How should I define a lambda in my method's arguments?

Question

Ι have the following custom Runnable:

class RandomSum extends Runnable {

   public void run() {
     float sum - 0;
     for(int i = 0; i<1000000;i++){
       sum+=Math.random();
     } 
   }
}

And I want to run it like that:

  RandomSum s =new RandomSum();
  s.retrieveCallback((sum)->{
    System.out.println(sum);
  });
  Thread thread = new Thread();
  thread.start();

But I do not know how I should define the method retrieveCallback in RandomSum that accepts a lambda?

Answer 1

You can define retrieveCallback within RandomSum as follows:

public void retrieveCallback(FeedbackHandler feedbackHandler) {
    int sum = ...; // Get the value however you like.
    feedbackHandler.handleFeedback(sum);
}

And then define this FeedbackHandler interface as:

public interface FeedbackHandler {
    void handleFeedback(int sum);
}

Essentially what happens when you pass lambda (sum) -> {...} to retrieveCallback is:

retrieveCallback(new FeedbackHandler() {
    @Override
    public void handleFeedback(int sum) {
        ...
    }
});