Passing interfaces as parameter in a class constructor [duplicate]

I was reading about the Strategy design pattern here.

They used one interface called strategy that contains one function called doOperation

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

---

Then they build two classes OperationAdd and OperationSubstract that implement this strategy interface

public class OperationAdd implements Strategy{
   @Override
   public int doOperation(int num1, int num2) {
      return num1 + num2;
   }
}

public class OperationSubstract implements Strategy{
   @Override
   public int doOperation(int num1, int num2) {
      return num1 - num2;
   }
}

---

After this, they defined a class called Context that contains private strategy interface and the constructor receives a strategy and fills it in the private attribute. Moreover, they did a function that calls the function of the strategy interface.

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);
   }
}

---

Finally, they did the final class called StrategyPatternDemo that called the Context class.

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));

   }
}

---

I understood the concept of the strategy pattern. However, I didn't understand how we can pass the classes OperationAdd and OperationSubstract in the constructor of the Context class while it receives an interface. as shown in Context context = new Context(new OperationAdd()); and context = new Context(new OperationSubstract());

How the class is cast to the interface?

Thanks in advance!