java - Can you add a mouse listener to an object in paintComponent

Question

Just a quick question, I was wondering if there is any way to add a mouse listener to a paint component? For example, say you drew a rectangle, could you make it so when you click that rectangle, it will then do something.

public void paintComponent(Graphics g) {
    g.drawRect(50, 50, 20, 20);
    //Do something when this rectangle is clicked on
}

Answer 1

To answer your topic question:

Can you add a mouse listener to an object in paintComponent

Yes, sort of. You can't do anything to objects that are declared within the paintComponent method, since their scope is limited to that method, but you can react to objects declared in the class that are drawn within the paintComponent method, and in fact this is commonly done with objects of classes that implement the Shape interface such as Rectangle2D.

For example, run this program, and click on the shapes within it:

package foo1;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.Stroke;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.swing.*;

@SuppressWarnings("serial")
public class GraphicsEg extends JPanel {
    private static final int PREF_W = 400;
    private static final int PREF_H = PREF_W;
    private static final Color SELECTED_COLOR = Color.RED;
    private static final Stroke SELECTED_STROKE = new BasicStroke(8f);
    private List<Shape> shapes = new ArrayList<>();
    private Map<Shape, Color> shapeColorMap = new HashMap<>();
    private Shape selectedShape = null;

    public GraphicsEg() {
        Shape shape = new Ellipse2D.Double(10, 10, 90, 90);
        shapeColorMap.put(shape, Color.GRAY);
        shapes.add(shape);

        shape = new Rectangle2D.Double(140, 140, 200, 200);
        shapeColorMap.put(shape, Color.BLUE);
        shapes.add(shape);

        shape = new RoundRectangle2D.Double(200, 200, 80, 80, 10, 10);
        shapeColorMap.put(shape, Color.GREEN);
        shapes.add(shape);

        addMouseListener(new MyMouseListener());
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        for (Shape shape : shapes) {
            Color color = shapeColorMap.get(shape);
            g2.setColor(color);
            g2.fill(shape);
        }

        if (selectedShape != null) {
            Graphics2D newG2 = (Graphics2D) g2.create();
            newG2.setColor(SELECTED_COLOR);
            newG2.setStroke(SELECTED_STROKE);
            newG2.draw(selectedShape);
            newG2.dispose(); // because this is a created Graphics object
        }
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(PREF_W, PREF_H);
    }

    private class MyMouseListener extends MouseAdapter {
        @Override
        public void mousePressed(MouseEvent e) {
            for (int i = shapes.size() - 1; i >= 0; i--) {
                if (shapes.get(i).contains(e.getPoint())) {
                    selectedShape = shapes.get(i);
                    repaint();
                    return;
                }
            }
        }
    }

    private static void createAndShowGui() {
        JFrame frame = new JFrame("GraphicsEg");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new GraphicsEg());
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGui();
            }
        });
    }
}