Question

This method shall take two objects of the same type and return one of those objects by random:

public static <T> T random(T o1, T o2)
{
   return Math.random() < 0.5 ? o1 : o2;
}

Now, why does the compiler accept two parameters with distinctive types?

random("string1", new Integer(10)); // Compiles without errors

EDIT:
Now that I know that both parameters are getting implicitly upcasted, I wonder why the compiler does complain when calling the following method:

public static <T> List<T> randomList(List<T> l1, List<T> l2) {
        return Math.random() < 0.5 ? l1 : l2;
    }

Call:

randomList(new ArrayList<String>(), new ArrayList<Integer>()); // Does not Compile

If those ArrayList Parameters are also getting upcasted to Object, why does it give me an error this time?

Answer 1

T is inferred to be Object, and both arguments are getting implicitly upcast.

Thus the code is equivalent to:

Main.<Object>random((Object)"string1", (Object)new Integer(10));

What may be even more surprising is that the following compiles:

random("string1", 10);

The second argument is getting auto-boxed into an Integer, and then both arguments are getting upcast to Object.