java - 令人惊讶的是，removeChangeListener(null) 不会抛出 NullPointerException

Question

public class Test {

    public static void main(String[] args) {
        try{
        JTabbedPane tab = new JTabbedPane();
        tab.removeChangeListener(null);
        }catch(Exception e){
            e.printStackTrace();
        }
    }

}

这不会导致 NullPointerException。

当我调用 tab.removeChangeListener(null) 时到底会发生什么？

Answer 1

what exactly happens when I call tab.removeChangeListener(null)?

正是这个:

在JTabbedPane中调用此方法:

public void removeChangeListener(ChangeListener l) {
    listenerList.remove(ChangeListener.class, l);
}

其中listenerList被声明为protected EventListenerLiSTListenerList = new EventListenerList();

删除方法是:

public synchronized <T extends EventListener> void remove(Class<T> t, T l) {
    if (l ==null) {
        // In an ideal world, we would do an assertion here
        // to help developers know they are probably doing
        // something wrong
        return;
    }
    ...
    ...

因此，删除 null 只是返回而不影响监听器