我是使用Netbeans和PropertyChangeListener的新手，在Java Bean中实现PropertyChangeSupport时，我遇到了同样（对我来说）的奇怪行为。
因此，我有一个名为TTTCell的bean，我在其中初始化了一个PropertyChangeSupport变量，然后实现了维护属性更改侦听器列表的函数。
package tttboard;
import java.awt.Color;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.beans.PropertyVetoException;
import java.beans.VetoableChangeListener;
import java.beans.VetoableChangeSupport;
public class TTTCell extends javax.swing.JPanel {
public static enum State
{
INITIAL,
PLAYER_X,
PLAYER_O,
WON,
DISABLE
}
// Variables
private State state;
private PropertyChangeSupport propChange = new PropertyChangeSupport(this);
public TTTCell() {
initComponents();
state = State.INITIAL;
}
@Override
public void addPropertyChangeListener(PropertyChangeListener listener) {
propChange.addPropertyChangeListener(listener); // it depends on that line whether the erroroccurs or not
}
@Override
public void removePropertyChangeListener(PropertyChangeListener listener) {
propChange.removePropertyChangeListener(listener);
}我现在将这个bean放在另一个名为TTTBoard的bean中，它给了我以下错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at tttboard.TTTCell.addPropertyChangeListener(TTTCell.java:55)
at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installListeners(SynthPanelUI.java:83)
at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installUI(SynthPanelUI.java:63)
at java.desktop/javax.swing.JComponent.setUI(JComponent.java:685)
at java.desktop/javax.swing.JPanel.setUI(JPanel.java:150)
at java.desktop/javax.swing.JPanel.updateUI(JPanel.java:126)
at java.desktop/javax.swing.JPanel.<init>(JPanel.java:86)
at java.desktop/javax.swing.JPanel.<init>(JPanel.java:109)
at java.desktop/javax.swing.JPanel.<init>(JPanel.java:117)
at tttboard.TTTCell.<init>(TTTCell.java:38)
at tttboard.TTTBoard.initComponents(TTTBoard.java:89)
at tttboard.TTTBoard.<init>(TTTBoard.java:27)
at tttboard.TTTBoard$2.run(TTTBoard.java:203)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)对我来说，这没有任何意义-PropertyChangeSupport变量是示例化的，为什么它应该是一个空指针？
仅供参考，我使用的是Netbeans IDE 11.1和openjdk 11.0.11
1条答案
按热度按时间kyks70gy1#
一个类的非静态字段直到该类进入初始化/构造时才被初始化。超类构造总是首先发生。
如果查看堆栈跟踪，可以看到异常是由于超类构造而发生的：
必须先完成超类构造函数（即JPanel构造函数），然后才能运行TTTCell字段初始化与构造函数.
JPanel构造函数调用其updateUI方法，该方法最终调用addPropertyChangeListener：
但是TTTCell还没有机会运行它的初始化器，它不仅包括
TTTCell()构造函数，还包括字段的初始化，包括下面这个：由于在调用addPropertyChangeListener覆盖时该行尚未运行（因为JPanel构造函数尚未完成），因此
propChange仍为空。解决办法很简单：移除addPropertyChangeListener和removePropertyChangeListener的覆写。它们没有任何好处。Component类别已经提供了这些方法的工作实作。没有任何东西可以阻止您使用它们。