java - How to move an image (animation)?

Question

I'm trying to move the boat on the x-axis (without the keyboard yet). How can I relate the movement/ animation to the boat.png and not to any other image?

public class Mama extends Applet implements Runnable {

    int width, height;
    int x = 200;
    int y = 200;
    int dx = 1;
    Image img1, img2, img3, img4;

    @Override
    public void init(){
        setSize(627, 373);
        Thread t = new Thread(this);
        img1 = getImage(getCodeBase(),"Background.png");
        img2 = getImage(getCodeBase(), "boat.png");
        img3 = getImage(getCodeBase(), "LeftPalm.png");
        img4 = getImage(getCodeBase(), "RightPalm.png");

    }

    @Override
    public void paint(Graphics g){
        g.drawImage(img1, 0, 0, this);
        g.drawImage(img2, 200, 200, this);
        g.drawImage(img3, 40, 100, this);
        g.drawImage(img4, 450, 130, this);
    }

    @Override
    public void run() {
        while(true){

            x += dx;
            repaint();
            try {
                Thread.sleep(17);
            } catch (InterruptedException e) {
                System.out.println("Thread generates an error.");
            }
        }
    }
} 

Answer 1

There are a few things that stand out...

The "Problem"

As has already been stated, you need to supply variable arguments to the image drawing process. g.drawImage(img2, x, y, this);, this will allow you define where the image should be painted.

While you've implemented Runnable, you've actually not started any threads to call it. This means, nothing is actually changing the variables.

In you start method, you should be calling something like new Thread(this).start().

Recommendations

Although you've tagged the question as Swing, you're using AWT components. This isn't recommended (in fact applets are generally discouraged as they are troublesome - IMHO). The other problem, as you are bound to find out shortly, is they are not double buffered, this generally leads to flickering when performing animation, which isn't desirable.

As a side note, it is also discouraged to override the paint method of top level containers like Applet. Top level containers tend to contain a number additional components, by overriding the paint method like this, you destroy this setup. Also, top level containers don't tend to be double buffered either.

The example below uses a JFrame, but it wouldn't take much to convert it to use a JApplet (just drop the AnimationPanel on to it. This is another reason why extending from top level containers is generally discouraged ;)

public class AnimatedBoat {

    public static void main(String[] args) {
        new AnimatedBoat();
    }

    public AnimatedBoat() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new AnimationPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }

        });
    }

    public class AnimationPane extends JPanel {

        private BufferedImage boat;
        private int xPos = 0;
        private int direction = 1;

        public AnimationPane() {
            try {
                boat = ImageIO.read(new File("boat.png"));
                Timer timer = new Timer(40, new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        xPos += direction;
                        if (xPos + boat.getWidth() > getWidth()) {
                            xPos = getWidth() - boat.getWidth();
                            direction *= -1;
                        } else if (xPos < 0) {
                            xPos = 0;
                            direction *= -1;
                        }
                        repaint();
                    }

                });
                timer.setRepeats(true);
                timer.setCoalesce(true);
                timer.start();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return boat == null ? super.getPreferredSize() : new Dimension(boat.getWidth() * 4, boat.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            int y = getHeight() - boat.getHeight();
            g.drawImage(boat, xPos, y, this);

        }

    }

}