getClass() of a Generic Method Parameter in a Java

Question

The following Java method fails to compile:

<T extends Number> void foo(T t)
{
    Class<? extends T> klass = t.getClass();
}

Error received is: Type mismatch: cannot convert from Class<capture#3-of ? extends Number> to Class<? extends T>

Can someone explain why Class<? extends T> is invalid, but Class<? extends Number> is fine?

Javadoc says:

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:

Number n = 0;
Class<? extends Number> c = n.getClass(); 

Answer 1

Because the T class' type doesn't extend from T. Instead, it extends from Number, exactly as you have declared yourself in <T extends Number>. Simple as that.

A better question would be:

Why doesn't the following compile?

<T extends Number> void foo(T t)
{
    Class<T> class1 = t.getClass();
}

The answer to that is that the Object#getClass() returns Class<?> with an unbounded wildcard ? because the object itself is not directly aware about its generic type which is been expected in an arbitrary method.