JLabel 不显示在 JPanel 上

Question

I already looked up this problem and found several threads. Tried all the solutions and none of them helped. I am trying to show a cross where my mouse is placed, the x and the y coordinate of my current mouse position is supposed to be shown in the top left and top right corner. In order to achieve this, I used two JLabels. Maybe I am overlooking something?

I played around with the standard Text I set in the Labels, positioning, different Layouts for my frame and panel - nothing helps. The following code should be good enough to get an understanding, I dont think it would be helpful if I left out something.

Fensterkreuz(){
    jl1 = new JLabel("0");
    jl2 = new JLabel("0");
    jl1.setSize(new Dimension(100,100));
    jl2.setSize(new Dimension(100,100));
    jl1.setFont(new Font ("Arial", Font.PLAIN, 15));
    jl2.setFont(new Font ("Arial", Font.PLAIN, 15));
    cP = new Point();
    this.add(jl1);
    this.add(jl2);
    addMouseMotionListener(this);       
    }

public void mouseDragged (MouseEvent e){
}
public void mouseMoved (MouseEvent e) {
    cP = e.getPoint();
    repaint();
}
public void paint (Graphics g){
    g.drawLine((cP.x),(cP.y-15), (cP.x),(cP.y+15));
    g.drawLine((cP.x-15),(cP.y), (cP.x+15),(cP.y));
    jl1.setText(String.valueOf(cP.x));
    jl2.setText(String.valueOf(cP.y));
}

public static void main (String [] args) {
    JFrame f = new JFrame();
    JComponent test = new Fensterkreuz();
    test.setOpaque(false);
    f.setVisible(true);
    f.setSize(1500,1000);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setContentPane(test);
}

Answer 1

You are overriding the paint() method. So, you need add super.paint(g); as the first line in your overridden paint() method.

To show the 2 labels properly, you need to add this.setLayout(new FlowLayout(FlowLayout.LEFT)); line.

I'm adding the full code with above changes here, so that you can run it and see the results yourself.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Fensterkreuz extends JComponent implements MouseMotionListener {

  private JLabel jl1;
  private JLabel jl2;
  private Point cP;

  Fensterkreuz(){
    jl1 = new JLabel("0");
    jl2 = new JLabel("0");
    jl1.setSize(new Dimension(100,100));
    jl2.setSize(new Dimension(100,100));
    jl1.setFont(new Font ("Arial", Font.PLAIN, 15));
    jl2.setFont(new Font ("Arial", Font.PLAIN, 15));
    cP = new Point();

    //this.setLayout(new FlowLayout(FlowLayout.LEFT));
    //this.add(jl1);
    //this.add(jl2);

    this.setLayout(new GridBagLayout());
    this.add(jl1, new GridBagConstraints(0, 0, 1, 1, 0.0, 1.0, GridBagConstraints.NORTHWEST,
        GridBagConstraints.NONE, new Insets(5, 5, 5, 5), 0, 0));
    this.add(jl2, new GridBagConstraints(1, 0, 1, 1, 1.0, 1.0, GridBagConstraints.NORTHEAST,
        GridBagConstraints.NONE, new Insets(5, 5, 5, 5), 0, 0));

    addMouseMotionListener(this);
  }

  public void mouseDragged (MouseEvent e){
  }
  public void mouseMoved (MouseEvent e) {
    cP = e.getPoint();
    repaint();
  }
  public void paint (Graphics g){
    super.paint(g);
    g.drawLine((cP.x),(cP.y-15), (cP.x),(cP.y+15));
    g.drawLine((cP.x-15),(cP.y), (cP.x+15),(cP.y));
    jl1.setText(String.valueOf(cP.x));
    jl2.setText(String.valueOf(cP.y));
  }

  public static void main (String [] args) {
    JFrame f = new JFrame();
    JComponent test = new Fensterkreuz();
    test.setOpaque(false);
    f.setVisible(true);
    f.setSize(1500,1000);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setContentPane(test);
  }
}

Answer 2

Put repaint() at the bottom of your main method. Repaint calls the Paint method you have but I think you also have to add your own overriding repaint method to stop “flickering”.