I'm writing a simple program to test out basic GUI. The program prints a letter in the middle of the screen and allows the user to move it with the arrow keys. Everything works fine, but when I try to center the letter at the start of the program, it seems that the getWidth
and getHeight
functions aren't returning the proper numbers.
Here's the snippet containing my Panel class
static class LinePanel extends JPanel{
int xCenter = getWidth() /2;
int yCenter = getHeight() /2;
private int x = xCenter;
private int y = yCenter;
private char keyChar = 'A';
public LinePanel(){
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();
}
});
}
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.setFont(new Font("TimesRoman", Font.PLAIN, 24));
g.drawString(String.valueOf(keyChar), x, y);
}
}
Why are my getWidth
and getHeight
functions returning '0'?
Thanks for any help
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.
I cannot tell the reason but:
A way to avoid this is to override your getPreferredSize()
function and return your preferred size.
Member variables are initialized when the object is created (after the constructors of any parent classes & before constructor body of the current class. Swing does not doing any sizing work when the object is first created. Therefore, when you call getWidth()
and getHeight()
, no values have been set yet.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With