MouseListener in Canvas not working

Question

I am having a problem with a game that I am trying to make in Java. I am trying to attach a MouseListener to my canvas, however, when I click on the canvas, nothing happens. I think I may be attaching the MouseListener to the wrong thing, but I don't know what to attach it to. I have tried attaching it to the JFrame and the canvas. Here is my code:

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;

import java.io.*;

import javax.imageio.ImageIO;
import javax.swing.*;
import java.util.Random;

public class Gravity extends Canvas {

    public static final int screenw = 1024;
    public static final int screenh = 768;

    public static Random gen = new Random();

    public static Boolean started = false;

    public static int[] starx = new int[100];
    public static int[] stary = new int[100];
    public static Color[] starc = new Color[100];

    public static JFrame frame;
    public static Gravity canvas;
    public static Image buffer;
    public static Graphics bg;

    public static int[] xh = new int[1000];
    public static int[] yh = new int[1000];

    public static int i = 0;

    public static Image title;

    public static ArrayList<Integer> ptx = new ArrayList<Integer>();
    public static ArrayList<Integer> pty = new ArrayList<Integer>();
    double x = 100;
    double y = 100;

    public Gravity(){
    }

    public void paint (Graphics g) {
        frame.addMouseListener(new MouseListener(){
            public void mouseClicked(MouseEvent e){
                started = true;
                System.out.println("Mouse was clicked");
            }

            public void mouseEntered(MouseEvent arg0) {}
            public void mouseExited(MouseEvent arg0) {}
            public void mousePressed(MouseEvent arg0) {}
            public void mouseReleased(MouseEvent arg0) {}
        });


        buffer = createImage(screenw, screenh);
        bg = buffer.getGraphics();

        int w = getWidth();
        int h = getHeight();

        double px = getWidth()/2; 
        double py = getHeight()/2;

        bg.setColor(Color.BLACK);
        bg.fillRect(0, 0, w, h); //black background

        for (int j=0; j < 100; j++){ //make stars
            starx[j] = gen.nextInt(w);
            stary[j] = gen.nextInt(h);
            starc[j] = new Color(gen.nextInt(100)+156, gen.nextInt(100)+156, gen.nextInt(100)+156);
            bg.setColor(starc[j]);
            bg.drawLine(starx[j], stary[j], starx[j]+2, stary[j]+2);
            bg.drawLine(starx[j], stary[j]+2, starx[j]+2, stary[j]);
        }

        try {
            title = ImageIO.read(new ByteArrayInputStream(Base64.decode(""))); //I have omitted the Base64 code for the image for my title screen
        } catch (IOException e) {
            e.printStackTrace();
        }

        bg.drawImage(title, 100, 100, null);
        g.drawImage(buffer, 0, 0, null);

        while (!started){
            try {
                Thread.sleep(50);
            } catch (InterruptedException e1) {
                e1.printStackTrace();
            }
        }

        double xvel = -15;
        double yvel = 10;

        for (int j=0; j < 100; j++){ //store stars
            starx[j] = gen.nextInt(w);
            stary[j] = gen.nextInt(h);
            starc[j] = new Color(gen.nextInt(100)+156, gen.nextInt(100)+156, gen.nextInt(100)+156);
        }

        Image test = createImage(200,200);
        Graphics testg = test.getGraphics();
        testg.drawLine(50,50,150,150);

        while(true){
            g.drawImage(buffer, 0,0, null);
            try {
                Thread.sleep(33);
            } catch (Exception e) {
                e.printStackTrace();
            }

            bg.setColor(Color.BLACK);
            bg.fillRect(0, 0, w, h); //black background


            for (int j=0; j < 100; j++){ //draw stars
                bg.setColor(starc[j]);
                bg.drawLine(starx[j], stary[j], starx[j]+2, stary[j]+2);
                bg.drawLine(starx[j], stary[j]+2, starx[j]+2, stary[j]);
            }

            bg.setColor(Color.BLUE);

            if (i > 0){
                for (int z=0; z < i-1; z++){
                    bg.drawLine(ptx.get(z), pty.get(z), ptx.get(z+1), pty.get(z+1));
                }
            }

            bg.setColor(Color.CYAN);
            bg.fillOval((int)px, (int)py, 25, 25); //planet

            bg.setColor(Color.RED);
            bg.fillRect((int)(x-5),(int)(y-5),10,10); //ship

            double fg = (5*50000)/(Math.pow(dist(x,y,px,py),2));

            double m = (y-py)/(x-px);
            double ms = Math.sqrt(Math.abs(m));
            if (m < 0) ms = -ms;

            double xchg = fg;
            double ychg = fg*ms;

            if (x > px){
                xchg = -xchg;
                ychg = -ychg;
            }

            xvel += xchg;
            yvel += ychg;

            x += xvel;
            y += yvel;

            ptx.add((int)x);
            pty.add((int)y);

            i++;
        }
    }

    public static void main(String[] args){

        canvas = new Gravity();
        frame = new JFrame();
        frame.setSize(screenw, screenh);
        frame.setResizable(false);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(canvas);

        frame.setVisible(true);
    }


    public static double dist(double x1, double y1, double x2, double y2){
        double x = x2-x1;
        double y = y2-y1;
        return Math.sqrt((x*x)+(y*y));
    }
}

Answer 1

Use a JComponent instead of a Canvas. You'll want to add the mouse listener to that object. You'll also need to set up the mouse listener in the constructor, not the paint() method.

Edit: You're doing to much in the paint() method as @AndrewThompson pointed out.

public Gravity() {
    addMouseListener(new MouseListener() {

        @Override
        public void mouseClicked(MouseEvent e) {
            System.out.println("Mouse was clicked");
        }

        @Override
        public void mouseEntered(MouseEvent arg0) {
        }

        @Override
        public void mouseExited(MouseEvent arg0) {
        }

        @Override
        public void mousePressed(MouseEvent arg0) {
        }

        @Override
        public void mouseReleased(MouseEvent arg0) {
        }
    });
}

@Override
public void paint(Graphics g) {

    buffer = createImage(screenw, screenh);
    bg = buffer.getGraphics();

    ...

    bg.setColor(Color.BLACK);
    bg.fillRect(0, 0, w, h); // black background

    for (int j = 0; j < 100; j++) { // make stars
        ...
    }

    bg.drawImage(title, 100, 100, null);
    g.drawImage(buffer, 0, 0, null);

    double xvel = -15;
    double yvel = 10;

    for (int j = 0; j < 100; j++) { // store stars
        ...
    }

    Image test = createImage(200, 200);
    Graphics testg = test.getGraphics();
    testg.drawLine(50, 50, 150, 150);

    g.drawImage(buffer, 0, 0, null);
    bg.setColor(Color.BLACK);
    bg.fillRect(0, 0, w, h); // black background

    for (int j = 0; j < 100; j++) { // draw stars
        ...
    }

    bg.setColor(Color.BLUE);

    if (i > 0) {
        for (int z = 0; z < i - 1; z++) {
            bg.drawLine(ptx.get(z), pty.get(z), ptx.get(z + 1), pty.get(z + 1));
        }
    }

    bg.setColor(Color.CYAN);
    bg.fillOval((int) px, (int) py, 25, 25); // planet

    bg.setColor(Color.RED);
    bg.fillRect((int) (x - 5), (int) (y - 5), 10, 10); // ship

    ....

    ptx.add((int) x);
    pty.add((int) y);
}

public static void main(String[] args) {
    ...

    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

    ...
}

Notes about your code:

1. Avoid making fields public static. 99.9% of the time this is not necessary, and usually leads to trouble later.
2. The painting code seems to be unnecessarily using buffers--the test Image is not used at all currently!
3. I removed the while(true) loop, but noticed you're drawing an image, modifying the image, and then drawing the image again. You can probably do this in one go.
4. You should be able to avoid using an Image buffer altogether since you're creating a new one each time paint() is called, and clearing it at the beginning. Just draw your graphics directly to g.
5. Avoid doing I/O from within the paint() method. Load your images during construction or in a background thread.

Answer 2

General tips, in point form:

1. Don't mix AWT (e.g. Canvas) with Swing (e.g. JFrame) components. Instead of the Canvas, use a JPanel and override paintComponent(Graphics) rather than paint(Graphics).
2. Don't call Thread.sleep(n) on the EDT. Instead use a Swing based Timer to call repaint()
3. Don't perform long running operations on the paint method, especially starting an infinite loop! Even the creation of the buffer image should only be done in the case the screen size changes. (left as 'TODO' - BNI)
4. Add the MouseListener once in the constructor or an init() method rather than every time paint is called.
5. Highly recommend all of Nate's numbered list of notes.

Try this code, and compare it carefully to the original to see the changes.

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Dimension;
import java.awt.event.*;
import java.util.ArrayList;

import java.io.*;

import javax.imageio.ImageIO;
import javax.swing.*;
import java.util.Random;

public class Gravity extends JPanel {

    public static final int screenw = 800;
    public static final int screenh = 600;

    public static Random gen = new Random();

    public static int[] starx = new int[100];
    public static int[] stary = new int[100];
    public static Color[] starc = new Color[100];

    public static Image buffer;
    public static Graphics bg;

    public static int[] xh = new int[1000];
    public static int[] yh = new int[1000];

    public static int i = 0;

    public static ArrayList<Integer> ptx = new ArrayList<Integer>();
    public static ArrayList<Integer> pty = new ArrayList<Integer>();
    double x = 100;
    double y = 100;

    Timer timer;

    public Gravity(){
        // set thre PREFERRED size!
        setPreferredSize(new Dimension(screenw, screenh));
        addMouseListener(new MouseListener(){
                    public void mouseClicked(MouseEvent e){
                        System.out.println("Mouse was clicked");
                        timer.start();
                    }

                    public void mouseEntered(MouseEvent arg0) {}
                    public void mouseExited(MouseEvent arg0) {}
                    public void mousePressed(MouseEvent arg0) {}
                    public void mouseReleased(MouseEvent arg0) {}
        });
        ActionListener animation = new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                repaint();
            }
        };
        timer = new Timer(50, animation);
    }

    @Override
    public void paintComponent(Graphics g) {
        buffer = createImage(screenw, screenh);
        bg = buffer.getGraphics();

        int w = getWidth();
        int h = getHeight();

        double px = getWidth()/2;
        double py = getHeight()/2;

        bg.setColor(Color.BLACK);
        bg.fillRect(0, 0, w, h); //black background

        for (int j=0; j < 100; j++){ //make stars
            starx[j] = gen.nextInt(w);
            stary[j] = gen.nextInt(h);
            starc[j] = new Color(gen.nextInt(100)+156, gen.nextInt(100)+156, gen.nextInt(100)+156);
            bg.setColor(starc[j]);
            bg.drawLine(starx[j], stary[j], starx[j]+2, stary[j]+2);
            bg.drawLine(starx[j], stary[j]+2, starx[j]+2, stary[j]);
        }

        g.drawImage(buffer, 0, 0, null);

        double xvel = -15;
        double yvel = 10;

        for (int j=0; j < 100; j++){ //store stars
            starx[j] = gen.nextInt(w);
            stary[j] = gen.nextInt(h);
            starc[j] = new Color(gen.nextInt(100)+156, gen.nextInt(100)+156, gen.nextInt(100)+156);
        }

        Image test = createImage(200,200);
        Graphics testg = test.getGraphics();
        testg.drawLine(50,50,150,150);

        g.drawImage(buffer, 0,0, null);
        try {
            Thread.sleep(33);
        } catch (Exception e) {
            e.printStackTrace();
        }

        bg.setColor(Color.BLACK);
        bg.fillRect(0, 0, w, h); //black background


        for (int j=0; j < 100; j++){ //draw stars
            bg.setColor(starc[j]);
            bg.drawLine(starx[j], stary[j], starx[j]+2, stary[j]+2);
            bg.drawLine(starx[j], stary[j]+2, starx[j]+2, stary[j]);
        }

        bg.setColor(Color.BLUE);

        if (i > 0){
            for (int z=0; z < i-1; z++){
                bg.drawLine(ptx.get(z), pty.get(z), ptx.get(z+1), pty.get(z+1));
            }
        }

        bg.setColor(Color.CYAN);
        bg.fillOval((int)px, (int)py, 25, 25); //planet

        bg.setColor(Color.RED);
        bg.fillRect((int)(x-5),(int)(y-5),10,10); //ship

        double fg = (5*50000)/(Math.pow(dist(x,y,px,py),2));

        double m = (y-py)/(x-px);
        double ms = Math.sqrt(Math.abs(m));
        if (m < 0) ms = -ms;

        double xchg = fg;
        double ychg = fg*ms;

        if (x > px){
            xchg = -xchg;
            ychg = -ychg;
        }

        xvel += xchg;
        yvel += ychg;

        x += xvel;
        y += yvel;

        ptx.add((int)x);
        pty.add((int)y);

        i++;
    }

    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame();

                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.getContentPane().add(new Gravity());
                frame.setResizable(false);
                frame.pack();

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


    public static double dist(double x1, double y1, double x2, double y2){
        double x = x2-x1;
        double y = y2-y1;
        return Math.sqrt((x*x)+(y*y));
    }
}

Answer 3

Your paint() method is tying up the Thread that handles the mouse click event, due to the while (!started) loop that never exits. started is never true because the MouseListener's mouseClicked() is never called because the paint() method is waiting for started to be true! If you remove that loop, the while (true) loop will have a similar effect. Any mouse events that happen while paint() is running will be queued until paint() returns.