How to draw a line on a existing jPanel in Swing (Java)?

Question

I want to draw a simple line on my existing jPanel called mypanel. I want to do it like this:

    mypanel.drawLine(0,0, 20, 35);

The numbers are the X and Y Position of Point 1 and the others are X and Y Position of Point 2, between Point 1 and Point 2 there should be my line. Is there a easy way without adding an additional jPanel on my jFrame? Thank you in advance.

Edit:

My GUI Code:

    import java.awt.Graphics;
import javax.swing.JPanel;

public class main_panel extends javax.swing.JFrame {

    public main_panel() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        panel = new javax.swing.JPanel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        panel.setBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(0, 0, 0)));

        javax.swing.GroupLayout panelLayout = new javax.swing.GroupLayout(panel);
        panel.setLayout(panelLayout);
        panelLayout.setHorizontalGroup(
            panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );
        panelLayout.setVerticalGroup(
            panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 163, Short.MAX_VALUE)
        );

        jButton1.setText("Set Values");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(panel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addGap(0, 280, Short.MAX_VALUE)
                        .addComponent(jButton1)))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(panel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jButton1)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

        panel = new JPanel() {
            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                g.drawLine(0, 0, 20, 35);
            }

        };
    }                                        

    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(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(main_panel.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 main_panel().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JPanel panel;
    // End of variables declaration                   

}

In your code, you create a 2nd JPanel but you add it to nothing, so of course it's not displayed. Just because you use the same panel variable has no effect, and in order for the 2nd JPanel to be seen, it must be added. You need to learn layout managers and learn to use the human-usable managers such as BorderLayout for this to work. — Hovercraft Full Of Eels, Jan 04 '16 at 18:08
But how can I add the line on my existing panel called `panel`? — Pascal, Jan 04 '16 at 18:14
Again, you can only add a line on a JPanel whose paintComponent method has been overridden -- Period, end of story, and please believe me -- I've done this for years.. Some may tell you to use `getGraphics()` on the JPanel, and if they do, they'd be doing you a disservice since that will result in a graphics object that does not persist. — Hovercraft Full Of Eels, Jan 04 '16 at 18:22
... and that JPanel must be on the GUI -- which means that you will need to read up on and learn the Swing layout managers: [tutorial](http://docs.oracle.com/javase/tutorial/uiswing/layout/index.html) — Hovercraft Full Of Eels, Jan 04 '16 at 18:30
@PascalAckermann I've added the complete (and tested) example based on your code. — Sergiy Medvynskyy, Jan 05 '16 at 09:54

Sergiy Medvynskyy · Accepted Answer · 2016-01-05T09:52:29.587

Something like this

myPanel = new JPanel() {
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(0,0, 20, 35);
    };

But your suggestion with two panels is better.

Here is the full GUI code (each click on "Set Values" will toggle the line).

import java.awt.Graphics;
import javax.swing.JPanel;

public class main_panel extends javax.swing.JFrame {

    private boolean drawLine;

    public main_panel() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        panel = new javax.swing.JPanel() {
            @Override
            protected void paintComponent(Graphics g) {
                super.paintComponent(g);
                if (drawLine) {
                    g.drawLine(0, 0, 20, 35);
                }
            }
        };
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        panel.setBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(0, 0, 0)));

        javax.swing.GroupLayout panelLayout = new javax.swing.GroupLayout(panel);
        panel.setLayout(panelLayout);
        panelLayout.setHorizontalGroup(
            panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );
        panelLayout.setVerticalGroup(
            panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 163, Short.MAX_VALUE)
        );

        jButton1.setText("Set Values");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(panel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addGap(0, 280, Short.MAX_VALUE)
                        .addComponent(jButton1)))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(panel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jButton1)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        drawLine = !drawLine;
        panel.repaint();
    }                                        

    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(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(main_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(main_panel.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 main_panel().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JPanel panel;
    // End of variables declaration                   

}

This doesn't work for me, the line doesn't appear on my panel. How would you solve it with two panels? — Pascal, Jan 04 '16 at 17:17
@PascalAckermann: define "doesn't work" including showing code. And what do you mean by "with two panels"? By itself and out of context that statement doesn't make sense. Please create and show us your [mcve]. — Hovercraft Full Of Eels, Jan 04 '16 at 17:24
I added exactly the code from @Sergiy Medvynskyy into my application, there are no errors the line just doesn't appear on the panel. With the solution with two panels I want to add another jPanel on my existing jPanel with the line on it, maybe it's easier to implement. — Pascal, Jan 04 '16 at 17:28
@PascalAckermann: see edit to answer. Without showing your pertinent code, we have no idea why Sergiy's answer isn't working. 1+ to this answer by the way for being correct. — Hovercraft Full Of Eels, Jan 04 '16 at 17:31
@PascalAckermann Override `getPreferredSize` to return the preferred size of the component so the layout manager doesn't make it `0x0` — MadProgrammer, Jan 04 '16 at 20:43

score 1 · Answer 2 · edited May 23 '17 at 12:04

Is there a easy way without adding an additional jPanel on my jFrame?

This statement is hard to interpret by itself and out of context. What you must do is override paintComponent as is noted above for the component that you want to display the line it, nothing more, and nothing less. How you do this will depend on the structure of your program, something we know little about at present.

Please look at this link for more details.

You also state:

I added exactly the code from @Sergiy Medvynskyy into my application, there are no errors the line just doesn't appear on the panel. With the solution with two panels I want to add another jPanel on my existing jPanel with the line on it, maybe it's easier to implement.

That's possible as long as your original JPanel is set up to receive a 2nd JPanel, in other words has a layout manager that will allow easy addition of the 2nd JPanel, and a location on it where the added JPanel won't cover anything else of importance. For instance if the first JPanel uses BorderLayout, then the second with the drawing code could be added BorderLayout.CENTER to display its drawing in the middle of the first.

But again, details are key. If you need more direct help, then show your GUI code, preferably an mcve.

In your posted code, you create a 2nd JPanel but you add it to nothing, so of course it's not displayed. Just because you use the same panel variable has no effect, and in order for the 2nd JPanel to be seen, it must be added. You need to learn layout managers and learn to use the human-usable managers such as BorderLayout for this to work.

How to draw a line on a existing jPanel in Swing (Java)?

2 Answers2