設計模式之策略模式

1.定義

策略模式：定義了算法家族，分別封裝起來，讓它們之間可以互相替換，此模式讓算法的變化，不會影響到使用算法的用戶。

同樣以計算器的例子來展示策略模式。

2.UML:

3.Code

Operation.class:

abstract class Operation{
   public double numberA;
   public double numberB;

   abstract double getResult();

}

子類：

OperationAdd.class:

class OperationAdd extends Operation{

   public double getResult{
      return numerA + numberB;
   }
}

OperationSub.class:

class OperationSub extends Operation{

   public double getResult{
      return numerA - numberB;
   }
}

OperationMul.class:

class OperationMul extends Operation{

   public double getResult{
      return numerA * numberB;
   }
}

OperationDiv.class:

class OperationDiv extends Operation{

   public double getResult{
      if(numberB == 0){
         throw new Exception("除數不能爲0");
      }
      return numerA / numberB;
   }
}

Context.class:

class Context{
    private Operation operation;
    public Context(Operation opera){
         operation = opera;
    }

    public double getResult(){
         return operation.getResult();
    }

}

Client.class:

public class Client{
   public static void main(String[] args){
       String operator = "+";

       Operation operation = null;
       switch(operator){
           case "+":
               operation = new OperationAdd(); 
               break;
           case "-":
               operation = new OperationSub(); 
               break;
           case "*":
               operation = new OperationMul(); 
               break;
           case "/":
               operation = new OperationDiv(); 
               break;
        }
       operation.numberA = 2;
       operation.numberB = 4;

       Context context = new Context(operation);
       double result = context.getResult();   
   } 
}

純粹的策略模式使得客戶端還需要處理一大堆的判斷，顯得很臃腫，可以將策略模式與簡單工廠模式結合，將判斷的代碼移到Context中，如下：

Context.class:

class Context{
    private Operation operation;
    public Context(String operator,String numberA,String numberB){
         switch(operator){
           case "+":
               operation = new OperationAdd(); 
               break;
           case "-":
               operation = new OperationSub(); 
               break;
           case "*":
               operation = new OperationMul(); 
               break;
           case "/":
               operation = new OperationDiv(); 
               break;
        }
        operation.numberA = numberA;
        operation.numberB = numberB;
    }

    public double getResult(){
         return operation.getResult();
    }

}

Client.class

public class Client{
   public static void main(String[] args){
       Context context = new Context("+",2,4);
       double result = context.getResult();   
   } 
}

4.總結

可以看到策略模式與簡單工廠模式非常非常相似~

策略模式與簡單工廠模式的區別：

簡單工廠模式：

Operation operation = OperationFactory.create("+"，2,4);
double result = operation.getResult();

策略模式：

Context context = new Context("+",2,4);
double result = context.getResult();

策略模式中用戶只需要關心Context一個對象即可，簡單工廠模式有OperationFactory和Operation兩個對象需要用戶去了解。

策略模式中Context類不止可以生成對象，而且還暴露出方法可以讓客戶端直接調用對象的方法，爲客戶端提供一條龍服務~因此對客戶端來說，策略模式的封裝性更好。