无法理解战略模式

Question

我从这里开始关注战略模式的一个例子

教程中的所有内容都很清楚但是:

public class Context {
   private Strategy strategy;

   public Context(Strategy strategy){
      this.strategy = strategy;
   }

   public int executeStrategy(int num1, int num2){
      return strategy.doOperation(num1, num2);
   }
}

因此Context类在其构造函数中需要一个Strategy参数.

战略的定义是:

public interface Strategy {
   public int doOperation(int num1, int num2);
}

上面是一个接口,Context Class需要一个Strategy类型的对象.在StrategyPatternDemo类中,我们执行以下操作:

public class StrategyPatternDemo {
   public static void main(String[] args) {
      Context context = new Context(new OperationAdd());        
      System.out.println("10 + 5 = " + context.executeStrategy(10, 5));

      context = new Context(new OperationSubstract());      
      System.out.println("10 - 5 = " + context.executeStrategy(10, 5));

      context = new Context(new OperationMultiply());       
      System.out.println("10 * 5 = " + context.executeStrategy(10, 5));
   }
}

我完全感到困惑,因为我们无法根据定义初始化接口:

接口在几个方面与类不同,包括:

您无法实例化接口.

这究竟是Context context = new Context(new OperationAdd());作为一个参数发送的public Context(Strategy strategy){ this.strategy = strategy; }

Answer 1

OperationAdd这些类OperationSubstract都OperationMultiply实现了该接口Strategy。因此，这些类的实例可以传递到Context需要类型对象的构造函数中Strategy。

由于OperationAdd、OperationSubstract和OperationMultiply实现接口Strategy，它们都是该类型。