我正在使用Swing制作GUI，它有几个功能：使用Graphics2D，JInternalFrame，JDesktopFrame，AffineTransform的自定义图形。我认为我已经接近让一切正常工作但我得到了空指针异常。我在这做错了什么？
这是我自包含的例子：
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
答案 0 :(得分：3)
你在这里获得NPE：
//Create a new internal frame.
protected void createRFFrame() {
frame.setLayout(new BorderLayout());
简单地说，NPE是正确的，frame为null，你只是从未初始化过这个变量。尝试将变量标记为final，您将看到一个永远不会初始化的编译器错误。我认为混淆可能是你有另一个叫做frame的变量，但是它有不同的范围而且没有关系：
private static void createAndShowGui() {
JFrame frame = new MainPanel();
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setLocationByPlatform(false);
frame.setVisible(true);
}
答案 1 :(得分：2)
您忘了在frame = new RFInternalFrame();之前致电frame.setLayout(new BorderLayout());。如果没有这一行，您将尝试设置尚未初始化的RFInternalFrame的布局 - 因此程序会打印出NPE。有关详细信息，请参阅What is a Null Pointer Exception, and how do I fix it?。