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Code is allowed which is rejected by javac. A method with a bounded generic type argument is assigned to a result with an unbounded generic type. The inferred type for the generic type argument should be unbounded resulting in a compile error attempting to call the method. import java.awt.Container; import java.util.Collection; public class Demo { private <T extends Container> Collection<T> foo() { return null; } private void showProblem() { Collection<?> result = foo(); } } /* Fails when compiled in javac: Demo.java:14: incompatible types; inferred type argument(s) ? do not conform to bounds of type variable(s) T found : <T>java.util.Collection<T> required: java.util.Collection<?> Collection<?> result = foo(); ^ 1 error */
This is a bug in Javac, it fails to infer that under-constrained T should be 'Container'. Note that in a little variable, it does fine. import java.awt.Container; import java.util.Collection; public class X { private <T extends Container> Collection<T> foo() { return null; } private void showProblem() { Collection<?> result = (Collection<Container>) foo(); } }
Previous comment should have said: "... Note that with a small variation, it does fine."
Note: the variation with extra cast is not strictly equivalent, as with the cast it does NOT consider the expected type (Collection<?>) into the inference stage (pass2). Technically though, the expected type doesn't bring any useful information for inference, and should just end up with the same final result, i.e. T is underconstrained and should default to be Container.
Inference from expected type with unbound wildcard (Collection<?>) is described in JLS 15.12.7, p.459. Added GenericTypeTest#test1137. Marking as invalid.
