基本上我正在制作一块用鼠标绘制的绘图板;有一组颜色按钮可供选择，这些颜色按钮是在自定义类ColoredButton中制作的，其颜色参数由setColor（int r，g，b）设置。
因此，在ColourToolbar类中，当您单击其中一个ColoredButtons时，它会将一个全局变量selectedCOlor设置为该按钮中的颜色集（如上所述），然后DrawBoard类使用getColor（）来获取颜色。 ColourToolbar类中的'selectedCOlor'变量。
这是类（我拿出了不需要的代码）：
public class ColoredButton extends JButton {
public Color color;
public ColoredButton(ImageIcon img){
super(img);
}
public void setColor(int r, int g, int b){
this.color = new Color(r,g,b);
}
public Color getColor(){
return this.color;
}
}
下一课：
public class ColourToolbar extends JPanel implements ActionListener{
public Color selectedColor;
public boolean colorSelected = false;
public ColoredButton pink,black,blue,green,orange,yellow,darkp,red,white;
public ColourToolbar(){
setBackground(Color.DARK_GRAY);
setLayout(new GridBagLayout());
Dimension size = getPreferredSize();
size.setSize(1024,80); //w, h
setPreferredSize(size);
// NOTE: There are 8 more blocks of codes like the one below, left for example.
ImageIcon blackicon = new ImageIcon(getClass().getResource("Icons/Colours/Black.png"));
black = new ColoredButton(new ImageIcon(blackicon.getImage().getScaledInstance(width, height, java.awt.Image.SCALE_SMOOTH)));
gbc.gridx = 7;
gbc.gridy = 0;
gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.weightx = 0.5;
black.setColor(255, 255, 255);
black.setOpaque(false);
black.setContentAreaFilled(false);
black.setBorderPainted(false);
black.setActionCommand("orange");
black.addActionListener(this);
gbc.insets = new Insets(0,0,0,0); //top, left, bottom, right
add(black, gbc);
}
@Override
public void actionPerformed(ActionEvent e){
// all colors: black, blue, green, orange, *pink, yellow, darkpink, *red, white
if("black".equalsIgnoreCase(e.getActionCommand())){
this.selectedColor = black.getColor();
System.out.println("black= " + selectedColor.toString());
this.colorSelected = true;
}
}
public Color getCurrColor(){
return this.selectedColor;
}
}
我得到NullPointerException的最后一个类：
public class DrawBoard extends JPanel implements MouseListener, MouseMotionListener{
public JLabel status;
private JLabel imgLabel; // this is where the drawing happens
private List<Point> points = new ArrayList<Point>();
private List<BufferedImage> lines = new ArrayList<BufferedImage>();
private static final int BI_WIDTH = 1024;
private static final int BI_HEIGHT = 800;
private BufferedImage bImage = new BufferedImage(BI_WIDTH, BI_HEIGHT,
BufferedImage.TYPE_INT_ARGB);
public Color currentColor;
public DrawBoard(){
Graphics2D g2d = bImage.createGraphics();
g2d.dispose();
Dimension size = getPreferredSize();
size.setSize(1024,800); //w, h
setPreferredSize(size);
status = new JLabel("default");
add(status, BorderLayout.SOUTH);
addMouseListener(this);
addMouseMotionListener(this);
imgLabel = new JLabel(new ImageIcon(bImage)) {
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
paintInLabel(g);
}
};
imgLabel.setOpaque(false);
setOpaque(false);
add(imgLabel, BorderLayout.CENTER);
}
private void paintInLabel(Graphics g) {
Graphics2D g2d = (Graphics2D) g;
g2d.setColor(getColor()); // this colour is when mouse is pressed
g2d.setStroke(new BasicStroke(5));
if (points.size() < 2) {
return;
}
for (int i = 1; i < points.size(); i++) {
int x1 = points.get(i - 1).x;
int y1 = points.get(i - 1).y;
int x2 = points.get(i).x;
int y2 = points.get(i).y;
g2d.drawLine(x1, y1, x2, y2);
}
}
// Where the drawing happens
@Override
public void mousePressed(MouseEvent e) {
status.setText("you pressed down the mouse");
this.pstart = e.getPoint();
points.add(e.getPoint());
}
@Override
public void mouseDragged(MouseEvent e) {
status.setText("you draged the mouse");
points.add(e.getPoint());
imgLabel.repaint();
}
@Override
public void mouseReleased(MouseEvent e) {
status.setText("you release the mouse click");
Graphics2D g2d = bImage.createGraphics();
g2d.setColor(getColor()); // this is the final colour -
g2d.setStroke(new BasicStroke(5));
if (points.size() >= 2) {
for (int i = 1; i < points.size(); i++) {
int x1 = points.get(i - 1).x;
int y1 = points.get(i - 1).y;
int x2 = points.get(i).x;
int y2 = points.get(i).y;
g2d.drawLine(x1, y1, x2, y2);
}
}
g2d.dispose();
points.clear();
imgLabel.repaint();
}
// End of where the drawing happens
// BELOW IN THIS METHOD IS WHERE it gets the NullPointerException
private Color getColor() {
ColourToolbar ct = new ColourToolbar();
System.out.println(ct.getCurrColor().toString());
return ct.getCurrColor();
}
private void setColor(Color col){
this.currentColor = col;
}
}
错误堆栈跟踪在这里：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at gui.DrawBoard.getColor(DrawBoard.java:162)
at gui.DrawBoard.paintInLabel(DrawBoard.java:78)
at gui.DrawBoard.access$000(DrawBoard.java:32)
at gui.DrawBoard$1.paintComponent(DrawBoard.java:67)
at javax.swing.JComponent.paint(JComponent.java:1054)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:585)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5228)
at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:295)
at javax.swing.RepaintManager.paint(RepaintManager.java:1206)
at javax.swing.JComponent.paint(JComponent.java:1040)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:78)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:115)
at java.awt.Container.paint(Container.java:1967)
at java.awt.Window.paint(Window.java:3877)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:781)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:728)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:677)
at javax.swing.RepaintManager.access$700(RepaintManager.java:59)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)
答案 0 :(得分：5)
好的，这就是问题所在：
ColourToolbar ct = new ColourToolbar();
System.out.println(ct.getCurrColor().toString());
getCurrColor()在selectedColor内返回ColourToolbar ...但你还没有得到一个selectedColor ......构造函数没有...把它设置为任何东西。因此值为null，当您致电toString()时，您将获得例外。
在你的描述中你说：
它设置了一个全局变量selectedCOlor
但selectedColor不是“全局”变量 - 它是实例变量;每个ColourToolbar实例都有一个单独的selectedColor变量。
为什么要在 new ColourToolbar上执行此操作？您真的不想从现有工具栏中获取所选颜色吗？或者如果你真的希望它是全局的（ick）它应该是一个静态变量，由静态方法获取，而不需要你创建一个新的实例。