How to click a button and have a line drawn on a panel in a frame.

**Krotus** · January 27th, 2019, 02:35 AM

Hello!

I'm a VB.net developer that is trying to learn Java. I am using IntelliJ on Windows 10.

Using Swing, I have created a simple JFrame with 2 buttons, Render and OK. There is also a JPanel in the center of the frame. Clicking "OK" exits the program. What I would like to do is click "Render" and have a line drawn onto the JPanel.

This is the code I have:

import javax.swing.*;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.Graphics;
import java.awt.Color;
 
public class MainForm extends JPanel{
    private JButton btnOK;
    private JPanel pnlMainPanel;
    private JButton btnRender;
    private JPanel pnlImage;
    private static int width = 800;
    private static int height = 600;
 
    public MainForm() {
 
        btnOK.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        });
 
        btnRender.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
 
            }
        });
    }
 
    public static void main(String[] args) {
 
        JFrame frame = new JFrame("Drawing Test");
 
        frame.setContentPane(new MainForm().pnlMainPanel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
 
}

What I don't know how to do, and cant figure out, is how to invoke the JPanel's paintComponent() from the action listener of the Render button. Thanks in advance for any help and/or suggestions.

**Norm** · January 27th, 2019, 06:46 AM

how to invoke the JPanel's paintComponent()

You don't call the paintComponent() directly. It is called by the JVM when the component needs to be painted or after the repaint method has been called.

**jim829** · January 27th, 2019, 02:01 PM

How long have you been trying to learn Java? I ask this because there is quite a bit to learn before doing painting or any graphics related programming.

Regards,
Jim

**Krotus** · January 28th, 2019, 12:11 AM

I've had a couple of Java courses for my CIS degree. Java I and Java II. Which doesn't mean much.

All of the examples I see for drawing on a JPanel show the paintComponent function inside of a class that extends JPanel with the drawing instructions inside the paintComponent. So I'm probably not even on the right track. Maybe what I really should be doing is drawing on a buffered image and then displaying that image in the panel.

**Norm** · January 28th, 2019, 06:42 AM

Either approach should work. The first one should be easier.

**jim829** · January 28th, 2019, 08:11 AM

You might want to start here --> https://docs.oracle.com/javase/tutor...ing/index.html

Regards,
Jim

**Krotus** · February 1st, 2019, 02:08 AM

So I've changed the code to this:

import java.awt.*;
import java.awt.image.*;
import java.awt.event.*;
import java.io.*;
import javax.swing.*;
import javax.imageio.ImageIO;
 
public class JPanelExample {
 
 
 
    public static void main(String[] arguments) throws IOException {
 
        JPanel panel = new JPanel();
 
        BufferedImage image = ImageIO.read(new File("C:\\Temp\\ic342.jpg"));
        JLabel label = new JLabel(new ImageIcon(image));
        panel.add(label);
 
        //Main window
        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame("JPanel Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
        //Add a button
        JButton btn = new JButton("Render");
        btn.setBounds(260,700,100, 65);
 
        //Add the listener to the JButton
        btn.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
 
            }
        });
 
        frame.setLayout(new BoxLayout( frame.getContentPane(), BoxLayout.Y_AXIS));
 
        //Add the Jpanel to the main window
        frame.add(panel);
 
        //Adding button onto the frame
        frame.add(btn);
 
        frame.pack();
        frame.setVisible(true);
 
    }
}

How would I add the code in the btn ActionListener to display the buffered image instead of it displaying when the form loads.

Nevermind - I figured it out. I have to call the frame.pack(); after I call panel.add(label); in the ActionListener.

**Norm** · February 1st, 2019, 06:14 AM

Is the code doing what you want now?

**Krotus** · February 1st, 2019, 11:50 PM

Yes, it is, thanks.

**Norm** · February 2nd, 2019, 06:37 AM

Ok. The code doesn't do what the title of this thread asks about. The action listener method is empty. There is no code that draws a line.

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