Java Swing中的空指针异常[关闭]

Posted 2023-03-27

【中文标题】Java Swing中的空指针异常[关闭]

【英文标题】：Null Pointer Exception in Java Swing [closed]

【发布时间】：2020-06-21 15:23:13

【问题描述】：

每当我尝试使用我在下面的代码中写入的任何按钮时，我都会不断收到此错误消息。奇怪的是我正在查看一段类似的代码，它并没有出现在那里，但我无法区分。

代码如下：

主窗口类

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class FinalPaintWindow extends JFrame 
    public static final int CANVAS_WIDTH = 640;
    public static final int CANVAS_HEIGHT = 480;

    ***ActionListener actionListener = new ActionListener() 
        FinalDrawSpace finalDrawSpace = new FinalDrawSpace();
        public void actionPerformed(ActionEvent e) 
            if (e.getSource() == clearBtn)
                finalDrawSpace.clear();
             else if (e.getSource() == blackBtn) 
                finalDrawSpace.black();
             else if (e.getSource() == rectangleBtn) 
                finalDrawSpace.rectangle();
            
        
    ;***

    private DrawCanvas FinalDrawSpace;
    JButton clearBtn, blackBtn, rectangleBtn;


    public FinalPaintWindow() 
        FinalDrawSpace = new DrawCanvas();
        FinalDrawSpace.setPreferredSize(new Dimension(CANVAS_WIDTH, CANVAS_HEIGHT));
        getContentPane().addComponentListener(new ComponentAdapter() 
            public void componentResized(ComponentEvent e) 
                Component c = (Component)e.getSource();
            
        );

        Container cp = getContentPane();
        cp.add(FinalDrawSpace, BorderLayout.CENTER);
        JPanel controls = new JPanel();

        clearBtn = new JButton("Clear");
        clearBtn.addActionListener(actionListener);
        blackBtn = new JButton("Black");
        blackBtn.addActionListener(actionListener);
        rectangleBtn = new JButton("Rectangle");
        rectangleBtn.addActionListener(actionListener);

        controls.add(clearBtn);
        controls.add(blackBtn);
        controls.add(rectangleBtn);

        cp.add(controls, BorderLayout.EAST);

        setDefaultCloseOperation(EXIT_ON_CLOSE);
        pack();
        setTitle("Final Paint");
        setVisible(true);
    

    private class DrawCanvas extends JPanel 
        @Override
        public void paintComponent(Graphics g) 
            super.paintComponent(g);
            setBackground(Color.WHITE);
        
    

    public static void main (String[] args) 
        SwingUtilities.invokeLater(new Runnable() 
            @Override
            public void run() 
                new FinalPaintWindow();
            
        );
    

图形类

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class FinalDrawSpace extends JComponent 

    public Image image;
    private Graphics2D graphics;
    private int newX;
    private int newY;
    private int oldX;
    private int oldY;

    public FinalDrawSpace() 
        setDoubleBuffered(false);
        addMouseListener(new MouseAdapter() 
            public void mousePressed(MouseEvent e) 
                oldX = e.getX();
                oldY = e.getY();
            
        );

        addMouseMotionListener(new MouseMotionAdapter() 
            public void mouseDragged(MouseEvent e) 
                newX = e.getX();
                newY = e.getY();

                if (graphics != null) 
                    graphics.drawLine(oldX, oldY, newX, newY);
                
            
        );
    

    protected void paintComponent(Graphics g) 
        if (image == null) 
            image = createImage(getSize().width, getSize().height);
            graphics = (Graphics2D) image.getGraphics();
            graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            clear();
        
        g.drawImage(image, 0, 0, null);
    

    ***public void clear() 
        graphics.setPaint(Color.white);
        graphics.fillRect(0, 0, getSize().width, getSize().height);
        graphics.setPaint(Color.black);
        repaint();
    

    public void rectangle() 
        graphics.drawRect(oldX, oldY, newX, newY);
    

    public void black()
        graphics.setPaint(Color.black);
    ***

带星号的区域是抛出 NullPointerException 的区域，我只是想不出办法解决它。有人能帮忙吗？

【问题讨论】：

你需要添加异常。

尽快发布堆栈跟踪以获得更好的帮助。

【参考方案1】：

***ActionListener actionListener = new ActionListener() 
    FinalDrawSpace finalDrawSpace = new FinalDrawSpace();

问题在于，在 ActionListener 中，您总是会创建 FinalDrawSpace 类的新实例。由于您直接调用 clear() 方法，因此不会初始化“图形”变量，因为从不调用 paintComponent() 方法。

摆脱：

//FinalDrawSpace finalDrawSpace = new FinalDrawSpace();

现在 ActionListener 代码将引用 FinalPaintWindow 类的“finalDrawSapce”变量。

还有：

private DrawCanvas FinalDrawSpace;

应该是：

private drawCanvas finalDrawSpace;

因为变量名不应以大写字符开头。