Java reflection, call method of the passed in object's class

Just start to learn and know about java reflection, would appreciate any help on this problem.

I'm trying to write a method and it looks like this:

    private <T> void myMethod (List<T> testSub) {
        testSub.forEach(s -> assertEquals(s.getSource(), "TEST"));
    }

But it shows an error because java don't know if testSub's class has getSource() this method and it want me to cast s.getSource() to a known class.

What I want is to somehow let java know that the the itmes in testSub's class has this getSource() method and it could be invoked and safe to call.

Thanks in advance!