Key Listeners

[wired-in=p]

I have been trying to design a simple key listener that listens to the enter key. here is my code;

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

public class pacman extends JFrame{
	public pacman()
	{
		super();
		setSize(500, 500);
		mypanel second = new mypanel();
		second.setFocusable(true);
		add(second);
		setVisible(true);
		setDefaultCloseOperation(3);
		validate();
	}
	public static void main(String args[])
	{
		pacman pac = new pacman();
		
	}
	public class mypanel extends JPanel
	{
		public int x;
		public int y;
		public mypanel()
		{
			super();
			 addKeyListener(new KeyAdapter(){
				public void pressedkey(KeyEvent e)
				{
					System.out.println("Here");
					if (e.getKeyCode()==KeyEvent.VK_UP)
					{
						y++;
					}
					else if (e.getKeyCode()==KeyEvent.VK_DOWN)
					{	
						y--;
					}
					else if (e.getKeyCode()==KeyEvent.VK_RIGHT)
					{
						x++;
					}
					else if (e.getKeyCode()==KeyEvent.VK_LEFT)
					{
						x--;
					}
					repaint();
				}
			});
		}
		public void paintComponent(Graphics g)
		{
			super.paintComponent(g);
			g.setColor(Color.yellow);
			x = this.getSize().width/2-15;
			y = this.getSize().height/2-15;
			g.fillOval(x, y, 30, 30);
			g.setColor(Color.black);
			this.setOpaque(true);
			this.setBackground(Color.black);	
		}
	}
}

I cannot understand why it will not work
I don't know whether i need to learn about key binding and setting the focus of the component

[Fubarable]

What API is pressedkey from? You can't make stuff up and expect it to work.

And yes, you should look into learning how to use key bindings.

Also: don't hard-code your x and y in the paintComponent method. What good will changing their values in your program if it's only going to be reset back to hard coded values whenever the component is painted?

Also: don't set background or opaque from within paintComponent. These should be set once in the constructor.

[wired-in=p]

what did i make up? and what do i need to add to make it work

[wired-in=p]

I still don't understand why the program won't run even after i set the focus to my panel.

[doWhile]

what did i make up? and what do i need to add to make it work

Read the API for KeyListener: pressedkey != keyPressed. For future reference, using the @Override annotation would have given you a compile time error and caught this error much earlier. And yes, look into using KeyBindings. Lastly, I recommend you take some time to read about java code conventions

[Fubarable]

Exactly, pressedkey doesn't exist. It may sound right, but since it was made up, it overrides no super method and the JVM won't respond to it as it would to a proper method override. To use these things you must use the references available including the API and the tutorials. That's what they're for.

[wired-in=p]

isn't pressed key a method in the interface KeyListener?

[Fubarable]

isn't pressed key a method in the interface KeyListener?

Again, please read the API; don't guess! It's online -- do you need the link?

[wired-in=p]

now the key pressed method works but the if e.getKeyCode == VK_UP or for any other arrow key won't work

[mine0926]

As suggested above, use Key Binding.
How to Use Key Bindings (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Other Swing Features)

[camickr]

> or for any other arrow key won't work

Define won't work. What do you expect to happen?

[JosAH]

Define won't work. What do you expect to happen?

Miracles; why ask? ;-)

kind regards,

Jos

[wired-in=p]

I read the key binding tutorial and it makes sense to me except for one thing.

component.getInputMap().put(KeyStroke.getKeyStroke ("SPACE"),
"pressed");
component.getInputMap().put(KeyStroke.getKeyStroke ("released SPACE"),
"released");
component.getActionMap().put("pressed",
pressedAction);
component.getActionMap().put("released",
releasedAction);
//where pressedAction and releasedAction are javax.swing.Action objects

I don't understand how to attach the "SPACE" string to the actual space key. I wouldn't know how to replace the space keystroke with any of the arrow keys in the code. Lastly, I don't know how to have the actions performed when a key is pressed or when its released.

[Fubarable]

There are several ways of getting the KeyStroke for key binding. Myself, I like to use KeyStroke.getKeyStroke(int, int), the first int is obtained by using the vk_ constants held by the KeyEvent class. For instance, assuming your code has a String constant for up:

   private static final String UP = "UP";

Then you could do one key binding like so:

      int condition = JComponent.WHEN_IN_FOCUSED_WINDOW;
      InputMap inputMap = getInputMap(condition);
      ActionMap actionMap = getActionMap();
      
      inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), UP);
      actionMap.put(UP, new AbstractAction() {
         public void actionPerformed(ActionEvent arg0) {
            y--;
            repaint();
         }
      });

For example:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;

import javax.swing.*;

@SuppressWarnings("serial")
public class MoveBall extends JPanel {
   private static final String UP = "UP";
   private static final String DOWN = "DOWN";
   private static final String LEFT = "LEFT";
   private static final String RIGHT = "RIGHT";
   private static final Color BACKGROUND = Color.black;
   private static final Color BALL_COLOR = Color.yellow;
   private static final int BALL_WIDTH = 30;
   private static final int PREF_WIDTH = 500;
   private static final int PREF_HEIGHT = PREF_WIDTH;
   private static final Dimension PREF_SIZE = new Dimension(PREF_WIDTH, PREF_HEIGHT);
   private int x = PREF_WIDTH / 2;
   private int y = PREF_HEIGHT / 2;
   
   public MoveBall() {
      setBackground(BACKGROUND);

      int condition = JComponent.WHEN_IN_FOCUSED_WINDOW;
      InputMap inputMap = getInputMap(condition);
      ActionMap actionMap = getActionMap();
      
      inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), UP);
      actionMap.put(UP, new AbstractAction() {
         public void actionPerformed(ActionEvent arg0) {
            y--;
            repaint();
         }
      });
      
      inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0), DOWN);
      actionMap.put(DOWN, new AbstractAction() {
         public void actionPerformed(ActionEvent arg0) {
            y++;
            repaint();
         }
      });
      
      // etc for left and right
      
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setColor(BALL_COLOR);
      g2.fillOval(x - BALL_WIDTH / 2, y - BALL_WIDTH / 2, 
            BALL_WIDTH, BALL_WIDTH);
   }
   
   @Override
   public Dimension getPreferredSize() {
      return PREF_SIZE;
   }

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

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

Edit: to get released vs pressed, look at the KeyStroke API for the appropriate method. Myself I'd use the getKeyStroke(int, int, boolean) for this.

[wired-in=p]

Thank you very much. One question, what is the 0 argument in KeyStroke.getKeyStroke for?

[Fubarable]

Thank you very much. One question, what is the 0 argument in KeyStroke.getKeyStroke for?

A good habit to get in to is to first check the API: KeyStroke API

[wired-in=p]

Thanks again, my program now works. however, i made the ball move one pixel when i press an arrow key. it was moving too slow so i changed it to 10 pixels, which is a better speed but it seems like it is jumping around. is there anyway i can lower the time it takes to press a key so the ball moves smoother?

[Fubarable]

Consider using a Timer for your game loop. You could have a right-arrow key-down change a moveRight boolean to true and key-up change it to false and the game loop would move the sprite to the right if (moveRight).