Compiler error type of object in for (Class obj : Items) - why?

Question

Hi all,
I'm new to Java generics and am in the process of converting some
legacy code. I have encountered a particular error which I do not
understand the reason for, since I think the compiler has enough
information about the code to work out type information. The following
code shows a test class which demonstrates this error:
import java.util.Collection;
public class Test
    interface Base {}
    static class Impl1 implements Base{}
    public <T extends Base> Collection<T> getCollection(Class<? extends Base> cls)
        return null;
    void someMethod()
        for ( Impl1 obj : getCollection( Impl1.class ))
            // *** compiler complains about the line above ***
            // compiler says:
            // Test.java:22: incompatible types
            // found   : Test.Base
            // required: Test.Impl1
            //        for ( Impl1 obj : getCollection( Impl1.class ))
            //                                        ^
    void someMethod2()
        Collection<Impl1> coln = getCollection(Impl1.class);
        for (Impl1 obj : coln)
            // do something here with obj
}Why should the compiler complain where it does ( the line marked with
asterisks) in someMethod, but does not reject the declaration statement
in someMethod2? I would have assumed both are the same and the
compiler has the same type information at its disposal to determine
the correct type.
Cheers,
Bonny

Answer 1

The Java 5 compiler not only looks at a method's name and arguments, but also at its return value in order to determine whether a method is callable and what type should be infered for its type parameters.
In your example the type parameter T is only used in the declaration of the method's return type. Note that there is no relation between T and the type of the argument cls!
Thus, the compiles needs to know what type of variable the return value will be assigned to.
Hence, someMethod2 can be resolved, whereas in someMethod there is no assignment of the return value. (Rather, an implicit call to Iterable.iterator()).
You can solve this problem by forming a relation between the method's argument, its return type and its type parameter. Just declare the method like this:
public <T extends Base> Collection<T> getCollection(Class<T> cls)
        return null;
    }