java - 尝试启动摆动计时器时出现空指针异常错误？

Question

当我尝试使用 timer.start(); 启动摇摆计时器时；它只是提出了一个我不知道如何修复的空指针异常错误。我叫错了吗？我见过它的例子有 timer.start(); 在实际的计时器方法中，但我不知道将它放在我的代码中的哪个位置。任何帮助，将不胜感激！！！！谢谢你。

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.Scanner;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class Board extends JPanel {
    private Timer timer;
    @Override
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.fillRect(30, 30, 640, 640);//makes a black square
        for(int i=30;i<=510; i+=160)//adds white columns
        {
            for(int j=30; j<=510; j+=160)
            {
                g.clearRect(i,  j,  80,  80);
            }
        }
        for(int i=110; i<=590; i+=160)//adds black columns
        {
            for(int j=110; j<=590; j+=160)
            {
                g.clearRect(i, j, 80, 80);
            }
        }
        g.setFont(new Font("Monospace", Font.BOLD, 30));
        g.setColor(Color.WHITE);
        g.drawString("a", 85, 660);
        g.drawString("c", 245, 660);
        g.drawString("e", 405, 660);
        g.drawString("g", 565, 660);
        g.drawString("7", 35, 140);
        g.drawString("5", 35, 300);
        g.drawString("3", 35, 460);
        g.drawString("1", 35, 620);
        
        g.setColor(Color.BLACK);
        g.drawString("b", 165, 660);
        g.drawString("d", 325, 660);
        g.drawString("f", 485, 660);
        g.drawString("h", 645, 660);
        g.drawString("8", 35, 60);
        g.drawString("6", 35, 220);
        g.drawString("4", 35, 380);
        g.drawString("2", 35, 540);
        timer.start();
    }
    public void showX(Graphics g)
    {
        g.setFont(new Font("wrongFont", Font.BOLD, 200)); 
        g.setColor(Color.RED);
        g.drawString("X", 35, 540);
    }
    public void boardImage()
    {
        JFrame frame=new JFrame();
        frame.setSize(600, 600);
        frame.getContentPane().add(new Board());
        frame.setLocationRelativeTo(null);
        frame.setBackground(Color.LIGHT_GRAY);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        CoordinateGame game=new CoordinateGame();
        frame.addMouseListener(new MouseAdapter() 
        {
            public void mouseClicked(MouseEvent e) 
            {
                int x=e.getX();
                int y=e.getY();
                    
            }
        }); 
    }
    public static void Timer()
    {
        class TimerListener implements ActionListener
        {
            int elapsedSeconds = 30;
            @Override
            public void actionPerformed(ActionEvent evt)
            {
                Timer timer = new Timer(1000, new TimerListener());
                elapsedSeconds--;
                System.out.println(":(");
                if(elapsedSeconds <= 0){
                    timer.stop();
                }
            }
        }
    }
    
}

标签： javaswingtimer