Java: removing "Comparable is a raw type" warning

Question

Suppose I have a method called foo taking 2 Object as parameter. Both objects are of the same type and both implements comparable interface.

void foo(Object first, Object second){

    if (!first.getClass().isInstance(second))   //first and second of the same type
        return;

    Comparable firstComparable = (Comparable)first;  //WARNING
    Comparable secondComparable = (Comparable)second;  //WARNING

    int diff = firstComparable.compareTo(secondComparable);  //WARNING
}

The first 2 warning are:

Comparable is a raw type. References to generic type Comparable should be parameterized

The last warning:

Type safety: The method compareTo(Object) belongs to the raw type Comparable. References to generic type Comparable should be parameterized

How could I refactor my code in order to remove these warnings?

EDIT: Can I do that without changing foo method's signature?

Answer 1

You have to tell the compiler that they are the same type and comparable. If you can't change the signature you can add a method for backward compatibility.

@SuppressWarnings("unchecked")
static void foo(Object first, Object second) {
    foo((Comparable) first, (Comparable) second);
}

static <T extends Comparable<T>> void foo(T first, T second){
    int diff = first.compareTo(second); // no warning.
}