在日常开发中,常常会作一些状态判断,用到swich-caseif-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
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;
    }
}
Method中,存在四个状态PostGetPutDelete。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
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;
    }
}
四个子类分别继承和override相应的方法。
PostMethod.java
package com.zj.subclass;
 
public class PostMethod extends Method {
    @Override
    public String getMethod() {
       return "This is POST method";
    }
 
    @Override
    public boolean passwordRequired() {
       return false;
    }
}
GetMethod.java
package com.zj.subclass;
 
public class GetMethod extends Method{
    @Override
    public String getMethod() {
       return "This is GET method";
    }
 
    @Override
    public boolean safeMethod() {
       return true;
    }
}
PutMethod.java
package com.zj.subclass;
 
public class PutMethod extends Method {
    @Override
    public String getMethod() {
       return "This is PUT method";
    }
}
DelMethod.java
package com.zj.subclass;
 
public class DelMethod extends Method{
    @Override
    public String getMethod(){
       return "This is DELETE method";
    }
}
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
package com.zj.state;
 
public interface MethodType {
    String getTypeDescription();
 
    String getMethodDescription();
 
    boolean isSafe();
 
    boolean isRequired();
}
Post.java
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;
    }
}
Get.java
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;
    }
}
Put.java
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;
    }
}
Delete.java
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;
    }
}
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
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());
       }
    }
}
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
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();
    }
}