Draw On Mouse Click

You can use the Mouse Listener to register mouse clicks. In this code we draw a blue square where we click in the screen.

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

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Click to Draw a Square");
        DrawingPanel panel = new DrawingPanel();

        // Set up the GUI
        frame.setSize(400, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(panel);
        frame.setVisible(true);
    }
}

class DrawingPanel extends JPanel {
    private int squareX = -50; // Initial position offscreen
    private int squareY = -50;
    private final int SQUARE_SIZE = 50; // Fixed size for the square

    public DrawingPanel() {
        // Add MouseListener to capture click events
        this.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                squareX = e.getX() - SQUARE_SIZE / 2; // Center the square on click
                squareY = e.getY() - SQUARE_SIZE / 2;
                repaint(); // Redraw the panel
            }
        });
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g); // Clear the panel for fresh drawing
        g.setColor(Color.BLUE); // Set the square's color
        g.fillRect(squareX, squareY, SQUARE_SIZE, SQUARE_SIZE); // Draw the square
    }
}