How to Draw a Line on a JPanel in Java When a Button is Clicked

Question

How can I draw a line on a JPanel using a button click in Java?

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

public class LineDrawer extends JPanel {
    private int x1 = 0, y1 = 0, x2 = 100, y2 = 100;

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(x1, y1, x2, y2);
    }

    public void drawLine() {
        // Update line coordinates
        x2 += 10;
        y2 += 10;
        repaint();  // Redraw the JPanel
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Line Drawer");
        LineDrawer panel = new LineDrawer();
        JButton button = new JButton("Draw Line");

        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                panel.drawLine();
            }
        });

        frame.add(panel, BorderLayout.CENTER);
        frame.add(button, BorderLayout.SOUTH);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

Answer

In Java, drawing on a JPanel in response to user actions such as button clicks involves overriding the appropriate painting methods and handling events. This guide demonstrates how to draw a line on a JPanel, updating the line's position with each button press.

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

public class LineDrawer extends JPanel {
    private int x1 = 0, y1 = 0, x2 = 100, y2 = 100;

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(x1, y1, x2, y2);
    }

    public void drawLine() {
        // Update line coordinates
        x2 += 10;
        y2 += 10;
        repaint();  // Redraw the JPanel
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("Line Drawer");
        LineDrawer panel = new LineDrawer();
        JButton button = new JButton("Draw Line");

        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                panel.drawLine();
            }
        });

        frame.add(panel, BorderLayout.CENTER);
        frame.add(button, BorderLayout.SOUTH);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

Causes

Understanding how to respond to button click events in Java.
Properly overriding the paintComponent method in a JPanel to handle custom drawing.

Solutions

Create a custom JPanel class that overrides paintComponent.
Use a JButton to trigger repainting and change line attributes.
Manage line coordinates to be flexible and responsive to button actions.

Common Mistakes

Mistake: Not calling repaint() after updating line coordinates.

Solution: Always call repaint() within the method that updates the line attributes to ensure the JPanel is redrawn.

Mistake: Forgetting to initialize JFrame properties before setting it visible.

Solution: Ensure that you set the size, default close operation, and visibility of the JFrame in the correct order.

Helpers

Java JPanel drawing
Java draw line button click
Java GUI programming
JPanel paintComponent
Drawing in Java Swing