Question

我正在尝试实现一个Java Stop Watch，其中我使用了一个Thread来连续监视时间。但我在我的代码中遇到了问题。我的Java代码是 -

  import java.awt.FlowLayout;
  import java.awt.GridBagConstraints;
  import java.awt.GridBagLayout;
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  import java.util.Calendar;

  import javax.swing.*;

  public class Prac2_StopWatch implements ActionListener{
      JFrame Time;
      JLabel TimeLabel;
      JButton StartStop;
      JButton Reset;
      StopWatch stThread;
      String prefix="<html><h1><body>";
      String suffix="</h1></body></html>";
      StopWatch myControl;

      Prac2_StopWatch()
     {
         Time = new JFrame();
         Time.setSize(275,275);
         Time.setTitle("Stop Watch");
         Time.setLayout(new GridBagLayout());
         GridBagConstraints gridConst = new GridBagConstraints();

         TimeLabel = new JLabel();
         TimeLabel.setText(prefix+"0:0:0"+suffix);

         myControl = new StopWatch(TimeLabel);
         myControl.start();

         StartStop = new JButton();
         Reset = new JButton();

         StartStop.setActionCommand("Start");
         StartStop.setText("Start");
         Reset.setText("Reset");

         StartStop.addActionListener(this);
         Reset.addActionListener(this);

         gridConst.gridx=0;
         gridConst.gridy=0;
         Time.add(StartStop,gridConst);

         gridConst.gridx=1;
         gridConst.gridy=0;
         Time.add(Reset,gridConst);

         gridConst.gridx=0;
         gridConst.gridy=1;
         Time.add(TimeLabel,gridConst);

         Time.setVisible(true);



      }
      public void actionPerformed(ActionEvent evt)
     {
         if(evt.getActionCommand()=="Start")
         {
            StartStop.setActionCommand("Stop");
            StartStop.setText("Stop");
            if(myControl.curMil==-1)
                 myControl.curMil=Calendar.getInstance().getTimeInMillis();
            else 
                myControl.curMil=Calendar.getInstance().getTimeInMillis()-      myControl.diff;
                myControl.count=true;

             }
            else if(evt.getActionCommand()=="Stop")
           {
              StartStop.setActionCommand("Start");
              StartStop.setText("Start");
              myControl.count=false;

           }
           else if(evt.getActionCommand()=="Reset")
           {
               StartStop.setActionCommand("Start");
               StartStop.setText("Start");
               myControl.count=false;
               myControl.curMil=-1;
               TimeLabel.setText(prefix+"0:0:0"+suffix);

           }
        }

        public static void main(String args[])
        {
             SwingUtilities.invokeLater(new Runnable(){
                public void run()
                {
                   new Prac2_StopWatch();
                }
             });

        }
   }

   class StopWatch extends Thread
   {
        long curMil;
        long diff;
        JLabel TimeLabel;
        boolean count;
        String prefix="<html><body><h1>";
        String suffix="</h1></body></html>";

        StopWatch(JLabel TimeLabel)
        {
           this.TimeLabel = TimeLabel;
           this.count=false;
           this.curMil=-1;
         }
       public void run()
       {
           while(true){
           System.out.print("Commenting this line will make stop watch useless");
           if(count){
                diff= Calendar.getInstance().getTimeInMillis() - curMil;
                TimeLabel.setText(prefix+((diff/60000)%60)+" : "+((diff/1000)%60)+" : "+(diff%1000)+suffix);
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                   e.printStackTrace();
              }
           }
        }
     }
 }

现在的问题是，当我对语句进行注释时，在StopWatch类的run方法中 -

System.out.println("Commenting this line will make stop watch useless");

第123行

我的秒表停止工作，即时钟总是显示＆＃34; 0：0：0＆＃34;即使按下“开始”，“停止＆＃39;或者＆＃39;重置＆＃39;。此外，将行123替换为除了System.out.println（）＆＃39;以外的任何其他语句。有同样的问题。

Answer 1

我看到两个主要问题：

~~你有一个紧凑的循环，一个暂停1毫秒会占用处理器~~ [编辑：这实际上没问题]
您正在进行Swing调用，此处setText(...)来自您的后台线程，这是您不应该做的事情，并且通过紧密循环可以完全冻结您的Swing GUI。
您在线程之间共享count布尔变量，因此必须将其声明为volatile，以便其状态在所有线程中都相同。

如果需要在后台线程中进行Swing状态更改，请使用SwingWorker并使用发布/进程对进行更改。

另外，请勿使用==或!=比较字符串。请改用equals(...)或equalsIgnoreCase(...)方法。了解==检查两个对象引用是否相同而不是您感兴趣的内容。另一方面，这些方法检查两个字符串是否具有相同的字符以相同的顺序，这是重要的。而不是

else if (evt.getActionCommand() == "Stop") {

DO

else if (evt.getActionCommand().equalsIgnoreCase("Stop")) {

如，

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Calendar;
import javax.swing.*;

public class Prac2_StopWatch implements ActionListener {
    JFrame time;
    JLabel timeLabel;
    JButton startStop;
    JButton reset;
    StopWatch stThread;
    String prefix = "<html><h1><body>";
    String suffix = "</h1></body></html>";
    StopWatch myControl;

    Prac2_StopWatch() {
        time = new JFrame();

        time.setSize(275, 275);
        time.setTitle("Stop Watch");
        time.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        time.setLayout(new GridBagLayout());
        GridBagConstraints gridConst = new GridBagConstraints();

        timeLabel = new JLabel();
        timeLabel.setText(prefix + "00:00:000" + suffix);

        myControl = new StopWatch(timeLabel);
        new Thread(myControl).start(); // StopWatch should implement Runnable
        // myControl.start();

        startStop = new JButton();
        reset = new JButton();

        startStop.setActionCommand("Start");
        startStop.setText("Start");
        reset.setText("Reset");

        startStop.addActionListener(this);
        reset.addActionListener(this);

        gridConst.gridx = 0;
        gridConst.gridy = 0;
        time.add(startStop, gridConst);

        gridConst.gridx = 1;
        gridConst.gridy = 0;
        time.add(reset, gridConst);

        gridConst.gridx = 0;
        gridConst.gridy = 1;
        time.add(timeLabel, gridConst);

        time.setVisible(true);
    }

    public void actionPerformed(ActionEvent evt) {
        if (evt.getActionCommand().equalsIgnoreCase("Start")) {
            startStop.setActionCommand("Stop");
            startStop.setText("Stop");
            if (myControl.getCurMil() == -1)
                myControl.setCurMil(Calendar.getInstance().getTimeInMillis());
            else
                myControl.setCurMil(Calendar.getInstance().getTimeInMillis() - myControl.getDiff());
            myControl.count = true;

        } else if (evt.getActionCommand().equalsIgnoreCase("Stop")) {
            startStop.setActionCommand("Start");
            startStop.setText("Start");
            myControl.count = false;

        } else if (evt.getActionCommand().equalsIgnoreCase("Reset")) {
            startStop.setActionCommand("Start");
            startStop.setText("Start");
            myControl.count = false;
            myControl.setCurMil(-1);
            timeLabel.setText(prefix + "00:00:000" + suffix);
        }
    }

    public static void main(String args[]) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Prac2_StopWatch();
            }
        });

    }
}

class StopWatch implements Runnable {
    private long curMil;
    private long diff;
    private JLabel timeLabel;

    // *************** this is key ****************
    volatile boolean count; // !! *********************

    String prefix = "<html><body><h1>";
    String suffix = "</h1></body></html>";

    StopWatch(JLabel TimeLabel) {
        this.timeLabel = TimeLabel;
        this.count = false;
        this.curMil = -1;
    }

    public long getCurMil() {
        return curMil;
    }

    public void setCurMil(long curMil) {
        this.curMil = curMil;
    }

    public long getDiff() {
        return diff;
    }

    public void run() {
        while (true) {
            // System.out.println("Commenting this line will make stop watch useless");
            if (count) {
                diff = Calendar.getInstance().getTimeInMillis() - curMil;

                // make Swing changes **on** the event thread only
                SwingUtilities.invokeLater(() -> {
                    int mSec = (int) (diff % 1000);
                    int sec = (int) ((diff / 1000) % 60);
                    int min = (int) ((diff / (60 * 1000)) % 60);
                    String text = String.format("%s%02d:%02d:%03d%s", prefix, min, sec, mSec, suffix);
                    timeLabel.setText(text);
                });
                try {
                    Thread.sleep(1);  // ** actually 1 is OK **
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Answer 2

移动

if(count){
  diff= Calendar.getInstance().getTimeInMillis() - curMil;
  TimeLabel.setText(prefix+((diff/60000)%60)+":"+((diff/1000)%60)+" : "+(diff%1000)+suffix);
}

在try-catch区块内。

附注：

JFrame窗口关闭时，您的程序不会停止。使用
```
Time.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
```

在java中比较字符串的正确方法是

if("string1".equals("string2")) {
    //do stuff
}

Java Thread的未知行为

2 个答案: