java.lang.NullPointerException | java.lang.NullPointerException我的时钟不工作

Question

我和我的一个 friend 必须为大学用 Java 编写一个游戏。我正在尝试在我们创建的工具栏中实现一个小时钟。当我运行代码时，出现此错误:

Exception in thread "Thread-0" java.lang.NullPointerException at GameTimer.run(GameTimer.java:29)

KodeKsToolBar.java

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.DateFormat;

import javax.swing.*;

public class KodeKsToolBar extends JToolBar{
    private static final long serialVersionUID = 1L;
    public static JLabel timeLabel;

    public KodeKsToolBar(GUI listener, Dimension size){
        //unimported Stuff hidden
        //...   
        JLabel timeLabel = new JLabel(GameTimer.currentTime);
        GameTimer t = new GameTimer();
        t.start();
        //clock = new Clock(this);
        //clock.start();

        setFloatable(false);
        add(ToolBarItem_NGame);
        add(ToolBarItem_Load);
        add(ToolBarItem_Save);
        add(ToolBarItem_Resign);
        add(toolBarItem_PauseResume);
        add(timeLabel);
    }
}

GameTimer.java:

import java.awt.*;
import java.awt.event.*;
import java.text.DateFormat;
import java.util.Date;



public class GameTimer extends Thread {
    static boolean running = true;
    int milliSeconds = 0;
    int seconds = 0;
    int minutes = 0;
    static String currentTime = "00:00";

    public void run() {
        while(running) {
            milliSeconds++;
            if(milliSeconds > 999){
                milliSeconds = 0;
                seconds++;
            }
            if(seconds > 59){
                seconds = 0;
                minutes++;
            }
            KodeKsToolBar.timeLabel.setText(getTime()); // <-- This is the line mentioned above, which causes the error
            try {
                Thread.sleep(10);
            } catch (InterruptedException e2) {
                e2.printStackTrace();
            }
        }
        try {
            Thread.sleep(10);
        } catch (InterruptedException e2) {
            e2.printStackTrace();
        }
    }

    public String getTime() {
        currentTime = minutes + ":" + seconds;
        return currentTime;
    }
}

Answer 1

您正在访问静态变量

KodeKsToolBar.timeLabel

但在 KodeKsToolBar 构造函数中初始化同名的局部变量

JLabel timeLabel = new JLabel(GameTimer.currentTime);

删除JLabel以保留。-

timeLabel = new JLabel(GameTimer.currentTime);

PS:正如 @Overcraft Full of Eels 所说，请记住，除了常量(声明为 final 的常量)之外，实例变量应该是非静态的。