Description Tom Hofmann 2005-01-05 10:08:28 EST

I20040104-1600-gtk

Consider the below snippet. The eclipse compiler claims that b.getOne() should
only return an Object, which is consistent with javac. Inlining the call to
B.getTwo produces an error...

It seems that the type bound resolution depends on the calling context: inside
B.getTwo the return type of getOne is resolved to be at least a Number. Outside
of B, however, the resolvation is not made.

Also, as soon as the type parameter is bound to a concrete type (vs. just
restricted), type resolvation succeeds again, as can be seen with type C.

It looks like, while stepping up the type hierarchy, the compiler does not
sidestep from the superinterface clause "extends A<T>" to the formal type
declaration "B<T extends Number>" to collect the type bound.


------------ snip ----------

package test1;

public interface A<E>{
	E getOne();
}


abstract class B<T extends Number> implements A<T> {
	Number getTwo() {
		return getOne(); // succeeds
	}
}

abstract class C extends B<Integer> {
}

class X {
	void foo(A a, B b, C c){
		Object o= a.getOne();
		Number n1= b.getOne(); // fails
		Number n2= b.getTwo(); // succeeds, but inlining fails
		Integer i = c.getOne(); // succeeds
	}
}

------

The error message from eclipse:

"Type mismatch: cannot convert from Object to Number"

Javac:
test1/A.java:20: incompatible types
found   : java.lang.Object
required: java.lang.Number
                Number n1= b.getOne();
                                   ^
1 error