在日常開發中,常常會作一些狀态判斷,用到 swich-case 與 if-else 。在面向對象的環境裡,有兩種方式可以替代它們。一種是使用繼承子類的多态,另一種是使用 state 模式。它們使用對象的間接性有效地擺脫了傳統的狀态判斷。 舉個例子。 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 newNoMethodTypeException(); } } public boolean safeMethod() { if ( _type == GET ) return true ; else return false; } public boolean passwordRequired() { if ( _type == POST ) return false ; else return true; } } |
類 Method 中,存在四個狀态 Post 、 Get 、 Put 和 Delete 。有一個 switch-case 判斷,用于輸出四種方法的描述資訊;兩個 if-else 判斷,分别判斷方法是否安全 ( 隻有 Get 方法是安全的 ) ,方法是否需要密碼 ( 隻有 Post 方法不需要密碼 ) 。
1. 使用繼承子類多态使用繼承子類多态的方式,通常對于某個具體對象,它的狀态是不可改變的 ( 在對象的生存周期中 ) 。
現在使用四個子類分别代表四種類型的方法。這樣就可以使用多态将各個方法的具體邏輯分置到子類中去了。 在抽象基類 Method 中可以提供建立子類執行個體的靜态方法,當然也可以使用 Simple Factory 模式。對于 getMethod() 方法,延遲到子類中實作;對于 safeMethod() 方法和 passwordRequired() 方法,提供一個預設的實作,這個實作應該符合絕大部分子類的要求,這樣的話,對于少數不符合預設實作的子類隻需 override 相應方法即可。 <<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 newGetMethod(); } public final static Method createPutMethod() { return newPutMethod(); } public final static Method createDeleteMethod() { return newDelMethod(); } abstract public String getMethod(); public boolean safeMethod() { return false; } public boolean passwordRequired() { return true; } } |
四個子類分别繼承和 override 相應的方法。 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" ; } } |
如果希望對象在生存周期内,可以變化自己的狀态,則可以選擇 state 模式。
這裡抽象狀态為一個接口 MethodType ,四種不同的狀态實作該接口。 <<interface>>MethodType.java
package com.zj.state; public interface MethodType { String getTypeDescription(); String getMethodDescription(); boolean isSafe(); booleanisRequired(); } |
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; } } |
此時,在類 Method 中儲存一個 field 表示 MethodType ,在某對象中,可以随時變化四種已知的狀态 ( 具體見 runAllMethods() 方法 ) 。 Method.java
package com.zj.state; public class Method { private MethodType _type ; public Method() { _type = null; } publicMethod(MethodType type) { _type = type; } public String getMethod() { return_type .getMethodDescription(); } public boolean safeMethod() { return_type .isSafe(); } public boolean passwordRequired() { return_type .isRequired(); } public voidchangeType(MethodType type) { _type = type; } public voidrunAllMethods() { 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.測試
在測試類中,分别使用上面 3 中機制展示結果。它們的結果應該是一緻的。 Client.java
package com.zj.utils; public class Client { static void print(String s) { System. out .println(s); } static voidprint(Boolean b) { System. out .println(b); } public static void main(String[] args) throwsNoMethodTypeException { print( "===original===" ); print( "===POST===" ); com.zj.original.Method post1 = newcom.zj.original.Method( com.zj.original.Method. POST ); print(post1.getMethod()); print(post1.safeMethod()); print(post1.passwordRequired()); print( "===GET===" ); com.zj.original.Method get1 = newcom.zj.original.Method( com.zj.original.Method. GET ); print(get1.getMethod()); print(get1.safeMethod()); print(get1.passwordRequired()); print( "===PUT===" ); com.zj.original.Method put1 = newcom.zj.original.Method( com.zj.original.Method. PUT ); print(put1.getMethod()); print(put1.safeMethod()); print(put1.passwordRequired()); print( "===DELETE===" ); com.zj.original.Method del1 = newcom.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===" ); newcom.zj.state.Method().runAllMethods(); } } |
轉載于:https://blog.51cto.com/zhangjunhd/68070