Clicking on a drawn object

Question

I've got a class called Shape which inherits from JPanel.

A number of sub-classes in turn extend the Shape classes, one for each type of shape.

Each shape has its own overriden paint() method, which draws the respective shape.

I would like to be able to click on any shape, and am trying to implement this logic for now. Please note that each shape has been added to an arrayList.

However, the contains statement always returns false, even when I have clearly clicked inside the shape.

Any ideas?

Answer 1

Never override paint() in JPanel rather paintComponent(..)

Im not quite sure I understand however I made a short example which I hope will help. Basically it is a simple JFrame with a DrawingPanel (my own class which extends JPanel and the shapes are drawn on). This panel will create shapes (only 2 for testing) add them to an ArrayList and draw them to the JPanel via paintComponent(..) and a for loop, it also has a MouseAdapter to check for user mouseClicked(..) evnets on the JPanel. When a click is made we iterate through each Shape in the ArrayList and check whether the Shape contains the point or not, and if so prints its class name and uses instance of to check what type of Shape is clicked and prints appropriate message:

Output (after clicking both shapes):

Clicked a java.awt.geom.Rectangle2D$Double

Clicked a rectangle

Clicked a java.awt.geom.Ellipse2D$Double

Clicked a circle

ShapeClicker.java:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Shape;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Rectangle2D;
import java.util.ArrayList;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class ShapeClicker {

    public ShapeClicker() {
        JFrame frame = new JFrame();
        frame.setTitle("Shape Clicker");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);

        initComponents(frame);

        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {

        //create frame and components on EDT
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new ShapeClicker();
            }
        });
    }

    private void initComponents(JFrame frame) {
        frame.add(new ShapePanel());
    }
}

//custom panel
class ShapePanel extends JPanel {

    private Shape rect = new Rectangle2D.Double(50, 100, 200, 100);
    private Shape cirlce = new Ellipse2D.Double(260, 100, 100, 100);
    private Dimension dim = new Dimension(450, 300);
    private final ArrayList<Shape> shapes;

    public ShapePanel() {
        shapes = new ArrayList<>();
        shapes.add(rect);
        shapes.add(cirlce);
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent me) {
                super.mouseClicked(me);
                for (Shape s : shapes) {
                    
                    if (s.contains(me.getPoint())) {//check if mouse is clicked within shape
                        
                        //we can either just print out the object class name
                        System.out.println("Clicked a "+s.getClass().getName());
                        
                        //or check the shape class we are dealing with using instance of with nested if
                        if (s instanceof Rectangle2D) {
                            System.out.println("Clicked a rectangle");
                        } else if (s instanceof Ellipse2D) {
                            System.out.println("Clicked a circle");
                        }
                        
                    }
                }
            }
        });
    }

    @Override
    protected void paintComponent(Graphics grphcs) {
        super.paintComponent(grphcs);
        Graphics2D g2d = (Graphics2D) grphcs;
        for (Shape s : shapes) {
            g2d.draw(s);
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return dim;
    }
}

Answer 2

If you are implementing Shape you have to implement the contains method yourself. The default implementation for Shape always returns false.

If your Shape is bounded by curves that you know how to intersect (or determine if a point is on one or the other side), you can use the even-odd rule. Cast a ray from the point tested in any direction not parallel to a straight line. If the number of intersections is odd, the point is inside. If the number of intersections is even, the point is outside.

The built-in classes implement this method, so you can use/extend the Polygon, Ellipse2D.Double or RoundRectangle2D.Double class and have a filled polygon / ellipse / round rectangle that knows its inside.