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?

Where do you want to execute the lambda in RandomSum class? Do you want to print the sum after the for loop? — Thiyagu
– Thiyagu, Commented Apr 10, 2021 at 12:47
Actually I want to pass the sum in a lamnda therefore yes I want to do it after the for loop. — Dimitrios Desyllas
– Dimitrios Desyllas, Commented Apr 10, 2021 at 12:48

Pieter12345 · Accepted Answer · 2021-04-10 12:52:25Z

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) {
        ...
    }
});

answered Apr 10, 2021 at 12:52

Pieter12345

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2 Comments

Dimitrios Desyllas Over a year ago

IC A lambda actually is a sinlgle-method anomymous class right?

Pieter12345 Over a year ago

Yes, where it is important that only one method can possibly match the signature that you pass in. Note that (int sum) -> {} is also possible (specifying the type of the argument), and that a lambda that returns something can be either (...) -> returnVal or (...) -> { ...; return returnVal; }

Thiyagu · Accepted Answer · 2021-04-10 12:50:36Z

One possible target type of the lambda you've shown is a Consumer. Define a method setCallback accepting a Consumer<Float> and store it as an instance variable. Then invoke it after the for loop.

class RandomSum implements Runnable {
    Consumer<Float> callback;
   
    public void setCallback(Consumer<Float> callback) {
        this.callback = callback;
    }

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

Caller side

RandomSum s =new RandomSum();
s.setCallback((sum)->{
    System.out.println(sum);
});

Or using method references,

s.setCallback(System.out::println);

Preferably you can pass the callback in the constructor.

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