I was trying to create a method reference to an arbitrary object, so I defined the following types:
interface I {
boolean get(Impl impl);
}
static class Impl {
public boolean get() {
return true;
}
}
Then I declared the method reference, like below:
I i = Impl::get;
When I call:
i.get(null);
I get a NullPointerException:
Exception in thread "main" java.lang.NullPointerException
Can someone explain why this happens even though the Impl reference is not used anywhere?
I think you misunderstood the meaning of this line:
I i = Impl::get;
I is a functional interface that represents a method that takes an Impl and returns a boolean, whereas get is a method that takes no parameters and returns a boolean. How does this conversion work? Well, the compiler realises that get is an instance method, and to call it you must need a Impl object. Isn't that just like a function having a parameter before it is called?
So the compiler can happily infer that you meant:
I i = impl -> impl.get();
Now the cause of the NPE should be clear.
In general, all instance methods can be thought of as static methods that take one extra parameter, of type T where T is the declaring type of that instance method.