什么是Java中的回调接口？

What is a call back interface in Java?

接口SETSAVER的这个代码片段是从有效的Java中获取的(避免过多的同步项67)

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public interface SetObserver<E> {
// Invoked when an element is added to the observable set
void added(ObservableSet<E> set, E element);
}

将SetObserver传递给addObserver()和removeObserver方法，如下所示：

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// Broken - invokes alien method from synchronized block!
public class ObservableSet<E> extends ForwardingSet<E> {
public ObservableSet(Set<E> set) {
super(set);
}

private final List<SetObserver<E>> observers =
new ArrayList<SetObserver<E>>();

public void addObserver(SetObserver<E> observer) {
synchronized (observers) {
observers.add(observer);
}
}

public boolean removeObserver(SetObserver<E> observer) {
synchronized (observers) {
return observers.remove(observer);
}
}

private void notifyElementAdded(E element) {
synchronized (observers) {
for (SetObserver<E> observer : observers)
observer.added(this, element);
}
}

bloch将SetObserver接口称为回调接口。Java中的回调接口是什么时候？