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.
