我想创建一个5秒后开始计时的秒表。没有计时器，它可以完美地工作，但是当我将TaskStopwatch（）方法放在Task类中，该方法负责在时间过去之后执行任务时，我得到一个错误。如果我对自己在做错的事情有任何解释，或者有更好的解决办法，我将感到高兴。这是我的代码。
import java.lang.reflect.InvocationTargetException;
import java.util.Timer;
import java.util.TimerTask;
import javax.swing.SwingUtilities;
public class Stopwatch extends javax.swing.JFrame implements Runnable{
private javax.swing.JLabel Display;
private javax.swing.JButton StartButton;
private javax.swing.JButton StopButton;
private final static java.text.SimpleDateFormat timerFormat = new java.text.SimpleDateFormat("mm : ss.SSS");
private long elapsed;
private long startTime;
private Thread updater;
private boolean isRunning= false;
private Timer timer;
public Stopwatch() {
initComponents();
}
private void initComponents() {
Display = new javax.swing.JLabel();
StartButton = new javax.swing.JButton();
StopButton = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
Display.setText("00 : 00.000");
StartButton.setText("Start");
StartButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
StartButtonActionPerformed(evt);
}
});
StopButton.setText("Stop");
StopButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
StopButtonActionPerformed(evt);
}
});
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(131, 131, 131)
.addComponent(StartButton)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
.addComponent(StopButton))
.addGroup(layout.createSequentialGroup()
.addGap(165, 165, 165)
.addComponent(Display)))
.addContainerGap(147, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(124, 124, 124)
.addComponent(Display)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(StartButton)
.addComponent(StopButton))
.addContainerGap(133, Short.MAX_VALUE))
);
pack();
setLocationRelativeTo(null);
}
private void StopButtonActionPerformed(java.awt.event.ActionEvent evt) {
stopStopwatch();
}
private void StartButtonActionPerformed(java.awt.event.ActionEvent evt) {
//startStopwatch();
new Stopwatch(5);
}
public static void main(String args[]) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new Stopwatch().setVisible(true);
}
});
}
//STOPWATCH
@Override
public void run() {
try{
while(isRunning){
SwingUtilities.invokeAndWait(displayUpdater);
Thread.sleep(10);
}
}catch(InterruptedException e){
e.printStackTrace();
} catch (InvocationTargetException ex) {
ex.printStackTrace();
}
}
private final Runnable displayUpdater= new Runnable() {
public void run(){
displayElapsedTime(System.currentTimeMillis() - Stopwatch.this.startTime);
}
};
private void displayElapsedTime (long elapsedTime1){
Display.setText(timerFormat.format(new java.util.Date(elapsedTime1)));
}
public void startStopwatch(){
System.out.println("2:Start stopwatch.");
startTime= System.currentTimeMillis();
isRunning = true;
updater = new Thread(this);
updater.start();
}
private void stopStopwatch(){
System.out.println("3:Stop stopwatch.");
elapsed = System.currentTimeMillis() - startTime;
isRunning= false;
try{
updater.join();
}catch(InterruptedException ie){
ie.printStackTrace();
}
displayElapsedTime(elapsed);
}
//TIMER
public Stopwatch(int seconds){
timer = new Timer();
timer.schedule(new Task(), seconds * 1000);
}
class Task extends TimerTask{
@Override
public void run() {
System.out.println("1:Timer has finished.");
timer.cancel();
startStopwatch();
}
}
}
这里是错误
java.lang.reflect.InvocationTargetException
at java.awt.EventQueue.invokeAndWait(Unknown Source)
at java.awt.EventQueue.invokeAndWait(Unknown Source)
at javax.swing.SwingUtilities.invokeAndWait(Unknown Source)
at test.Stopwatch.run(Stopwatch.java:105)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at test.Stopwatch.displayElapsedTime(Stopwatch.java:122)
at test.Stopwatch.access$1(Stopwatch.java:121)
at test.Stopwatch$1.run(Stopwatch.java:117)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
当您使用秒表的构造函数作为计时器时，您的Display元素为空。由于某种原因，您创建了一个新的秒表并且不对其进行初始化。因此，您要更改的Display元素为null。
//TIMER
public StopWatch(int seconds) {
timer = new Timer();
timer.schedule(new Task(), seconds * 1000);
}
构造函数应包含initComponents（），以免出现nullpointer异常。但是我不确定它在功能上是否可以满足您的要求。
//TIMER
public StopWatch(int seconds) {
initComponents();
timer = new Timer();
timer.schedule(new Task(), seconds * 1000);
}
这样的结果是您将不再看到错误。但是在原始屏幕上也不会发生任何事情。如果要查看正在发生的事情，则需要使其可见。因此它应该看起来像这样。
//TIMER
public StopWatch(int seconds) {
initComponents();
setVisible(true);
timer = new Timer();
timer.schedule(new Task(), seconds * 1000);
}
这会使新窗口可见，所以要使用单独的秒表。您可能想要或不想要哪个？
堆栈跟踪显示方法displayElapsedTime中存在NullPointerException。这是一种单行方法，因此查找问题应该不那么困难。添加Display和timerFormat的调试打印。其中之一未正确初始化