当X扩展Y时，javax不适用于参数Y

Question

事件处理程序。java:

public abstract class EventHandler<E> {
    public abstract void HandleEvent(Object sender, E e);
}

Foo。java:

private OnProgressEventHandler m_OnProgressEventHandler = new OnProgressEventHandler();
private class OnProgressEventHandler extends EventHandler<ProgressEventArgs> {
    public void HandleEvent(Object sender, ProgressEventArgs args) {
        /*...*/
    }
}

ProgressEventArgs。java:

public class ProgressEventArgs extends EventArgs {
    /*...*/
}

条。java:

public void addProgressEventListener(EventHandler<EventArgs> listener) {
    m_OnProgress.addListener(listener);
}

这一行：

myBarObject.addProgressEventListener(m_OnProgressEventHandler); 

原因：

The method addProgressEventListener(EventHandler) in the type Bar is not applicable for the arguments (Foo.OnProgressEventHandler)

我不明白它为什么不编译，因为OnProgressEventHandler扩展了EventHandler<ProgressEventArgs>。我肯定我遗漏了一些琐碎的东西，但我不明白为什么

Answer 1

改变

public void addProgressEventListener(EventHandler<EventArgs> listener) {
    m_OnProgress.addListener(listener);
}

到

public void addProgressEventListener(EventHandler<? extends EventArgs> listener) {
    m_OnProgress.addListener(listener);
}