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
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package com.zj.original;
import com.zj.utils.NoMethodTypeException;
public class Method {
private int _type;
public static final int POST = 0;
public static final int GET = 1;
public static final int PUT = 2;
public static final int DELETE = 3;
public Method(int type) {
_type = type;
}
public String getMethod() throws NoMethodTypeException {
switch (_type) {
case POST:
return "This is POST method";
case GET:
return "This is GET method";
case PUT:
return "This is PUT method";
case DELETE:
return "This is DELETE method";
default:
throw new NoMethodTypeException();
}
}
public boolean safeMethod() {
if (_type == GET)
return true;
else
return false;
}
public boolean passwordRequired() {
if (_type == POST)
return false;
else
return true;
}
}
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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
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package com.zj.subclass;
public abstract class Method {
public final static Method createPostMethod() {
return new PostMethod();
}
public final static Method createGetMethod() {
return new GetMethod();
}
public final static Method createPutMethod() {
return new PutMethod();
}
public final static Method createDeleteMethod() {
return new DelMethod();
}
abstract public String getMethod();
public boolean safeMethod() {
return false;
}
public boolean passwordRequired() {
return true;
}
}
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PostMethod.java
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package com.zj.subclass;
public class PostMethod extends Method {
@Override
public String getMethod() {
return "This is POST method";
}
@Override
public boolean passwordRequired() {
return false;
}
}
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package com.zj.subclass;
public class GetMethod extends Method{
@Override
public String getMethod() {
return "This is GET method";
}
@Override
public boolean safeMethod() {
return true;
}
}
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package com.zj.subclass;
public class PutMethod extends Method {
@Override
public String getMethod() {
return "This is PUT method";
}
}
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package com.zj.subclass;
public class DelMethod extends Method{
@Override
public String getMethod(){
return "This is DELETE method";
}
}
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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
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package com.zj.state;
public interface MethodType {
String getTypeDescription();
String getMethodDescription();
boolean isSafe();
boolean isRequired();
}
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package com.zj.state;
public class Post implements MethodType{
public String getMethodDescription() {
return "This is POST method";
}
public String getTypeDescription() {
return "===POST===";
}
public boolean isRequired() {
return false;
}
public boolean isSafe() {
return false;
}
}
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package com.zj.state;
public class Get implements MethodType{
public String getMethodDescription() {
return "This is GET method";
}
public String getTypeDescription() {
return "===GET===";
}
public boolean isRequired() {
return true;
}
public boolean isSafe() {
return true;
}
}
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package com.zj.state;
public class Put implements MethodType{
public String getMethodDescription() {
return "This is PUT method";
}
public String getTypeDescription() {
return "===PUT===";
}
public boolean isRequired() {
return true;
}
public boolean isSafe() {
return false;
}
}
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package com.zj.state;
public class Delete implements MethodType{
public String getMethodDescription() {
return "This is DELETE method";
}
public String getTypeDescription() {
return "===DELETE===";
}
public boolean isRequired() {
return true;
}
public boolean isSafe() {
return false;
}
}
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Method.java
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package com.zj.state;
public class Method {
private MethodType _type;
public Method() {
_type = null;
}
public Method(MethodType type) {
_type = type;
}
public String getMethod() {
return _type.getMethodDescription();
}
public boolean safeMethod() {
return _type.isSafe();
}
public boolean passwordRequired() {
return _type.isRequired();
}
public void changeType(MethodType type) {
_type = type;
}
public void runAllMethods() {
MethodType[] types = new MethodType[] { new Post(), new Get(),
new Put(), new Delete() };
for (MethodType type : types) {
System.out.println(type.getTypeDescription());
changeType(type);
System.out.println(getMethod());
System.out.println(safeMethod());
System.out.println(passwordRequired());
}
}
}
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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
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package com.zj.utils;
public class Client {
static void print(String s) {
System.out.println(s);
}
static void print(Boolean b) {
System.out.println(b);
}
public static void main(String[] args) throws NoMethodTypeException {
print("===original===");
print("===POST===");
com.zj.original.Method post1 = new com.zj.original.Method(
com.zj.original.Method.POST);
print(post1.getMethod());
print(post1.safeMethod());
print(post1.passwordRequired());
print("===GET===");
com.zj.original.Method get1 = new com.zj.original.Method(
com.zj.original.Method.GET);
print(get1.getMethod());
print(get1.safeMethod());
print(get1.passwordRequired());
print("===PUT===");
com.zj.original.Method put1 = new com.zj.original.Method(
com.zj.original.Method.PUT);
print(put1.getMethod());
print(put1.safeMethod());
print(put1.passwordRequired());
print("===DELETE===");
com.zj.original.Method del1 = new com.zj.original.Method(
com.zj.original.Method.DELETE);
print(del1.getMethod());
print(del1.safeMethod());
print(del1.passwordRequired());
print("===subclass===");
print("===POST===");
com.zj.subclass.Method post2 = com.zj.subclass.Method
.createPostMethod();
print(post2.getMethod());
print(post2.safeMethod());
print(post2.passwordRequired());
print("===GET===");
com.zj.subclass.Method get2 = com.zj.subclass.Method.createGetMethod();
print(get2.getMethod());
print(get2.safeMethod());
print(get2.passwordRequired());
print("===PUT===");
com.zj.subclass.Method put2 = com.zj.subclass.Method.createPutMethod();
print(put2.getMethod());
print(put2.safeMethod());
print(put2.passwordRequired());
print("===DELETE===");
com.zj.subclass.Method del2 = com.zj.subclass.Method
.createDeleteMethod();
print(del2.getMethod());
print(del2.safeMethod());
print(del2.passwordRequired());
print("===state===");
new com.zj.state.Method().runAllMethods();
}
}
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本文出自 “子 孑” 博客，请务必保留此出处http://zhangjunhd.blog.51cto.com/113473/68070