Why does this generic method need a cast on the return value?

"david.karr" <davidmichaelkarr@gmail.com>
Thu, 28 Jan 2010 15:04:48 -0800 (PST)
I'm constructing a generic method with a couple of type parameters,
and the method returns an instance of one of them. The function
parameters provided should allow the type parameters to be
determined. I've gotten all compile errors out of the generic method,
but I'm confused by what happens when I call the method. Eclipse
seems to think the method returns "Object", so it says it can't
convert the response without a cast.

The following is a very simplified version of my class, just to
demonstrate the problem. Note that I have two generic methods and
calls to them, one of them that doesn't display this symptom, and the
other that does. The cast on the return value of "submitRequest()" is
what I'm trying to get rid of.

package generics;

public class Generics {

    public Generics() {
        Bar bar;
        try {
            bar = doit(new Foo(), Bar.class);
        } catch (Exception ex) {

        GenericResponseInfo genericResponseInfo =
            // Why does this line need a cast?
            (GenericResponseInfo) submitRequest(new GenericRequestInfo
(), 1000,
                new ThingieFactory() {
            public Thingie<GenericRequestInfo> createElem() {
                return null;
            new ThingieFactory() {
                public Thingie<GenericResponseInfo> createElem() {
                    return null;


    private <A,B> B doit(A a, Class<B> bclass) throws
InstantiationException, IllegalAccessException {
        return null;

    private <REQINFO, RESPINFO> RESPINFO submitRequest
(REQINFO requestInfo,
            int readTimeout,
            ThingieFactory<REQINFO> requestFactory,
            ThingieFactory<RESPINFO> responseFactory) {

        Thingie<RESPINFO> responseInfoElem =

        RESPINFO result = responseInfoElem.getValue();

        return (result);

    private static interface ThingieFactory<T> {
        Thingie<T> createElem();

    private static class Thingie<T> {
        public T getValue() { return null; }

    private static class GenericRequestInfo {

    private static class GenericResponseInfo {

    public static class Foo {

    public static class Bar {

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