I do not get the difference between:
<V extends SomeInterface> BinaryOperator<V> method() {...} and:
BinaryOperator<? extends SomeInterface> method() {...} The whole snippet:
import java.util.function.BinaryOperator; import java.util.function.Function; import java.util.stream.Collectors; import java.util.stream.Stream; class Scratch { interface SomeInterface { boolean isEditable(); } static class One implements SomeInterface { public String specificMethod() { return "Hello"; } @Override public boolean isEditable() { return false; } } public static void main(String[] args) { Stream.of(new One()) .collect(Collectors.toMap( // cannot resolve method 'specificMethod' in 'SomeInterface' in collector: key -> key.specificMethod(), Function.identity(), chooseNotEditable() )); } // works: // static <V extends SomeInterface> BinaryOperator<V> chooseNotEditable() { // // Doesn't work: // static BinaryOperator<? extends SomeInterface> chooseNotEditable() { return (current, other) -> current.isEditable() ? current : other; } } I'm doing this because I want to have one generic mergeFunction to many implementations of SomeInterface. The commented solution works, but I want to understand why the version with the wildcard is not working.
