I obviously don't understand positioning panels

[javatard]

I suck so so badly at Java. I can't even grasp this simple concept

All Im trying to do is create a frame, then create a panel inside it and set the size and position of that panel, THATS IT
I have created a border on the panel and instead of the border showing around the sized panel, it shows around the whole frame! I dont know whats going on

import java.awt.*;
import javax.swing.*;
import java.awt.font.*;
import java.awt.geom.*;


public class RushHour
{
	public static void main(String[] args)
	{
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				RushHourFrame frame = new RushHourFrame();
				frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
				frame.setVisible(true);
			}
		});
	}
}

class RushHourFrame extends JFrame
{
	public RushHourFrame()
	{
		setTitle("Super Happy Fun Time Rush Hour Clone");
		setBounds(300,300, 300, 400);
		
		// Create panel
		gridPanel grid = new gridPanel();
		
		grid.setBounds(20,20,200,200);
		grid.setBorder(BorderFactory.createLineBorder(Color.pink));
		
		add(grid);
	}


}

class gridPanel extends JPanel
{
	public void paintComponent( Graphics g ) 
	{
		//super.paintComponent( g );
	    // Draw the vertical and horizontal lines:
		for(int i=0; i<rows; i++)
		{
			g.drawLine(i*cellsize,0, i*cellsize,(rows-1)*cellsize);
			g.drawLine(0,i*cellsize, (rows-1)*cellsize,i*cellsize);
		}
	}
	
	private final int rows = 6;
	private final int cellsize = 40;
}

Help is immensely appreciated!

[neilcoffey]

Frames (and containers in general-- e.g. JPanels) have a layout that dictates how things are positioned inside them. You don't normally set specific hard-coded coordinates with setBounds() (though with a slight modification you can).

The problem here is that by default, a JFrame has a layout of type BorderLayout. BorderLayout expects to handle the exact coordinates of the child components (the panel in this case) itself -- i.e. it won't take notice of your call to setBounds(). BorderLayout lays out your components in terms of fixed locations: NORTH, SOUTH, EAST, WEST and CENTER. Here it's taking your component as the CENTER component and making it fill the frame (because you haven't told it anything different).

So you have a couple of options:
(1) Tell the JFrame not to use any particular layout -- call setLayout(null) on it.
(2) Arguably the preferred option -- just let the component fill the frame, or use another more appropriate layout to suit your needs. If you do this, it's best to call explicitly:

add(grid, BorderLayout.CENTER);

[javatard]

Thanks for your reply Neil
I wasn't aware JFrame defaulted to BorderLayout, things at least make a little more sense now! (I thought it was FlowLayout like JFrames, I was wrong).

So when I setLayout to null, things work as I intended
When I use BorderLayout, things are not working

Here is the changed code

class RushHourFrame extends JFrame
{
	public RushHourFrame()
	{
		setTitle("Super Happy Fun Time Rush Hour Clone");
		setBounds(300,300, 300, 400);
		// By default, frame has BorderLayout manager
		
		// Create panel
		JPanel centerPanel = new JPanel();
		add(centerPanel, BorderLayout.CENTER);
		
		gridPanel grid = new gridPanel();
		grid.setSize(200,200);
		grid.setBorder(BorderFactory.createLineBorder(Color.pink));
		
		centerPanel.add(grid);
	}
}

The border is no longer around the whole frame but its now just a tiny square near the top :confused: