Description Mauro Molinari 2017-06-02 06:15:45 EDT

I have the following example and I'd like to know if the current JDT behaviour is fine. Using Eclipse Neon.3 (4.6.3) with JDT 3.12.3.v20170301-0400.

public class TestLambda {

  public static void main(String[] args) throws InterruptedException {
    final List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
    final List<Callable<Void>> callables = strings.stream().map(new Function<String, Callable<Void>>() {

      @Override
      public Callable<Void> apply(final String s) {
        return new Callable<Void>() {

          @Override
          public Void call() throws Exception {
            System.out.println(s);
            return null;
          }
        };
      }
    }).collect(Collectors.toList());
    Executors.newFixedThreadPool(2).invokeAll(callables);
  }
}

If I convert new Function<>(){...} to lambda expression, with Ctrl+1, I get:

public class TestLambda {

  public static void main(String[] args) throws InterruptedException {
    final List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
    final List<Callable<Void>> callables = strings.stream().map(s -> new Callable<Void>() {

      @Override
      public Void call() throws Exception {
        System.out.println(s);
        return null;
      }
    }).collect(Collectors.toList());
    Executors.newFixedThreadPool(2).invokeAll(callables);
  }
}

But JDT complains: Type mismatch: cannot convert from List<Object> to List<Callable<Void>>.

If I roll back and then do the following:
- Ctrl+1 on new Callable<Voide>(){...} to convert to lambda expression
- Cltr+1 on new Function<>(){...} to convert to lambda expression
I get:

public class TestLambda {

  public static void main(String[] args) throws InterruptedException {
    final List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
    final List<Callable<Void>> callables = strings.stream().map(s -> () -> {
      System.out.println(s);
      return null;
    }).collect(Collectors.toList());
    Executors.newFixedThreadPool(2).invokeAll(callables);
  }
}

But then JDT complains twice:
- The target type of this expression must be a functional interface
- Cannot infer type argument(s) for <R> map(Function<? super T,? extends R>)

The latter problem may be a consequence of the former, but I'm not sure.