Java：paintComponent没有被调用

Question

My full code (drag and drop code) (only class Surface and draw() function is manually):

import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.*;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
import javax.swing.text.DefaultFormatter;


class Surface extends JPanel {
int x1,y1,x2,y2;

public Surface(){

}

public Surface(int x1,int y1,int x2,int y2){
    this.x1=x1;
    this.y1=y1;
    this.x2=x2;
    this.y2=y2;
}

private void doDrawing(Graphics g) { // 3
    Graphics2D g2d = (Graphics2D) g;

    g2d.drawLine(x1, y1, x2, y2);
}


@Override
public void paintComponent(Graphics g) { // 2
    super.paintComponent(g);
    doDrawing(g);
}
}
public class NewJFrame extends JFrame {

int x1,y1,x2,y2;

/**
 * Creates new form NewJFrame
 */
public NewJFrame() {
    initComponents();
    draw();
}

private void draw(){
    xSpinner.addChangeListener(new ChangeListener() {
        @Override
        public void stateChanged(ChangeEvent e) {
            x1=(int) ((JSpinner)e.getSource()).getValue();
            add(new Surface(x1,y1,x2,y2));

        }
    });
}

/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    controlPanel = new javax.swing.JPanel();
    xSpinner = new javax.swing.JSpinner();
    ySpinner = new javax.swing.JSpinner();
    widthSpinner = new javax.swing.JSpinner();
    heightSpinner = new javax.swing.JSpinner();
    xLabel = new javax.swing.JLabel();
    yLabel = new javax.swing.JLabel();
    widthLabel = new javax.swing.JLabel();
    heightLabel = new javax.swing.JLabel();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    xLabel.setText("x");

    yLabel.setText("y");

    widthLabel.setText("width");

    heightLabel.setText("height");

    javax.swing.GroupLayout controlPanelLayout = new javax.swing.GroupLayout(controlPanel);
    controlPanel.setLayout(controlPanelLayout);
    controlPanelLayout.setHorizontalGroup(
        controlPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, controlPanelLayout.createSequentialGroup()
            .addGap(36, 36, 36)
            .addComponent(xLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 6, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(xSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, 40, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGap(21, 21, 21)
            .addComponent(yLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 6, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(ySpinner, javax.swing.GroupLayout.PREFERRED_SIZE, 40, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 63, Short.MAX_VALUE)
            .addComponent(widthLabel)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(widthSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, 40, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGap(31, 31, 31)
            .addComponent(heightLabel)
            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
            .addComponent(heightSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, 40, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGap(17, 17, 17))
    );
    controlPanelLayout.setVerticalGroup(
        controlPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(controlPanelLayout.createSequentialGroup()
            .addGap(22, 22, 22)
            .addGroup(controlPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                .addComponent(ySpinner, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(widthSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(xLabel)
                .addComponent(yLabel)
                .addComponent(widthLabel)
                .addComponent(heightLabel)
                .addComponent(xSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addComponent(heightSpinner, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
            .addContainerGap(258, Short.MAX_VALUE))
    );

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(controlPanel, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(controlPanel, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
    );

    pack();
}// </editor-fold>                        

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {
    /* Set the Nimbus look and feel */
    //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
    /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */
    try {
        for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
            if ("Nimbus".equals(info.getName())) {
                javax.swing.UIManager.setLookAndFeel(info.getClassName());
                break;
            }
        }
    } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>

    /* Create and display the form */
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new NewJFrame().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
private javax.swing.JPanel controlPanel;
private javax.swing.JLabel heightLabel;
private javax.swing.JSpinner heightSpinner;
private javax.swing.JLabel widthLabel;
private javax.swing.JSpinner widthSpinner;
private javax.swing.JLabel xLabel;
private javax.swing.JSpinner xSpinner;
private javax.swing.JLabel yLabel;
private javax.swing.JSpinner ySpinner;
// End of variables declaration                   
}

I'm trying to draw line with x,y value get from spinner

private void draw(){
    xSpinner.addChangeListener(new ChangeListener() {
        @Override
        public void stateChanged(ChangeEvent e) {
            x1=(int) ((JSpinner)e.getSource()).getValue();
            add(new Surface(x1,y1,x2,y2));

        }
    });
}

But paintComponent function in class Surface seem not being called.
Please help

Answer 1

Problems:

• Your controlPane is using GroupLayout, a layout that doesn't play nice when adding new components mid-stream
• You shouldn't even be adding a new component in your ChangeListener as it makes no sense. Why continuously add new JPanels when all you want to do is change what is drawn? Instead add your Surface JPanel on GUI creation and in the ChangeListener change the x1, y1, x2, y2 fields and call repaint() on it.

Myself, I'd get rid of all the windows builder generated code, and simply make Surface the central JPanel of the GUI. I'd create another JPanel that holds everything, give it a BorderLayout, add a single Surface instance to it BorderLayout.CENTER, and then in the ChangeListener, change this Surface's state and call repaint on it.

Answer 2

I've solved it. I forgot to set size and location for Surface(JPanel)