bad return type in lambda expression

Question

The following code compiles fine in IntelliJ and Eclipse, but the JDK compiler 1.8.0_25 complains. First, the code.

import java.util.function.Predicate;

public          


        

Answer 1

If a lambda expression appears in a target type with wildcards (as in most cases)

  Consumer<? super Boolean> consumer = b->{...}

the question arises - what's the type of the lambda expression; in particular, the type of b.

Of course, there could be many choices due to the wildcards; e.g. we could explicitly choose

  Consumer<? super Boolean> consumer = (Object b)->{...}

However, implicitly, b should be inferred as Boolean. This makes sense since the consumer should only be fed with Boolean anyway.

http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.27.3

If T is a wildcard-parameterized functional interface type and the lambda expression is implicitly typed, then the ground target type is the non-wildcard parameterization of T

(This probably assumes that wildcards are use properly variance-wise on the target type; we might find some hilarious examples if the assumption doesn't hold)

Answer 2

<? super E> includes Object, which isn't a Boolean. ie

MyPredicate<? super E>

Could be a

MyPredicate<Object>

and you can't return an Object as a boolean.

Try changing your lambda to:

MyStream.<Boolean> create().filter(b -> Boolean.TRUE.equals(b));

which will compile and execute without error not matter the type of the stream, but will return true for elements that are true Boolean values.