This is a real-world example from a 3rd party library API, but simplified.

Compiled with Oracle JDK 8u72

Consider these two methods:

<X extends CharSequence> X getCharSequence() { return (X) "hello"; } <X extends String> X getString() { return (X) "hello"; }

Both report an "unchecked cast" warning - I get why. The thing that baffles me is why can I call

Integer x = getCharSequence();

and it compiles? The compiler should know that Integer does not implement CharSequence . The call to

Integer y = getString();

gives an error (as expected)

incompatible types: inference variable X has incompatible upper bounds java.lang.Integer,java.lang.String

Can someone explain why would this behaviour be considered valid? How would it be useful?

The client does not know that this call is unsafe - the client's code compiles without warning. Why wouldn't the compile warn about that / issue an error?

Also, how is it different from this example:

<X extends CharSequence> void doCharSequence(List<X> l) { } List<CharSequence> chsL = new ArrayList<>(); doCharSequence(chsL); // compiles List<Integer> intL = new ArrayList<>(); doCharSequence(intL); // error

Trying to pass List<Integer> gives an error, as expected:

method doCharSequence in class generic.GenericTest cannot be applied to given types; required: java.util.List<X> found: java.util.List<java.lang.Integer> reason: inference variable X has incompatible bounds equality constraints: java.lang.Integer upper bounds: java.lang.CharSequence