat.use1(impl1);cat.use2(impl1);cat.use3(impl1);Dog dog = new Dog();dog.use1(impl2);dog.use2(impl2);dog.use3(impl2);}
}class Cat {public void use1(InOutAble inOutAble) {inOutAble.openDoor();}public void use2(InOutAble inOutAble) {inOutAble.pushGoods();}public void use3(InOutAble inOutAble) {inOutAble.writeGoodsName();}
}class Dog {public void use1(InOutAble inOutAble) {inOutAble.openDoor();}public void use2(InOutAble inOutAble) {inOutAble.popGoods();}public void use3(InOutAble inOutAble) {inOutAble.writePeopleName();}
}/*** 存取东西的接口*/
interface InOutAble {// 类型1void openDoor(); // 打开门// 类型2void pushGoods(); // 放入货物void writeGoodsName(); // 登记货物名字// 类型3void popGoods(); // 取出货物void writePeopleName(); // 登记取货人的名字
}class InOutAbleImpl1 implements InOutAble {@Overridepublic void openDoor() {System.out.println("InOutAbleImpl1 - openDoor");}@Overridepublic void pushGoods() {System.out.println("InOutAbleImpl1 - pushGoods");}@Overridepublic void writeGoodsName() {System.out.println("InOutAbleImpl1 - writeGoodsName");}@Overridepublic void popGoods() {System.out.println("InOutAbleImpl1 - popGoods");}@Overridepublic void writePeopleName() {System.out.println("InOutAbleImpl1 - writePeopleName");}
}class InOutAbleImpl2 implements InOutAble {@Overridepublic void openDoor() {System.out.println("InOutAbleImpl2 - openDoor");}@Overridepublic void pushGoods() {System.out.println("InOutAbleImpl2 - pushGoods");}@Overridepublic void writeGoodsName() {System.out.println("InOutAbleImpl2 - writeGoodsName");}@Overridepublic void popGoods() {System.out.println("InOutAbleImpl2 - popGoods");}@Overridepublic void writePeopleName() {System.out.println("InOutAbleImpl2 - writePeopleName");}
}

public class InterfaceSegregation {public static void main(String[] args) {OpenPushImpl impl1 = new OpenPushImpl();OpenPopImpl impl2 = new OpenPopImpl();Cat cat = new Cat();cat.use1(impl1);cat.use2(impl1);cat.use3(impl1);Dog dog = new Dog();dog.use1(impl2);dog.use2(impl2);dog.use3(impl2);}
}class Cat {public void use1(OpenPushImpl openPushImpl) {openPushImpl.openDoor();}public void use2(OpenPushImpl openPushImpl) {openPushImpl.pushGoods();}public void use3(OpenPushImpl openPushImpl) {openPushImpl.writeGoodsName();}
}class Dog {public void use1(OpenPopImpl openPopImpl) {openPopImpl.openDoor();}public void use2(OpenPopImpl openPopImpl) {openPopImpl.popGoods();}public void use3(OpenPopImpl openPopImpl) {openPopImpl.writePeopleName();}
}interface OpenDoorAble {void openDoor(); // 打开门
}interface PushAble {void pushGoods(); // 放入货物void writeGoodsName(); // 登记货物名字
}interface PopAble {void popGoods(); // 取出货物void writePeopleName(); // 登记取货人的名字
}class OpenPushImpl implements OpenDoorAble, PushAble {@Overridepublic void openDoor() {System.out.println("OpenPushImpl - openDoor");}@Overridepublic void pushGoods() {System.out.println("OpenPushImpl - pushGoods");}@Overridepublic void writeGoodsName() {System.out.println("OpenPushImpl - writeGoodsName");}
}class OpenPopImpl implements OpenDoorAble, PopAble {@Overridepublic void openDoor() {System.out.println("OpenPopImpl - openDoor");}@Overridepublic void popGoods() {System.out.println("OpenPopImpl - popGoods");}@Overridepublic void writePeopleName() {System.out.println("OpenPopImpl - writePeopleName");}
}

三、依赖倒置（倒转）

🍬 ① 高层模块不应该依赖低层模块（类），二者都应该依赖于抽象（接口）
🍬 ② 抽象（接口）不应该依赖细节（实现类），而是细节依赖于抽象
🍬 ③ 依赖倒置的中心思想是：面向接口（抽象）编程
🍬 ④ 依赖倒置设计理念：相对于细节的多变性，抽象的东西要稳定得多。以抽象为基础搭建的架构比以细节为基础搭建的架构要稳定
🍬 ⑤ 使用接口或抽象类的作用是：制定规范（协议）；把展现细节的任务交给接口的实现类

接口 ➡️ 抽象
实现类 ➡️ 细节

public class DependencyInversion {public static void main(String[] args) {Person person = new Person();person.sendMessage(new QQMessage("用QQ问候一下小明"));person.sendMessage(new WechatMessage("用微信问候一下小明"));}
}class Person {public void sendMessage(QQMessage qqMessage) {System.out.println(qqMessage.buildMessage());}public void sendMessage(WechatMessage wechatMessage) {System.out.println(wechatMessage.buildMessage());} 
}class QQMessage {private String message;public QQMessage(String message) {this.message = message;}public String buildMessage() {return "QQ Message: " + message;}
}class WechatMessage {private String message;public WechatMessage(String message) {this.message = message;}public String buildMessage() {return "Wechat Message: " + message;}
} 

😰 假如版本升级，还想发送 抖音消息 的话：① 需要创建一个 TiktokMessage 类；② 需要在 Person 类中再重载一个 sendMessage(TiktokMessage tiktokMessage) 方法
😰 每次版本升级，增加新的发送消息的方式的时候对代码的改动非常大。假如不止一个用户（不仅仅只有 Person 类），哪改动就更加巨大了

面向接口编程：

public class DependencyInversion {public static void main(String[] args) {Person person = new Person();person.sendMessage(new QQMessage("用QQ问候一下小明"));person.sendMessage(new WechatMessage("用微信问候一下小明"));}
}class Person {public void sendMessage(IMessage iMessage) {System.out.println(iMessage.buildMessage());}
}interface IMessage {String buildMessage();
}/*** 发送 QQ 消息*/
class QQMessage implements IMessage {private String message;public QQMessage(String message) {this.message = message;}@Overridepublic String buildMessage() {return "QQ Message: " + message;}
}/*** 发送微信消息*/
class WechatMessage implements IMessage {private String message;public WechatMessage(String message) {this.message = message;}public String buildMessage() {return "Wechat Message: " + message;}
}

四、里氏替换

继承优点： 🍬 提高代码的复用性（子类继承父类后可使用父类的非 private 关键字修饰的的成员）

public class Animal {public void eat() {System.out.println("Animal - public void eat()");}protected void drink() {System.out.println("Animal - protected void drink()");}void play() {System.out.println("Animal - void play()");}
}class Dragon extends Animal {public void use() {eat(); // Animal - public void eat()drink(); // Animal - protected void drink()play(); // Animal - void play()}public static void main(String[] args) {Dragon dragon = new Dragon();dragon.use();}
}

继承缺点: 🍬 ① 继承关系过去庞大的话，整个代码结构会很乱
🍬 ② 代码耦合性变高
🍬 ③ 代码稳定性变差（子类可以重写父类的方法。重写之后，运行时究竟调用的是父类的方法还是子类重写的方法很难判断）
🍬 ④ 如果有多处直接使用父类方法的实现，但凡父类方法修改了，所有依赖该父类方法的地方都得考虑兼容性（考虑代码是否会产生 bug）

public class Main {public static void main(String[] args) {Parent parent = new Parent();parent.m2(new ArrayList<>());Son son = new Son();son.m2(new ArrayList<>());/*Parent - m2(ArrayList)Parent - m2(ArrayList)*/}
}class Parent {public void m1() {System.out.println("Parent - m1()");}public void m2(ArrayList list) {System.out.println("Parent - m2(ArrayList)");}
}class Son extends Parent {// 不符合里氏替换原则（子类不应该重写父类的非抽象方法）@Overridepublic void m1() {System.out.println("Son - m1()");}// 子类重载父类的方法（子类重载的方法的参数类型要比父类被重载的方法的参数类型大）// 这样才符合里氏替换原则, 子类增加代码（如新增一个方法）不会影响父类方法的使用public void m2(List list) {System.out.println("Son - m2(List)");}
}

五、迪米特法则（Law of Demeter）

🍬 只与你的直接朋友交谈，不与陌生人交谈【降低类与类之间的耦合】

A ➡️ B ➡️ C
① A 和 B 是直接朋友
② B 和 C 是直接朋友
③ A 和 C 是陌生人
④ 若 A 想使用 C 中的方法，需要通过 B

🍬 直接朋友:
① 当前对象本身
② 当前对象的成员变量
③ 当前对象的成员方法的返回类型
④ 当前对象的成员方法的参数

class Parent {public void m() {// 在方法内部 new 出来的是非直接朋友User user = new User();}
} class User {}

六、开闭

🎄 Open Close Principe：软件对象（类、模块、方法等）应该对扩展开放，对修改关闭
🎄 用抽象构建框架，用实现扩展细节
🎄 开放 服务方 的拓展，关闭 消费方 的修改

public class OpenClosePrincipe {public static void main(String[] args) {// 对消费方的修改关闭// 尽量少修改原先的代码MilkTeaFactory factory = new MilkTeaFactory();factory.makeMilkTea(MilkTeaType.APPLE);factory.makeMilkTea(MilkTeaType.BANANA);}
}interface MilkTeaAble {}class AppleMilkTea implements MilkTeaAble {public AppleMilkTea() {System.out.println("苹果奶茶");}
}class BananaMilkTea implements MilkTeaAble {public BananaMilkTea() {System.out.println("香蕉奶茶");}
}enum MilkTeaType {APPLE, BANANA
}class MilkTeaFactory {public MilkTeaAble makeMilkTea(MilkTeaType type) {switch (type) {case APPLE:return new AppleMilkTea();case BANANA:return new BananaMilkTea();}return null;}
}

public class OpenClosePrincipe {// 修改依赖的类型（消费方代码没有修改）private static MilkTeaAble factory = new WatermelonMilkTeaFactory();public static void main(String[] args) {factory.milkTea();}
}interface MilkTeaAble {void milkTea();
}class AppleMilkTeaFactory implements MilkTeaAble {@Overridepublic void milkTea() {System.out.println("苹果奶茶");}
}class BananaMilkTeaFactory implements MilkTeaAble {@Overridepublic void milkTea() {System.out.println("香蕉奶茶");}
}class WatermelonMilkTeaFactory implements MilkTeaAble {@Overridepublic void milkTea() {System.out.println("西瓜奶茶");}
}

七、合成复用

📖 通过对象 组合、聚合、依赖 达成代码复用，而不是继承

class Animal {public void eat() {System.out.println("Animal - eat()");}public void drink() {System.out.println("Animal - drink()");}public void play() {System.out.println("Animal - play()");}
}/*** 继承（不推荐, 不符合合成复用原则）*/
class People1 extends Animal {public void use() {eat();drink();play();}
}/*** 依赖（推荐）*/
class People2 {public void use(Animal animal) {animal.eat();animal.drink();animal.play();}
}/*** 聚合（推荐）*/
class People3 {private Animal animal;public void setAnimal(Animal animal) {this.animal = animal;}public void use() {animal.eat();animal.drink();animal.play();}
}/*** 组合（推荐）*/
class People4 {private Animal animal = new Animal();public void use() {animal.eat();animal.drink();animal.play();}
}