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Thread: Why isn't this rectangle moving?

  1. #1
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    Question Why isn't this rectangle moving?

    Ok so I am trying to make a galaga game and I'm not getting very far at all. I just tried to make a rectangle be the spaceship and have you able to move it left and right, but it won't work. I added a keylistener and told it to add 5 to x when right arrowkey is hit and subtract 5 from x when the left arrow key is hit but the rectangle doesn't move when I run this program! Here is my code of what I did:

    package galaga;
     
    import java.awt.Color;
    import java.awt.Graphics;
    import java.awt.Image;
    import java.awt.Rectangle;
    import java.awt.event.KeyEvent;
    import java.awt.event.KeyListener;
    import java.awt.event.MouseEvent;
    import java.awt.event.MouseListener;
     
    import javax.swing.JFrame;
     
    public class mainmethod extends JFrame{
    	int x = 200, y= 300;
    	KL bob = new KL();
    	public mainmethod() {
    		super("Galaga");
    		setVisible(true);
    		setLocation(150,150);
    		setSize(400,400);
    		addKeyListener(bob);
    	}
     
    	public class KL implements KeyListener{
     
     
    		public void keyPressed(KeyEvent e) {
    			if(e.getKeyCode() == KeyEvent.VK_LEFT) {
    			x = x-5;
    			}
    			if(e.getKeyCode() == KeyEvent.VK_RIGHT) {
    			x = x+5;
    			}
    		}
    		public void keyReleased(KeyEvent e) {
     
     
    		}
     
    		@Override
    		public void keyTyped(KeyEvent e) {
     
     
    		}
    	}
    	public void paint(Graphics g) {
    		Image dbImage = createImage(getWidth(), getHeight());
    		Graphics dbGraphics = dbImage.getGraphics();
    		paintComponent(dbGraphics);
    		g.drawImage(dbImage, 0, 0, this);
    		}
     
    	public void paintComponent(Graphics g){
    		g.setColor(Color.CYAN);
    		Rectangle blarp = new Rectangle(x, y, 150, 150);
    		g.fillRect((int) blarp.getX(), (int) blarp.getY(), 90, 50);
     
    	}
     
     
     
     
     
     
     
     
     
     
    }

    Please help me and thanks if you take the time to read this


  2. #2
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    Default Re: Why isn't this rectangle moving?

    Quote Originally Posted by mkrage View Post
    Why isn't this rectangle moving?
    1. After changing x in the KeyListener routine you don't redraw(), so KeyListener has no effect on what you see.

    2. x and y are not used in the drawImage method, so no matter what you do with the value of x, nothing changes. (Hint: give initial values of x and y so that the rectangle is somewhere within the frame. Then use x,y in drawImage)



    Cheers!

    Z

  3. #3
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    Default Re: Why isn't this rectangle moving?

    Also you don't want to draw directly into a JFrame. Instead draw in the paintComponent override method of a JPanel or other class derived from JComponent. Be sure to call the super method first. Finally, your KeyListener won't work if the component listened to doesn't have the focus. For Swing applications it's usually better to use Key Bindings and not KeyListeners.

  4. #4
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    Default Re: Why isn't this rectangle moving?

    Quote Originally Posted by Zaphod_b View Post
    1. After changing x in the KeyListener routine you don't redraw(), so KeyListener has no effect on what you see.

    2. x and y are not used in the drawImage method, so no matter what you do with the value of x, nothing changes. (Hint: give initial values of x and y so that the rectangle is somewhere within the frame. Then use x,y in drawImage)



    Cheers!

    Z
    Ok for number 1 do i put repaint() (redraw() didnt work) inside the keylistener or where do i put it?
    And for number 2 idk what u mean by initialize x. Does that just mean do x=150 in the paint or paintcomponent method. Sorry I do not understand your basic helpful message. I am still a beginner to programing and am not taking a class, i am just doing this on free time but thank you soooooooo much for trying to help!


    -mkrage

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    Default Re: Why isn't this rectangle moving?

    Quote Originally Posted by curmudgeon View Post
    Also you don't want to draw directly into a JFrame. Instead draw in the paintComponent override method of a JPanel or other class derived from JComponent. Be sure to call the super method first. Finally, your KeyListener won't work if the component listened to doesn't have the focus. For Swing applications it's usually better to use Key Bindings and not KeyListeners.
    Ok first of all thank you for your help. Second its better if I draw on a JPanel an add it on and i am not familiar with key bindings and how they work! If you could tell a brief explanation on how they work and are set up and what parameter they take that would be much appreciated!

    Thanks for your help

    -mkrage

  6. #6
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    Default Re: Why isn't this rectangle moving?

    Quote Originally Posted by mkrage View Post
    I am still a beginner to programing
    That's OK. No one was born knowing this stuff, you know.

    First of all, you should heed curmudgeon's advice. (His screen name is a misnomer. Don't be fooled; he's not curmudgeonly at all. Well, not usually.)

    Anyhow...

    What I wanted to do was to get you to the point were you could actually see results you expect with the program you started with. I mean, this stuff is fun only when "something" works, right? Look for tutorial material to see better examples of the usual way of writing Swing graphics applications.

    In the meanwhile, look at documentation for the methods that you use. For example drawImage.

    So...

    Quote Originally Posted by mkrage View Post
    ...idk what u mean by initialize x...
    That means give the instance fields x (and y) valid values in the mainmethod class.

    First of all, you have this in the paint method:
            g.drawImage(dbImage, 0, 0, this);
    That does not depend on x and y, so changing x won't move the image.

    For now, try changing it to
            g.drawImage(dbImage, x, y, this);

    Next you have this to initialize the variables x and y.
    public class mainmethod extends JFrame{
    	int x = 200, y= 300;

    But the way you have set the frame size, that's outside the frame (won't show up on the screen).
    For now, change the initialization to something like
        int x = 75, y= 75;

    Finally, at the end of the keyPressed method, you need to repaint, otherwise, the new value of x won't be immediately effective on what you are seeing.

    You can even instrument that method to make sure the arrow keys are changing the value of x:
            public void keyPressed(KeyEvent e) {
                System.out.println("Entering keypressed(): x = " + x);
                if(e.getKeyCode() == KeyEvent.VK_LEFT) {
                x = x - 5;
                }
                if(e.getKeyCode() == KeyEvent.VK_RIGHT) {
                x = x + 5;
                }
                System.out.println("Repainting with x = " + x);
                repaint();
            }

    Anyhow once you see that it can be done, it's time to get on to the better way(s) of doing things that curmudgeon suggested.


    Cheers!

    Z
    Last edited by Zaphod_b; October 22nd, 2012 at 10:51 AM.

  7. #7
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    Default Re: Why isn't this rectangle moving?

    Yes thank you so much! I now have a moving rectangle! I now understand what you have to do to initialize and how you must use repaint! Thank you so much!

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