Robin Han Robin Han - 2 months ago 10
Java Question

Why UnaryFunction<Object> can be casted to UnaryFunction<T>?

When I read Effective Java item 27, the type casting between

UnaryFunction<Object>
and
UnaryFunction<T>
confused me.

interface UnaryFunction<T> {
T apply(T t);
}

public class Main {
private static final UnaryFunction<Object> IDENTITY = new UnaryFunction<Object>() {
public Object apply(Object t) {
return t;
}
};

@SuppressWarnings("unchecked")
public static <T> UnaryFunction<T> identityFunction() {
return (UnaryFunction<T>) IDENTITY;
}

public static void main(String ... args) {
UnaryFunction<A> identityA = Main.identityFunction();
A a = identityA.apply(new A());
}

}

class A {}


Why
UnaryFunction<Object>
can be casted to
UnaryFunction<T>
?


I know the generic type will be erased after complier. So
(UnaryFunction<T>) IDENTITY
will eventually be
(UnaryFunction<Object>) IDENTITY
, this will be working in Runtime.

But directly casting
UnaryFunction<Object>
to
UnaryFunction<A>
is not allowed by compiler.

UnaryFunction<A> identityA = (UnaryFunction<A>)IDENTITY;
//incompatible types: UnaryFunction<java.lang.Object> cannot be converted to UnaryFunction<A>


And there is no inheritance relationship between
UnaryFunction<Object>
and
UnaryFunction<T>
. So why
UnaryFunction<Object>
can be casted to
UnaryFunction<T>
?

Answer

All generic types, without a bound (... extends ..., is considered Object due to type erasure. So this cast might be valid for some values of T. @SuppressWarnings("unchecked") tells the compiler you are doing the right thing so the warning is suppressed.

Casting to a specific type is problematic. Let's assume you are dealing with List<Object> which you cast to List<A> where A is a class. In this case list might have elements which are a supertype of A so this cast is unsound hence the compiler does not allow it. In case of the function (UnaryFunction<Object>) it is implied that this can return an Object. Say your code was IDENTITY = t -> new Object(); in which case casting to UnaryFunction<A> would be unsound as it returns an Object. In the case it is UnaryFunction<T> there is some type T which satisfies the cast, that is, when T is an Object.

For some background reading on this see: Liskov substitution principle which deals with subtyping of functions.