java 在 Jframe 中画一个圆

Drawing a circle into a Jframe

提问by Dragongeek

I'm trying to draw a small circle into a gray square JFrame as an indicator of stick positions for an RC remote.

我正在尝试在灰色方形 JFrame 中绘制一个小圆圈，作为 RC 遥控器摇杆位置的指示器。

I've got two classes:

我有两个班级：

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);
    }
}

And I've got a second class:

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;
}

}

}

The current program draws the GUI just fine however when I attempt to run the drawCircle() it falls apart and gives me errors. I'm not sure what I've done wrong, I don't have a lot of experience with 2d drawing.

当前程序可以很好地绘制 GUI，但是当我尝试运行 drawCircle() 时它会崩溃并给我错误。我不确定我做错了什么，我对 2d 绘图没有很多经验。

So my questions:

所以我的问题：

-Is this the right approach to this? -How can I get the drawCircle to draw circles within the gray Jframes?

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

Error Message:

错误信息：

java.lang.NullPointerException
at experiments.StickWidget.drawCircle(StickWidget.java:59)
at experiments.GUI2.<init>(GUI2.java:44)
at experiments.GUI2.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.access0(EventQueue.java:103)
at java.awt.EventQueue.run(EventQueue.java:706)
at java.awt.EventQueue.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain.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)

采纳答案by ControlAltDel

Draw a circle by overridding paint / paintComponent (if using swing)

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);
}

To draw custom circles, you can use a (Buffered)Image to draw on first

要绘制自定义圆圈，您可以先使用 (Buffered)Image 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);
  }
}

回答by maskacovnik

To debug the problem try to put code:

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);
}

I would use Canvas instead of JPanel to paint with Graphics on it.

我会使用 Canvas 而不是 JPanel 在其上绘制图形。