java - Swing GUI 中的空指针异常

Question

我正在使用 Swing 制作一个 GUI，它有几个功能:使用 Graphics2D、JInternalFrame、JDesktopFrame、AffineTransform 自定义图形。我认为我已经接近让一切正常工作，但我遇到了空指针异常。我在这里做错了什么？

这是我的独立示例:

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.awt.geom.AffineTransform;
import java.awt.geom.Ellipse2D;
import java.util.*;

public class MainPanel extends JFrame implements ActionListener{

    private static final double version = 1.0;
    private JDesktopPane desktop;
    public static RFInternalFrame frame;

    private java.util.List<Point> POINT_LIST = Arrays.asList(
            //Top Row
            new Point(50, 30),
            new Point(70, 30),
            new Point(90, 30),
            new Point(110, 30),
            new Point(130, 30),
            new Point(150, 30),
            new Point(170, 30),
            new Point(190, 30),
            new Point(210, 30),
            new Point(230, 30),

            //Circle of Radios
            new Point(140, 60),
            new Point(120, 80),
            new Point(100, 100),
            new Point(100, 120),
            new Point(120, 140),
            new Point(140, 160),
            new Point(160, 140),
            new Point(180, 120),
            new Point(180, 100),
            new Point(160, 80));

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

    private static void createAndShowGui() {
        JFrame frame = new MainPanel();
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.setLocationByPlatform(false);
        frame.setVisible(true);
    }

    public MainPanel() {
        super("MainPanel " + version);

        //Make the big window be indented 50 pixels from each edge
        //of the screen.
        int inset = 50;
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        setBounds(inset, inset,
                screenSize.width - inset * 7,
                screenSize.height - inset * 4);

        //Set up the GUI.
        desktop = new JDesktopPane(); //a specialized layered pane
        desktop.setBackground(Color.DARK_GRAY);

        createRFFrame(); //create first RFFrame
        createScenarioFrame(); //create ScenarioFrame

        setContentPane(desktop);
        setJMenuBar(createMenuBar());

        //Make dragging a little faster but perhaps uglier.
        desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);
    }

    protected JMenuBar createMenuBar() {
        JMenuBar menuBar = new JMenuBar();

        //Set up the lone menu.
        JMenu menu = new JMenu("File");
        menu.setMnemonic(KeyEvent.VK_D);
        menuBar.add(menu);

        //Set up the first menu item.
        JMenuItem menuItem = new JMenuItem("Add Panel");
        menuItem.setMnemonic(KeyEvent.VK_N);
        menuItem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_N, ActionEvent.ALT_MASK));
        menuItem.setActionCommand("new");
        menuItem.addActionListener(this);
        menu.add(menuItem);

        //Set up the second menu item.
        menuItem = new JMenuItem("Quit");
        menuItem.setMnemonic(KeyEvent.VK_Q);
        menuItem.setAccelerator(KeyStroke.getKeyStroke(
                KeyEvent.VK_Q, ActionEvent.ALT_MASK));
        menuItem.setActionCommand("quit");
        menuItem.addActionListener(this);
        menu.add(menuItem);

        return menuBar;
    }

    //React to menu selections.
    public void actionPerformed(ActionEvent e) {
        if ("new".equals(e.getActionCommand())) { //new
            createRFFrame();
        } else {
            //quit
            quit();
        }
    }

    /*
     * ActivateAllAction activates all radios on the panel, essentially changes the color
     * of each ellipse from INACTIVE to ACTIVE
     */
    private class ActivateAllAction extends AbstractAction {
        public ActivateAllAction(String name) {
            super(name);
            int mnemonic = (int) name.charAt(1);
            putValue(MNEMONIC_KEY, mnemonic);
        }

        /*
         * This will find the selected tab and extract the DrawEllipses instance from it
         * Then for the actionPerformed it will call activateAll() from DrawEllipses
         */
        @Override
        public void actionPerformed(ActionEvent e) {
            Component comp = desktop.getSelectedFrame();
            if (comp instanceof DrawEllipses){
                DrawEllipses desktop = (DrawEllipses) comp;
                desktop.activateAll();
            }
        }
    }

    /*
     * DeactivateAllAction deactivates all radios on the panel, essentially changes the color
     * of each ellipse from ACTIVE to INACTIVE
     */
    private class DeactivateAllAction extends AbstractAction {
        public DeactivateAllAction(String name) {
            super(name);
            int mnemonic = (int) name.charAt(0);
            putValue(MNEMONIC_KEY, mnemonic);
        }

        /*
         * This will find the selected tab and extract the DrawPanel2 instance from it
         * Then for the actionPerformed it will call activateAll() from DrawEllipses
         */
        @Override
        public void actionPerformed(ActionEvent e) {
            Component comp = desktop.getSelectedFrame();
            if (comp instanceof DrawEllipses){
                DrawEllipses desktop = (DrawEllipses) comp;
                desktop.deactivateAll();
            }
        }
    }

    /*
     * Define a JPanel that will hold the activate and deactivate all JButtons
     */
    protected JPanel btnPanel() {
        JPanel btnPanel = new JPanel();

        btnPanel.setBorder(BorderFactory.createLoweredSoftBevelBorder());

        //Set the layout of the frame to a grid bag layout
        btnPanel.setLayout(new GridBagLayout());

        //Creates constraints variable to hold values to be applied to each aspect of the layout
        GridBagConstraints c = new GridBagConstraints();

        //Column 1
        c.gridx = 0;
        btnPanel.add(new JButton(new ActivateAllAction("Activate All")));

        //Column 2
        c.gridx = 1;
        btnPanel.add(new JButton(new DeactivateAllAction("Deactivate All")));
        return btnPanel;
    }

    //not used currently
    protected JPanel drawPanel() {
        JPanel drawPanel = new JPanel();
        drawPanel.setBorder(BorderFactory.createLoweredSoftBevelBorder());
        DrawEllipses drawEllipses = new DrawEllipses(POINT_LIST);
        drawPanel.add(drawEllipses);

        return drawPanel;

    }

    //Create a new internal frame.
    protected void createRFFrame() {
        frame.setLayout(new BorderLayout());

        DrawEllipses drawEllipses = new DrawEllipses(POINT_LIST);
        frame.add(drawEllipses);
        frame.add(btnPanel(), BorderLayout.SOUTH);

        frame.setVisible(true);
        desktop.add(frame);

        try {
            frame.setSelected(true);
        } catch (java.beans.PropertyVetoException e) {}
    }

    protected void createScenarioFrame() {
        ScenarioInternalFrame frame = new ScenarioInternalFrame();
        frame.setLayout(new BorderLayout());

        frame.setVisible(true);
        desktop.add(frame);

        try {
            frame.setSelected(true);
        } catch (java.beans.PropertyVetoException e) {}
    }

    //Quit the application.
    protected void quit() {
        System.exit(0);
    }

}

@SuppressWarnings("serial")
class DrawEllipses extends JPanel {
    private double translateX; //
    private double translateY; //
    protected double scale; //
    private static final int OVAL_WIDTH = 15;
    private static final Color INACTIVE_COLOR = Color.RED;
    private static final Color ACTIVE_COLOR = Color.green;
    private java.util.List<Point> points; //
    private java.util.List<Ellipse2D> ellipses = new ArrayList<>();
    private Map<Ellipse2D, Color> ellipseColorMap = new HashMap<>();

    public DrawEllipses(java.util.List<Point> points) {
        this.points = points; //
        translateX = 0; //
        translateY = 0; //
        scale = 1; //
        setOpaque(true); //
        setDoubleBuffered(true); //


        for (Point p : points) {
            int x = p.x - OVAL_WIDTH / 2;
            int y = p.y - OVAL_WIDTH / 2;
            int w = OVAL_WIDTH;
            int h = OVAL_WIDTH;
            Ellipse2D ellipse = new Ellipse2D.Double(x, y, w, h);
            ellipses.add(ellipse);
            ellipseColorMap.put(ellipse, INACTIVE_COLOR);
        }

        MyMouseAdapter mListener = new MyMouseAdapter();
        addMouseListener(mListener);
        addMouseMotionListener(mListener);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        AffineTransform tx = new AffineTransform(); //
        tx.translate(translateX, translateY); //
        tx.scale(scale, scale); //

        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        for (Ellipse2D ellipse : ellipses) {
            g2.setColor(ellipseColorMap.get(ellipse));
            g2.fill(ellipse);
        }
    }

    private class MyMouseAdapter extends MouseAdapter {
        @Override
        public void mousePressed(MouseEvent e) {
            for (Ellipse2D ellipse : ellipses) {
                if (ellipse.contains(e.getPoint())) {
                    Color c = ellipseColorMap.get(ellipse);
                    c =  (c == INACTIVE_COLOR) ? ACTIVE_COLOR : INACTIVE_COLOR;
                    ellipseColorMap.put(ellipse, c);
                }
            }
            repaint();
        }
    }

    //Used for button click action to change all ellipses to ACTIVE_COLOR
    public void activateAll(){
        for (Ellipse2D ellipse : ellipses){
            ellipseColorMap.put(ellipse, ACTIVE_COLOR);
        }
        repaint();
    }

    //Used for button click action to change all ellipses to INACTIVE_COLOR
    public void deactivateAll(){
        for (Ellipse2D ellipse : ellipses){
            ellipseColorMap.put(ellipse, INACTIVE_COLOR);
        }
        repaint();
    }
}

class RFInternalFrame extends JInternalFrame implements ComponentListener {
    protected static double scale = 1; //
    static int openFrameCount = 0;
    static final int xOffset = 300, yOffset = 0;

    public RFInternalFrame() {
        super("RF Panel #" + (++openFrameCount),
                true, //resizable
                true, //closable
                true, //maximizable
                true);//iconifiable

        setSize(300, 300);
        setMinimumSize(new Dimension(300, 300));
        addComponentListener(this);

        if (openFrameCount == 1) {

            setLocation(0,0);
        }
        else if (openFrameCount <= 4) {

            //Set the window's location.
            setLocation(xOffset * (openFrameCount - 1), yOffset * (openFrameCount - 1));
        }
        else if (openFrameCount == 5) {

            setLocation(xOffset - 300, yOffset + 300);
        }
        else if (openFrameCount == 6) {

            setLocation(xOffset + 600, yOffset + 300);
        }
    }

    //////////////////////////////////////////////////
    @Override
    public void componentResized(ComponentEvent e) {
        String str = "";
        if (getWidth() < 300) {
            str = "0." + getWidth();
        } else {
            str = "1." + (getWidth() - 300);
            System.out.println(getWidth() - 300);
        }
        double dou = Double.parseDouble(str);
        MainPanel.frame.scale = dou;
    }

    @Override
    public void componentMoved(ComponentEvent componentEvent) {

    }

    @Override
    public void componentShown(ComponentEvent componentEvent) {

    }

    @Override
    public void componentHidden(ComponentEvent componentEvent) {

    }
}

class ScenarioInternalFrame extends JInternalFrame {
    static int openFrameCount = 0;
    static final int xOffset = 300, yOffset = 300;

    public ScenarioInternalFrame() {
        super("Test Scenario" + (++openFrameCount),
                true, //resizable
                true, //closable
                true, //maximizable
                true);//iconifiable

        //...Create the GUI and put it in the window...

        //...Then set the window size or call pack...
        setSize(600, 300);

        //Set the window's location.
        setLocation(xOffset, yOffset);
    }
}

这是堆栈跟踪:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at MainPanel.createRFFrame(MainPanel.java:205)
    at MainPanel.<init>(MainPanel.java:69)
    at MainPanel.createAndShowGui(MainPanel.java:48)
    at MainPanel.access$000(MainPanel.java:8)
    at MainPanel$1.run(MainPanel.java:42)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Process finished with exit code 0

Answer 1

您可以在此处获取 NPE:

//Create a new internal frame.
protected void createRFFrame() {
    frame.setLayout(new BorderLayout()); 

简单地说，NPE 是正确的，frame 为空，你只是从未初始化过这个变量。尝试将变量标记为final，您将看到编译器错误，表明它从未初始化。我认为令人困惑的可能是您确实有另一个称为框架的变量，但它具有不同的范围并且没有关系:

private static void createAndShowGui() {
    JFrame frame = new MainPanel();
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setLocationByPlatform(false);
    frame.setVisible(true);
}