Thread: why paintComponent method is not invoked?

Hello guys

Below code can't display text "Hello world", I guess it's because paintComponent method is not invoked, but why it is not invoked?

Thanks heaps

package xxx;
 
import java.awt.*;
import javax.swing.*;
 
class TextPanel extends JPanel
{
// override the paintComponent method
public void paintComponent(Graphics g)
{
super.paintComponent(g);
Font f = new Font("SansSerif", Font.BOLD, 14);
Font fi = new Font("SansSerif", Font.BOLD + Font.ITALIC, 14);
FontMetrics fm = g.getFontMetrics(f);
 
int x = 75;
int y = 100;
g.setFont(f);
g.drawString("Hello, ", x, y);
x += fm.stringWidth("Hello, ");
g.setFont(fi);
g.drawString("World!", x, y);
} //paintComponent
 
public static void main(String[] args)
{
JFrame f = new JFrame("Your Frame");
f.show();
}
}

Hi ice,
maybe you lost this code:

public static void main(String[] args)
{
JFrame f = new JFrame("Your Frame");
f.setSize(300, 300);
TextPanel panel = new TextPanel();
f.add(panel);
f.show();
}

If you don't set size for f, f is too small to see "Hello Word!" String

Tell me somethig!
Bye

Hi guys

Thank you for your attention, I have absorbed member Lucasantos and Kevinworkman's advice: please see below code. but paintComponent method is still not invoked.
Does anyone know why?

Thanks heaps

////////////////////////////////////////////////////
package xxx;
 
import java.awt.*;
import javax.swing.*;
 
class TextPanel extends JPanel
{
// override the paintComponent method
public void paintComponent(Graphics g)
{
super.paintComponent(g);
Font f = new Font("SansSerif", Font.BOLD, 14);
Font fi = new Font("SansSerif", Font.BOLD + Font.ITALIC, 14);
FontMetrics fm = g.getFontMetrics(f);
 
int x = 75;
int y = 100;
g.setFont(f);
g.drawString("Hello, ", x, y);
x += fm.stringWidth("Hello, ");
g.setFont(fi);
g.drawString("World!", x, y);
} //paintComponent
 
public static void main(String[] args)
{
JFrame f = new JFrame("Your Frame");
f.setSize(300, 300);
TextPanel panel = new TextPanel();
f.add(panel);
f.show();
}
 
}

Please surround your code with the [highlight=java][/highlight] tags, it makes it much easier to read and help troubleshoot.
Some tips:
1) f.show is deprecated, you should use JFrame.setVisible(boolean b)
2) JFrame.getContentPane().add is more proper than JFrame.add
But neither of these is the problem. Some tips for the problem at hand
1) Add some System.out.println statements in the paintComponent to let you know it is being called.
2) Set the preferred size of the JPanel, and call pack() on the JFrame before setting it to visible. Make sure the size you set incorporates what you are drawing, so you can see the results without having to resize the JFrame.

Hi Copeg

Thank for very much your advice, now I add your tips in and put the code inside the tag. But it still shows a completely empty frame after I run it.

Any more thought?

package xxx;
 
import java.awt.*;
import javax.swing.*;
 
class TextPanel extends JPanel
{
// override the paintComponent method
public void paintComponent(Graphics g)
{
super.paintComponent(g);
Font f = new Font("SansSerif", Font.BOLD, 14);
Font fi = new Font("SansSerif", Font.BOLD + Font.ITALIC, 14);
FontMetrics fm = g.getFontMetrics(f);
 
int x = 75;
int y = 100;
g.setFont(f);
g.drawString("Hello, ", x, y);
x += fm.stringWidth("Hello, ");
g.setFont(fi);
g.drawString("World!", x, y);
} //paintComponent
 
public static void main(String[] args)
{
JFrame f = new JFrame("Your Frame");
f.pack();
            f.setVisible(true);
            f.setSize(300, 300);
            TextPanel panel = new TextPanel();
            f.getContentPane().add(panel);}
 
}

Hi Darryl
Thank you for interesting in this thread, I run the code in Netbean, it only comes out an empty window. So I guess the method is not called?