Question

我正在尝试将一个小圆圈绘制成灰色方形JFrame，作为RC遥控器杆位的指示器。

我有两节课：

public class GUI2 extends JFrame {

private JPanel contentPane;

/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                GUI2 frame = new GUI2();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}
public GUI2() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setLayout(new BorderLayout(0, 0));
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    StickWidget Left = new StickWidget();
    Left.setName("Left Stick");
    Left.drawCircle(10, 10);
    contentPane.add(Left.draw(), BorderLayout.EAST);
    StickWidget Right = new StickWidget();
    Right.setName("Right Stick");
    contentPane.add(Right.draw(), BorderLayout.WEST);
    }
}

我有第二堂课：

public class StickWidget extends JPanel {

private javax.swing.JPanel panel_1;
private String NameOfStick;
private int xpos;
private int ypos;

public void initcomponents()
{
panel_1 = new javax.swing.JPanel();
}

public StickWidget() {
    //do nothing?
}

public Component draw()
{
    initcomponents();
    JPanel main = new JPanel();
    main.setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
    Dimension D1 = new Dimension(120, 150);
    main.setMinimumSize(D1);
    main.setMaximumSize(D1);
    main.setPreferredSize(D1);
    main.setSize(D1);
    main.setLayout(new MigLayout("", "[grow]", "[grow][grow]"));
    JPanel panel = new JPanel();
    main.add(panel, "cell 0 0,grow");

    JLabel StickName = new JLabel(NameOfStick);
    panel.add(StickName);


    panel_1.setBackground(Color.LIGHT_GRAY);
    Dimension d = new Dimension(100, 100);
    panel_1.setMinimumSize(d);
    panel_1.setMaximumSize(d);
    main.add(panel_1, "cell 0 1,grow");
    return main;
}

public void drawCircle(int x, int y)
{
    Graphics2D g2d = (Graphics2D)panel_1.getGraphics();
    g2d.fillOval(x, y, 10, 10);
}
public void setName(String s)
{
    NameOfStick = s;
}

}

当前程序绘制GUI就好了但是当我尝试运行drawCircle（）时它会崩溃并给我错误。我不确定我做错了什么，我对2D绘图没有太多经验。

所以我的问题：

- 这是正确的方法吗？ - 如何让drawCircle在灰色Jframes中绘制圆圈？

错误讯息：

java.lang.NullPointerException
at experiments.StickWidget.drawCircle(StickWidget.java:59)
at experiments.GUI2.<init>(GUI2.java:44)
at experiments.GUI2$1.run(GUI2.java:23)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:312)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:745)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:715)
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)

Answer 1

通过覆盖paint / paintComponent（如果使用swing）绘制一个圆圈

public void paintComponent(Graphics g) {
  g.drawOval(0,0,50,50);
}

要绘制自定义圆圈，您可以使用（缓冲）图像在第一个

上绘制

public class DrawCircles extends JPanel {
  Image im = new BufferedImage(...);
  public void drawCircle(int x, int y) {
    im.getGraphics().drawOval(x,y,50,50); //TODO: should really dispose the Graphics object after drawing the oval
    repaint();
  }

  public void paintComponent(Graphics g) {
    g.drawImage(im,0,0,null);
  }
}

Answer 2

要调试问题，请尝试输入代码：

public void drawCircle(int x, int y)
{
    if(panel_1==null){
        System.err.println("Panel is null");
        return;
    }
    Graphics2D g2d = (Graphics2D)panel_1.getGraphics();
    if(g2d==null){
        System.err.println("Graphics is null");
        return;
    }
    g2d.fillOval(x, y, 10, 10);
}

我会使用Canvas而不是JPanel来绘制图形。

在Jframe中绘制一个圆圈

2 个答案: