Casting to an inner class with generics

Question

Consider the following code:

public class Outer<T>  {

    public class Inner{
    }

    public static <T> Outer<T>.Inner get(){
        Object o = new Object();
        return (Outer<T>.Inner)o;
    }

    public static void main(String[] args) throws Exception {
        Outer.<String>get();
    }
}

This code compiles successfully in Eclipse, but fails to compile in javac:

Outer.java:10: ')' expected
        return (Outer<T>.Inner)o;
                        ^
Outer.java:10: ';' expected
        return (Outer<T>.Inner)o;
                         ^
Outer.java:10: illegal start of expression
        return (Outer<T>.Inner)o;
                              ^
3 errors

Is this a bug in javac or Eclipse?

If I change the cast to (Outer.Inner)o it compiles, although there is a warning:

Eclipse:

Outer.Inner is a raw type. References to generic type Outer<T>.Inner should be parameterized

javac:

Outer.java:10: warning: [unchecked] unchecked conversion
found   : Outer.Inner
required: Outer<T>.Inner
        return (Outer.Inner)o;
               ^
1 warning

Javac version: 1.6.0_21

Answer 1

I have found that this is a bug in the javac compiler that has since been fixed. JDK 7b100 compiles this fine.

See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6665356

Answer 2

The most amusing thing is that, unless there is something that I miss about Java generics, both

return (Outer<T>.Inner) o;

And

return (Outer.Inner) o;

Both compile to the same bytecode.

The problem for the first line happens at parsing - meaning that javac and Eclipse do not use the same parser for Java source code. You should ask a question about what differences there are between Eclipse JDT's java parser and javac's. (Or post a bug at Eclipse).

If you insist on keeping that behaviour (I would suggest refactoring Inner to a static inner class), you could use @SuppressWarning with a field assignation (in order to restrict the @SuppressWarning to the smallest scope possible).

@SuppressWarnings({"rawtypes","unchecked"})
Outer<T>.Inner casted = (Outer.Inner)o;
return casted;

EDIT: OK, I believe I got it - Eclipse's JDT parses the Java code before passing it to the compiler - and their parser can make sense of such a cast, while (at least your and my version of) javac's cannot. (And after that, Eclipse directly passes the parsed code to compilation). Before filing a bug, look at how the last version of Java 6 behaves.