Java 8 uses generic type method references

I have the problem of combining Java 8 method references with generic types I have simplified my problem and made it clear The following code failed:

public static void main(String[] args) {
    new Mapper(TestEvent::setId);
}

private static class Mapper<T> {
    private BiConsumer<TestEvent,T> setter;
    private Mapper(BiConsumer<TestEvent,T> setter) { this.setter = setter; }
}

private static class TestEvent {
    public void setId(Long id) { }
}

But if I change the constructor call to

BiConsumer<TestEvent,Long> consumer = TestEvent::setId;
    new Mapper(consumer);

Everything worked. Can anyone explain why

I know that if I delete the generic type (T) and replace it with long, it won't work in solving my real problem

Solution

At present, you are trying to use the original mapper type, which will erase all kinds of things

Once you start using generic types, everything is fine - type inference can help you:

new Mapper<>(TestEvent::setId);

Add < > all these are needed for your code compilation