Swing components have no width or height until they’ve been rendered. This occurs if you call pack()
or setVisible(true)
on a root container. Consider placing your x y int initialization code in the componentResized
method of a ComponentListener that is added to your JPanel.
e.g.,
import java.awt.event.*;
import java.awt.*;
import javax.swing.*;
public class TestLinePanel {
private static void createAndShowGui() {
LinePanel mainPanel = new LinePanel();
JFrame frame = new JFrame("TestLinePanel");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(mainPanel);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
static class LinePanel extends JPanel {
private static final int PREF_W = 400;
private static final int PREF_H = PREF_W;
private char keyChar="A";
private int x;
private int y;
private boolean xySet = false;
public LinePanel() {
setFocusable(true);
addKeyListener(new KeyAdapter() {
public void keyPressed(KeyEvent e) {
switch (e.getKeyCode()) {
case KeyEvent.VK_DOWN:
y += 10;
break;
case KeyEvent.VK_UP:
y -= 10;
break;
case KeyEvent.VK_LEFT:
x -= 10;
break;
case KeyEvent.VK_RIGHT:
x += 10;
break;
default:
keyChar = e.getKeyChar();
}
repaint();
}
});
addComponentListener(new ComponentAdapter() {
@Override
public void componentResized(ComponentEvent e) {
if (!xySet) {
int xCenter = getWidth() / 2;
int yCenter = getHeight() / 2;
x = xCenter;
y = yCenter;
requestFocusInWindow();
xySet = true;
}
}
});
}
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setFont(new Font("TimesRoman", Font.PLAIN, 24));
g.drawString(String.valueOf(keyChar), x, y);
}
}
}
You’ll also want to use key bindings rather than a KeyListener to capture your key strokes.