Java APIs

I've got my compiler warning me of unnecessary casts, and this method generates one of the warnings:

    public static<T> T doIt(T input) {
        Object result = null;
        return (T) result; // Warning: Unnecessary cast to type T for expression of type Object
    }

Can anybody explain why it doesn't like me casting to type T? I get a compiler error if I don't make the cast.

Thanks.