我正在尝试为我之前制作的游戏制作一个GUI,并且在运行时遇到了一个小问题。
我希望将输出打印到扩展JPanel中的JTextField。但是,当我运行它时,它会出现这个错误:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Classic.print(Classic.java:509)
at Classic.play(Classic.java:43)
at Karma.actionPerformed(Karma.java:134)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
以下是适用的代码:
(我将跳过Karma.java:134,因为这只是播放按钮。)
Classic.java
import static javax.swing.SwingUtilities.invokeLater; //This is to show where invokeLater comes from
...
public class Classic extends Game {
private static JFrame gui;
private static GUIClassic newContentPane;
...
public void play() {
invokeLater(Classic::startGUI);
//The next line is "Classic.java:43
//length, difficulty, and log are all strings that were initialized when the game was instantiated
print("Selected Options:\nLength: " + length + "\nDifficulty: " + difficulty + "\nOutput Log? " + log + "\n");
...
}
private static void startGUI() {
gui = new JFrame("Karma :: Classic Mode");
gui.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
newContentPane = new GUIClassic();
newContentPane.setOpaque(true);
gui.setContentPane(newContentPane);
gui.pack();
gui.setVisible(true);
}
private static void print(String text) {
newContentPane.appendOutput(text);
}
GUIClassic.java(用于内容窗格的类) [编辑:这是上下文的完整类内容。]
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class GUIClassic extends JPanel implements ActionListener {
private JTextArea output;
private JTextField input;
private boolean inputReady;
private String inputText;
public GUIClassic() {
super();
setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
output = new JTextArea(15, 15);
output.setEditable(false);
JScrollPane outputScroll = new JScrollPane(output);
input = new JTextField("",40);
add(outputScroll);
add(Box.createRigidArea(new Dimension(0,5)));
add(input);
}
public void actionPerformed(ActionEvent e) {
inputText = input.getText();
inputReady = true;
input.setText("");
}
public boolean isInputReady() {
return inputReady;
}
public String getInput() {
if (!isInputReady())
return null;
inputReady = false;
return inputText;
}
public void appendOutput(String addition) {
output.append(addition + "\n");
}
}
有趣的是,GUI会在异常发生后弹出并保持运行状态。它只是不会将输出打印到JTextArea并卡住。
如果您需要更多上下文代码,请告诉我,我会添加它。
编辑:有人指出某些字段可能未初始化。我相应地解决了这些问我试图找出导致异常的确切原因,但失败了。它不应该是新的内容,而不是新的内容。或者'输出'因为这些都会在程序启动时出现在屏幕上。
答案 0 :(得分:2)
您在创建JTextArea之前调用print。 invokeLater在打印调用之前运行。最好的修复方法(IMO)是在Runnable.run方法的末尾打印输出语句,该方法首先调用Classic.startGUI,通过invokeLater启动,而不是单独调用Classic :: startGUI构造函数
invokeLater(Classic::startGUI);
//The next line is "Classic.java:43
//length, difficulty, and log are all strings that were initialized when the game was instantiated
print("Selected Options:\nLength: " + length + "\nDifficulty: " + difficulty + "\nOutput Log? " + log + "\n");
答案 1 :(得分:1)
检查一下,这是你的错误:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at Classic.print(Classic.java:509)
这是您的Classic#print
private static void print(String text) {
newContentPane.appendOutput(text);
}
在这里,唯一会抛出一个场景
NullPointerException
是newContentPane
等于null
。
是的,您可能有一些初始化newContentPane
的代码,但在您致电Classic#print
之前没有及时发生。
那么,为什么不及时发生呢?就像每个人都试图告诉你的那样,它与使用invokeLater
。