Java Help! - Overclock.net

pauldovi · 04-09-08

I don't understand where paintComponent(Graphics g) is being called in this program. Where is graphics g ever made?

Code:

import javax.swing.*;
import java.awt.Graphics;

public class TestPanelDrawing extends JFrame
{
     public TestPanelDrawing()
     {
          add(new NewPanel());
     }

     public static void main(String[] args)
     {
          TestPanelDrawing frame = new TestPanelDrawing();

          frame.setTitle("TestPanelDrawing");
          frame.setLocationRelativeTo(null);
          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          frame.setSize(100,100);
          frame.setVisible(true);
     }
}

class NewPanel extends JPanel
{
     protected void paintComponent(Graphics g)
     {
          super.paintComponent(g);
          g.drawLine(0,0,50,50);
          g.drawString("Banner",0,40);
     }
}

kdbolt70 · 04-09-08

Its been a while since I've done java, but it looks like that paintComponent function is just overloaded from JPanel. Therefore, whenever a JPanel's paintComponent function would be called, the NewPanel's is instead. The graphics object is probably created in the constructor of the JPanel... or somewhere similar. Any Java nuts want to chime in?

Licht · 04-10-08

Its an argument for the function. 'Graphics' is used as a data type and 'g' is the assigned name of the value. For example.

Code:

paintComponent(20)

'g' would then be equal to 20 when executing the code. Thats just an example i didn't read it well enough to get the jist.

I'm not 100% sure i'm just guessing from what i know, and what i know comes from C#, not Java.

pauldovi · 04-10-08

Quote:

Originally Posted by Licht

Its an argument for the function. 'Graphics' is used as a data type and 'g' is the assigned name of the value. For example.

Code:

paintComponent(20)

'g' would then be equal to 20 when executing the code. Thats just an example i didn't read it well enough to get the jist.

I'm not 100% sure i'm just guessing from what i know, and what i know comes from C#, not Java.

The thing is, the program never explicitly creates a graphics object, g and it never explicitly calls the method.

Licht · 04-10-08

Quote:

Originally Posted by pauldovi

The thing is, the program never explicitly creates a graphics object, g and it never explicitly calls the method.

Very puzzling.

h33b · 04-10-08

i'm not familiar much with java, but could it be a parent method called from the two imports at the top of the code?

rabidgnome229 · 04-10-08

Java does that stuff behind the scenes. When you do something with graphics it makes a new thread which is invisible to you. Somewhere in that thread it creates the graphics object and calls paintComponent. You don't need to know the context in which it is called, but you do need to specify what happens when the call is made