I am following an example of strategy pattern from [here](http://www.tutorialspoint.com/design_pattern/strategy_pattern.htm)

Everything in the tutorial is clear but this: 

```
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);
   }
}

```

So the Context class expects a Strategy argument in its constructor. 

The definition of Strategy is: 

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

```

The above being an interface, the Context Class expects an object of type Strategy. In the StrategyPatternDemo class we do:

```
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));
   }
}

```

I am utterly confused as we cant init an interface according to the definition: 

An interface is different from a class in several ways, including:

You cannot instantiate an interface. 

How exactly is this `Context context = new Context(new OperationAdd());` sent as an argument to `public Context(Strategy strategy){
 this.strategy = strategy;
 }`