修改if-else策略模式

Question

我在java中有以下if-else分支。

if (str.equals("a")) { A;} 
else if (str.equals("b")) { B;} 
else if (str.equals("c")) { C;} 
else if (str.length == 5) { D;} 
else { E;} 

如何將此代碼修改爲戰略模式？

Answer 1

這裏使用工廠策略模式的一個例子：

public interface Strategy { 
    public Object[] execute(Object[] args); 
} 

public class StrategyFactory { 

    public enum Name { 
     REVERSE, STRINGIFY, DUPLICATE; 
    } 

    private StrategyFactory() { 
     // never instantiate; only use static factory methods 
    } 

    public static Strategy getStrategyReverse() { 
     return new Strategy() { 
      public Object[] execute(Object[] args) { 
       Object[] reversed = new Object[args.length]; 
       for (int i = 0; i < args.length; i++) { 
        reversed[i] = args[args.length - i - 1]; 
       } 
       return reversed; 
      } 
     }; 
    } 

    public static Strategy getStrategyStringify() { 
     return new Strategy() { 
      public Object[] execute(Object[] args) { 
       String[] stringified = new String[args.length]; 
       for (int i = 0; i < args.length; i++) { 
        stringified[i] = String.valueOf(args[i]); 
       } 
       return stringified; 
      } 
     }; 
    } 

    public static Strategy getStrategyDuplicate() { 
     return new Strategy() { 
      public Object[] execute(Object[] args) { 
       Object[] duplicated = new Object[2 * args.length]; 
       for (int i = 0; i < args.length; i++) { 
        duplicated[i * 2] = args[i]; 
        duplicated[i * 2 + 1] = args[i]; 
       } 
       return duplicated; 
      } 
     }; 
    } 

    public static Strategy getStrategy(String name) { 
     return getStrategy(Name.valueOf(name)); 
    } 

    public static Strategy getStrategy(Name name) { 
     switch (name) { 
      case REVERSE: 
       return getStrategyReverse(); 
      case STRINGIFY: 
       return getStrategyStringify(); 
      case DUPLICATE: 
       return getStrategyDuplicate(); 
      default: 
       throw new IllegalStateException("No strategy known with name " + name); 
     } 
    } 
} 

public class Main { 
    public static void main(String[] args) { 

     Strategy strategy = StrategyFactory.getStrategy("DUPLICATE"); 
     System.out.println(Arrays.toString(strategy.execute(args))); 
    } 
} 

Answer 2

你必須考慮面向對象編程。使用多態性。 對於戰略模式， 定義接口併爲實現接口的類提供不同的實現。選擇上下文並多級決定班級。 http://en.wikipedia.org/wiki/Strategy_pattern

但是，對於您的if-else正確的模式對應於'工廠模式'。 http://en.wikipedia.org/wiki/Factory_method_pattern