Remove warning: Set Listener ? element Listener T usage

Question

Hello,
I have a class using listeners for diffent classes. I don't know how to remove the unchecked warning.
The class stores Listener for differen classes. When called with an object of a registered class the method doIt from the registered XListener should be called with the object.
Here comes the code:
public class XSupport {
    private Set<Listener<?>> listeners = new HashSet<Listener<?>>();
    public <T> void addXListener(Class<T> clazz, XListener<T> xListener) {
        listeners.add(new Listener<T>(clazz, xListener));
    @SuppressWarnings("unchecked")
    public <T> void doIt(T data) {
        Class<?> dataClass  = data.getClass();
        for (Listener listener : listeners)
            if (dataClass.equals(listener.getClazz()))
                listener.getXListener().doIt(data);
    private class Listener<T> {
        private Class<T> clazz;
        private XListener<T> xListener;
        public Listener(Class<T> clazz, XListener<T> xListener) {
            this.clazz = clazz;
            this.xListener = xListener;
        public Class<T> getClazz() {
            return clazz;
        public XListener<T> getXListener() {
            return xListener;
}Rem.: Listener also has equals and hashCode which I did not copy here
The XListener interface is definied like this:
public interface XListener<T> {
    public void doIt(T data);
}I suppose the for loop with raw Listener is the problem, but I can't use Listener<?> neither Listener<T> there.

Answer 1

Kurt76 wrote:
I have a class using listeners for diffent classes. I don't know how to remove the unchecked warning.Your code "anonymizes" the generic type of incoming listeners by adding them to a wildcarded Set. What you would need here is to "re-set" the type information by time of usage, which you cannot do reflectively.
I'd suggest a small redesign as follows:
public class XSupport {
    private Set<Listener<?>> listeners = new HashSet<Listener<?>>();
    public <T> void addXListener(Class<T> clazz, XListener<T> xListener) {
        listeners.add(new Listener<T>(clazz, xListener));
    public void doIt(Object data) {
        for (Listener<?> listener : listeners) {
            listener.visit(data);
    private class Listener<T> {
        private Class<T> clazz;
        private XListener<T> xListener;
        public Listener(Class<T> clazz, XListener<T> xListener) {
            this.clazz = clazz;
            this.xListener = xListener;
        public void visit(Object data) {
            if (clazz.isInstance(data)) {
             xListener.doIt(clazz.cast(data));
}Edit: Just as I missed that one: was the use of "equals" deliberate? If so, you have to change the visit() method implementation to check on "clazz.equals(data.getClass())". An equal on classes will only match if both are the same class (uses object identity). That is, you only can use concrete classes for adding listeners, no interfaces or abstract classes, nor make use of class hierarchies.
Edited by: stefan.schulz on Oct 13, 2007 11:15 AM