Java compiler rejects variable declaration with parameterized inner class

Question

I have some Groovy code which works fine in the Groovy bytecode compiler, but the Java stub generated by it causes an error in the Java compiler. I think this is probably yet another bug in the Groovy stub generator, but I really can't figure out why the Java compiler doesn't like the generated code.

Here's a truncated version of the generated Java class (please excuse the ugly formatting):

@groovy.util.logging.Log4j() public abstract class AbstractProcessingQueue
<T>  extends nz.ac.auckland.digitizer.AbstractAgent  implements
    groovy.lang.GroovyObject {
        protected int retryFrequency;
        protected java.util.Queue<nz.ac.auckland.digitizer.AbstractProcessingQueue.ProcessingQueueMember<T>> items;
        public AbstractProcessingQueue
        (int processFrequency, int timeout, int retryFrequency) {
             super ((int)0, (int)0);
    }

    private enum ProcessState
      implements
    groovy.lang.GroovyObject {
        NEW, FAILED, FINISHED;
    }

    private class ProcessingQueueMember<E>  extends java.lang.Object  implements
    groovy.lang.GroovyObject {
        public ProcessingQueueMember
        (E object) {}
    }
}

The offending line in the generated code is this:

protected java.util.Queue<nz.ac.auckland.digitizer.AbstractProcessingQueue.ProcessingQueueMember<T>> items;

which produces the following compile error:

[ERROR] C:\Documents and Settings\Administrator\digitizer\target\generated-sources\groovy-stubs\main\nz\ac\auckland\digitizer\AbstractProcessingQueue.java:[14,96] error: improperly formed type, type arguments given on a raw type

The column index of 96 in the compile error points to the <T> parameterization of the ProcessingQueueMember type. But ProcessingQueueMember is not a raw type as the compiler claims, it is a generic type:

private class ProcessingQueueMember
<E>  extends java.lang.Object  implements
groovy.lang.GroovyObject { ...

I am very confused as to why the compiler thinks that the type Queue<ProcessingQueueMember<T>> is invalid. The Groovy source compiles fine, and the generated Java code looks perfectly correct to me too. What am I missing here? Is it something to do with the fact that the type in question is a nested class?

(in case anyone is interested, I have filed this bug report relating to the issue in this question)

Edit: Turns out this was indeed a stub compiler bug- this issue is now fixed in 1.8.9, 2.0.4 and 2.1, so if you're still having this issue just upgrade to one of those versions. :)

Answer 1

Since the ProcessingQueueMember class is a non-static inner class of AbstractProcessingQueue, its body can refer to the T type parameter of AbstractProcessingQueue. Thus, a non-raw reference to the ProcessingQueueMember must supply both the type arguments. For example,

protected java.util.Queue<AbstractProcessingQueue<T>.ProcessingQueueMember<T>> items;

will compile. This code is probably excessively generic. I believe you actually want one of these two alternatives:

1. Make ProcessingQueueMember<E> static (i.e. a nested class as opposed to an inner class)
2. Remove the type parameter from ProcessingQueueMember

I do not know anything about the groovy stub generator, but maybe there is some way to annotate your groovy code to express this?