架构 之 使用子类多态 and 使用state模式（使用面向对象技术替代switch-case和if-else）

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在日常开发中，常常会作一些状态判断，用到swich-case与if-else。在面向对象的环境里，有两种方式可以替代它们。一种是使用继承子类的多态，另一种是使用state模式。它们使用对象的间接性有效地摆脱了传统的状态判断。

举个例子。

Method.java

[align=left]package com.zj.original;[/align] [align=left] [/align] [align=left]import com.zj.utils.NoMethodTypeException;[/align] [align=left] [/align] [align=left]public class Method {[/align] [align=left] private int _type;[/align] public static final int POST = 0; public static final int GET = 1; public static final int PUT = 2; public static final int DELETE = 3; [align=left] [/align] public Method(int type) { [align=left] _type = type;[/align] [align=left] }[/align] [align=left] [/align] public String getMethod() throws NoMethodTypeException { switch (_type) { [align=left] case POST:[/align] [align=left] return "This is POST method";[/align] [align=left] case GET:[/align] [align=left] return "This is GET method";[/align] [align=left] case PUT:[/align] [align=left] return "This is PUT method";[/align] [align=left] case DELETE:[/align] [align=left] return "This is DELETE method";[/align] [align=left] default:[/align] [align=left] throw new NoMethodTypeException();[/align] [align=left] }[/align] [align=left] }[/align] [align=left] [/align] public boolean safeMethod() { [align=left] if (_type == GET)[/align] [align=left] return true;[/align] [align=left] else[/align] [align=left] return false;[/align] [align=left] }[/align] [align=left] [/align] public boolean passwordRequired() { [align=left] if (_type == POST)[/align] [align=left] return false;[/align] [align=left] else[/align] [align=left] return true;[/align] [align=left] }[/align] [align=left]}[/align]

类Method中，存在四个状态Post、Get、Put和Delete。有一个switch-case判断，用于输出四种方法的描述信息；两个if-else判断，分别判断方法是否安全(只有Get方法是安全的)，方法是否需要密码(只有Post方法不需要密码)。

1.使用继承子类多态

使用继承子类多态的方式，通常对于某个具体对象，它的状态是不可改变的(在对象的生存周期中)。



现在使用四个子类分别代表四种类型的方法。这样就可以使用多态将各个方法的具体逻辑分置到子类中去了。

在抽象基类Method中可以提供创建子类实例的静态方法，当然也可以使用Simple Factory模式。对于getMethod()方法，延迟到子类中实现；对于safeMethod()方法和passwordRequired()方法，提供一个默认的实现，这个实现应该符合绝大部分子类的要求，这样的话，对于少数不符合默认实现的子类只需override相应方法即可。

<<abstract>>Method.java

[align=left]package com.zj.subclass;[/align] [align=left] [/align] public abstract class Method { [align=left] [/align] public final static Method createPostMethod() { [align=left] return new PostMethod();[/align] [align=left] }[/align] [align=left] [/align] public final static Method createGetMethod() { [align=left] return new GetMethod();[/align] [align=left] }[/align] [align=left] [/align] public final static Method createPutMethod() { [align=left] return new PutMethod();[/align] [align=left] }[/align] [align=left] [/align] public final static Method createDeleteMethod() { [align=left] return new DelMethod();[/align] [align=left] }[/align] [align=left] [/align] abstract public String getMethod(); [align=left] [/align] public boolean safeMethod() { [align=left] return false;[/align] [align=left] }[/align] [align=left] [/align] public boolean passwordRequired() { [align=left] return true;[/align] [align=left] }[/align] }

四个子类分别继承和override相应的方法。

PostMethod.java

[align=left]package com.zj.subclass;[/align] [align=left] [/align] public class PostMethod extends Method { [align=left] @Override[/align] [align=left] public String getMethod() {[/align] [align=left] return "This is POST method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] @Override[/align] public boolean passwordRequired() { [align=left] return false;[/align] [align=left] }[/align] }

GetMethod.java

[align=left]package com.zj.subclass;[/align] [align=left] [/align] [align=left]public class GetMethod extends Method{[/align] [align=left] @Override[/align] [align=left] public String getMethod() {[/align] [align=left] return "This is GET method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] @Override[/align] public boolean safeMethod() { [align=left] return true;[/align] [align=left] }[/align] }

PutMethod.java

[align=left]package com.zj.subclass;[/align] [align=left] [/align] public class PutMethod extends Method { [align=left] @Override[/align] [align=left] public String getMethod() {[/align] [align=left] return "This is PUT method";[/align] [align=left] }[/align] [align=left]}[/align]

DelMethod.java

[align=left]package com.zj.subclass;[/align] [align=left] [/align] [align=left]public class DelMethod extends Method{[/align] [align=left] @Override[/align] [align=left] public String getMethod(){[/align] [align=left] return "This is DELETE method";[/align] [align=left] }[/align] }

2.使用state模式

如果希望对象在生存周期内，可以变化自己的状态，则可以选择state模式。



这里抽象状态为一个接口MethodType，四种不同的状态实现该接口。

<<interface>>MethodType.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public interface MethodType {[/align] [align=left] String getTypeDescription();[/align] [align=left] [/align] [align=left] String getMethodDescription();[/align] [align=left] [/align] [align=left] boolean isSafe();[/align] [align=left] [/align] [align=left] boolean isRequired();[/align] }

Post.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public class Post implements MethodType{[/align] [align=left] public String getMethodDescription() {[/align] [align=left] return "This is POST method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] public String getTypeDescription() {[/align] [align=left] return "===POST===";[/align] [align=left] }[/align] [align=left] [/align] public boolean isRequired() { [align=left] return false;[/align] [align=left] }[/align] [align=left] [/align] public boolean isSafe() { [align=left] return false;[/align] [align=left] }[/align] }

Get.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public class Get implements MethodType{[/align] [align=left] public String getMethodDescription() {[/align] [align=left] return "This is GET method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] public String getTypeDescription() {[/align] [align=left] return "===GET===";[/align] [align=left] }[/align] [align=left] [/align] public boolean isRequired() { [align=left] return true;[/align] [align=left] }[/align] [align=left] [/align] public boolean isSafe() { [align=left] return true;[/align] [align=left] }[/align] [align=left]}[/align]

Put.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public class Put implements MethodType{[/align] [align=left] public String getMethodDescription() {[/align] [align=left] return "This is PUT method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] public String getTypeDescription() {[/align] [align=left] return "===PUT===";[/align] [align=left] }[/align] [align=left] [/align] public boolean isRequired() { [align=left] return true;[/align] [align=left] }[/align] [align=left] [/align] public boolean isSafe() { [align=left] return false;[/align] [align=left] }[/align] [align=left]}[/align]

Delete.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public class Delete implements MethodType{[/align] [align=left] public String getMethodDescription() {[/align] [align=left] return "This is DELETE method";[/align] [align=left] }[/align] [align=left] [/align] [align=left] public String getTypeDescription() {[/align] [align=left] return "===DELETE===";[/align] [align=left] }[/align] [align=left] [/align] public boolean isRequired() { [align=left] return true;[/align] [align=left] }[/align] [align=left] [/align] public boolean isSafe() { [align=left] return false;[/align] [align=left] }[/align] }

此时，在类Method中保存一个field表示MethodType，在某对象中，可以随时变化四种已知的状态(具体见runAllMethods()方法)。

Method.java

[align=left]package com.zj.state;[/align] [align=left] [/align] [align=left]public class Method {[/align] [align=left] private MethodType _type;[/align] [align=left] [/align] [align=left] public Method() {[/align] [align=left] _type = null;[/align] [align=left] }[/align] [align=left] [/align] [align=left] public Method(MethodType type) {[/align] [align=left] _type = type;[/align] [align=left] }[/align] [align=left] [/align] [align=left] public String getMethod() {[/align] [align=left] return _type.getMethodDescription();[/align] [align=left] }[/align] [align=left] [/align] public boolean safeMethod() { [align=left] return _type.isSafe();[/align] [align=left] }[/align] [align=left] [/align] public boolean passwordRequired() { [align=left] return _type.isRequired();[/align] [align=left] }[/align] [align=left] [/align] public void changeType(MethodType type) { [align=left] _type = type;[/align] [align=left] }[/align] [align=left] [/align] public void runAllMethods() { [align=left] MethodType[] types = new MethodType[] { new Post(), new Get(),[/align] new Put(), new Delete() }; [align=left] for (MethodType type : types) {[/align] [align=left] System.out.println(type.getTypeDescription());[/align] [align=left] changeType(type);[/align] [align=left] System.out.println(getMethod());[/align] [align=left] System.out.println(safeMethod());[/align] [align=left] System.out.println(passwordRequired());[/align] [align=left] }[/align] [align=left] }[/align] }

3.测试

在测试类中，分别使用上面3中机制展示结果。它们的结果应该是一致的。

Client.java

[align=left]package com.zj.utils;[/align] [align=left] [/align] [align=left]public class Client {[/align] static void print(String s) { [align=left] System.out.println(s);[/align] [align=left] }[/align] [align=left] [/align] static void print(Boolean b) { [align=left] System.out.println(b);[/align] [align=left] }[/align] [align=left] [/align] public static void main(String[] args) throws NoMethodTypeException { [align=left] print("===original===");[/align] [align=left] print("===POST===");[/align] [align=left] com.zj.original.Method post1 = new com.zj.original.Method([/align] [align=left] com.zj.original.Method.POST);[/align] [align=left] print(post1.getMethod());[/align] [align=left] print(post1.safeMethod());[/align] [align=left] print(post1.passwordRequired());[/align] [align=left] print("===GET===");[/align] [align=left] com.zj.original.Method get1 = new com.zj.original.Method([/align] [align=left] com.zj.original.Method.GET);[/align] [align=left] print(get1.getMethod());[/align] [align=left] print(get1.safeMethod());[/align] [align=left] print(get1.passwordRequired());[/align] [align=left] print("===PUT===");[/align] [align=left] com.zj.original.Method put1 = new com.zj.original.Method([/align] [align=left] com.zj.original.Method.PUT);[/align] [align=left] print(put1.getMethod());[/align] [align=left] print(put1.safeMethod());[/align] [align=left] print(put1.passwordRequired());[/align] [align=left] print("===DELETE===");[/align] [align=left] com.zj.original.Method del1 = new com.zj.original.Method([/align] [align=left] com.zj.original.Method.DELETE);[/align] [align=left] print(del1.getMethod());[/align] [align=left] print(del1.safeMethod());[/align] [align=left] print(del1.passwordRequired());[/align] [align=left] [/align] [align=left] print("===subclass===");[/align] [align=left] print("===POST===");[/align] [align=left] com.zj.subclass.Method post2 = com.zj.subclass.Method[/align] [align=left] .createPostMethod();[/align] [align=left] print(post2.getMethod());[/align] [align=left] print(post2.safeMethod());[/align] [align=left] print(post2.passwordRequired());[/align] [align=left] print("===GET===");[/align] [align=left] com.zj.subclass.Method get2 = com.zj.subclass.Method.createGetMethod();[/align] [align=left] print(get2.getMethod());[/align] [align=left] print(get2.safeMethod());[/align] [align=left] print(get2.passwordRequired());[/align] [align=left] print("===PUT===");[/align] [align=left] com.zj.subclass.Method put2 = com.zj.subclass.Method.createPutMethod();[/align] [align=left] print(put2.getMethod());[/align] [align=left] print(put2.safeMethod());[/align] [align=left] print(put2.passwordRequired());[/align] [align=left] print("===DELETE===");[/align] [align=left] com.zj.subclass.Method del2 = com.zj.subclass.Method[/align] [align=left] .createDeleteMethod();[/align] [align=left] print(del2.getMethod());[/align] [align=left] print(del2.safeMethod());[/align] [align=left] print(del2.passwordRequired());[/align] [align=left] [/align] [align=left] print("===state===");[/align] [align=left] new com.zj.state.Method().runAllMethods();[/align] [align=left] }[/align] }

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