Java Object Movement

Question

I am trying to get a circle to move around the keyboard input. I can’t move the object at all. Can someone help me figure out what is wrong? Here is my code:

import java.awt.Color; import java.awt.Graphics; import java.awt.event.KeyEvent; import java.awt.event.KeyListener; import javax.swing.JPanel; public class AlienInvader extends JPanel implements KeyListener{ Constants constant = new Constants(); public void update() { constant.x += constant.xvel; addKeyListener(this); } public void paintComponent(Graphics g) { super.paintComponent(g); g.setColor(Color.MAGENTA); g.fillOval(constant.x, constant.y, 30, 30); repaint(); } @Override public void keyPressed(KeyEvent e) { System.out.println(constant.x); switch(e.getKeyCode()) { case KeyEvent.VK_LEFT: constant.xvel = -1; break; case KeyEvent.VK_RIGHT: constant.xvel = 1; break; } } @Override public void keyReleased(KeyEvent e) { switch(e.getKeyCode()) { case KeyEvent.VK_LEFT: constant.xvel = -1; break; case KeyEvent.VK_RIGHT: constant.xvel = 1; break; } } @Override public void keyTyped(KeyEvent arg0) { // TODO Auto-generated method stub } } 

I'm not sure what I'm doing wrong. I thought this was due to the fact that I did not call the update method, but when I added the if statement to paintComponent (so that it only calls itself once) and tried it, I was out of luck.

Answer 1

To get started, do not call repaint within any paintXxx method. Drawing methods are usually called in response to a repaint call, so you create a nasty, never ending, never consuming hell resource loop.

Secondly, the KeyListener only responds to key events when 1- The component to which they are registered is configured 2- When the component to which they are registered has focus.

In this case, they are a bad choice. Use key bindings instead

Third, you do not provide a preferredSize hint for use by layout managers. This may or may not be bad in your case, but it is possible that the component will be laid out with a size of 0x0

Example

Sort of....

 import java.awt.BorderLayout; import java.awt.Color; import java.awt.Dimension; import java.awt.EventQueue; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.event.KeyEvent; import javax.swing.AbstractAction; import javax.swing.ActionMap; import javax.swing.InputMap; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.KeyStroke; import javax.swing.Timer; import javax.swing.UIManager; import javax.swing.UnsupportedLookAndFeelException; public class MoveCircle { public static void main(String[] args) { new MoveCircle(); } public MoveCircle() { EventQueue.invokeLater(new Runnable() { @Override public void run() { try { UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) { } JFrame frame = new JFrame("Testing"); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setLayout(new BorderLayout()); frame.add(new TestPane()); frame.pack(); frame.setLocationRelativeTo(null); frame.setVisible(true); } }); } public class TestPane extends JPanel { private int xDelta = 0; private int keyPressCount = 0; private Timer repaintTimer; private int xPos = 0; private int radius = 10; public TestPane() { InputMap im = getInputMap(WHEN_IN_FOCUSED_WINDOW); ActionMap am = getActionMap(); im.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0, false), "pressed.left"); im.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0, false), "pressed.right"); im.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0, true), "released.left"); im.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0, true), "released.right"); am.put("pressed.left", new MoveAction(-2, true)); am.put("pressed.right", new MoveAction(2, true)); am.put("released.left", new MoveAction(0, false)); am.put("released.right", new MoveAction(0, false)); repaintTimer = new Timer(40, new ActionListener() { @Override public void actionPerformed(ActionEvent e) { xPos += xDelta; if (xPos < 0) { xPos = 0; } else if (xPos + radius > getWidth()) { xPos = getWidth() - radius; } repaint(); } }); repaintTimer.setInitialDelay(0); repaintTimer.setRepeats(true); repaintTimer.setCoalesce(true); } @Override public Dimension getPreferredSize() { return new Dimension(200, 200); } @Override protected void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2d = (Graphics2D) g.create(); g2d.setColor(Color.RED); g2d.drawOval(xPos, 0, radius, radius); g2d.dispose(); } public class MoveAction extends AbstractAction { private int direction; private boolean keyDown; public MoveAction(int direction, boolean down) { this.direction = direction; keyDown = down; } @Override public void actionPerformed(ActionEvent e) { xDelta = direction; if (keyDown) { if (!repaintTimer.isRunning()) { repaintTimer.start(); } } else { repaintTimer.stop(); } } } } } 

For instance...