Java 把一个圆画成一个框架

我试图在一个灰色的正方形框架中画一个小圆圈，作为RC遥控器斗杆位置的指示器

我有两门课：

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);
    }
}

我还有第二节课：

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;
}

}

当前程序可以很好地绘制GUI，但是当我尝试运行drawCircle（）时，它会崩溃并产生错误。我不确定我做错了什么，我没有很多二维绘图的经验

所以我的问题是：

-这是正确的方法吗？ -如何让drawCircle在灰色框架内绘制圆

错误消息：

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)

java.lang.NullPointerException
在experiments.StickWidget.drawCircle（StickWidget.java:59）
位于experiments.GUI2.（GUI2.java:44）
在experiments.GUI2$1.run（GUI2.java:23）
在java.awt.event.InvocationEvent.dispatch（InvocationEvent.java:312）中
位于java.awt.EventQueue.dispatchEventImpl（EventQueue.java:745）
在java.awt.EventQueue.access$300（EventQueue.java:103）
在java.awt.EventQueue$3.run（EventQueue.java:706）
在java.awt.EventQueue$3.run（EventQueue.java:704）
位于java.security.AccessController.doPrivileged（本机方法）
位于java.security.ProtectionDomain$1.doIntersectionPrivilege（ProtectionDomain.java:76）
位于java.awt.EventQueue.dispatchEvent（EventQueue.java:715）
位于java.awt.EventDispatchThread.pumpOneEventForFilters（EventDispatchThread.java:242）
在java.awt.EventDispatchThread.PumpeEventsforFilter（EventDispatchThread.java:161）
位于java.awt.EventDispatchThread.PumpeEventsforHierarchy（EventDispatchThread.java:150）
位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:146）
位于java.awt.EventDispatchThread.pumpEvents（EventDispatchThread.java:138）
在java.awt.EventDispatchThread.run（EventDispatchThread.java:91）

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

要绘制自定义圆，可以先使用（缓冲）图像进行绘制

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);
  }
}

---

要调试问题，请尝试放置代码：

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);
}

---

我会使用画布而不是JPanel在上面绘制图形。

你能包括错误吗？@maskacovnik Ive添加了它

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);
}