Thread: Confused about JPanel.getY()

I am a complete newcomer to Java, so apologies for this basic question. I have a JPanel sitting inside of a JFrame. I call the panel's getY() method, and expect the result to give me the y-coordinate of the panel's top edge in the frame's coordinate system. But it isn't even close. The below example demonstrates what I'm encountering. First it prints (after "XXX") the panel's y-coordinate according to getY() (and its border thickness, for good measure). Then, whenever the mouse moves, it prints the (x,y) coords of the mouse pointer in the frame's coordinate system.

According the output printed after XXX, the panel's y-coordinate is 58 and its border thickness if 4. But moving the mouse around on my screen, I'm still well inside the button panel at y-coordinate 58. The frame's border seems to start around y-coordinate 85 or so.

I assume this has something to do with the fact that I'm using a layout manager, but perhaps I'm mistaken. In any event, how do I get the correct y-coordinate for the top of the panel in question?

Thanks in advance for help!

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
 
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.BoxLayout;
import javax.swing.border.Border;
import javax.swing.BorderFactory;
 
public class Example extends JFrame implements MouseMotionListener {
 
    public Example() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
	// Get Pane and set layout
	Container pane = getContentPane();
	pane.setLayout(new BoxLayout(pane, BoxLayout.Y_AXIS));
 
	// Add Buttons
	JPanel buttonPanel = new JPanel();
	buttonPanel.setLayout(new BoxLayout(buttonPanel, BoxLayout.Y_AXIS));
	pane.add(buttonPanel);
 
	JButton button1 = new JButton("Button1");
	button1.setAlignmentX(Component.CENTER_ALIGNMENT);
        buttonPanel.add(button1);
 
	JButton button2 = new JButton("Button2");
	button2.setAlignmentX(Component.CENTER_ALIGNMENT);
        buttonPanel.add(button2);
 
	// Set up the drawing area
	JPanel drawingPanel = new JPanel();
	drawingPanel.setAlignmentY(Component.BOTTOM_ALIGNMENT);
	Border raisedBorder = BorderFactory.createRaisedBevelBorder();
	Border loweredBorder = BorderFactory.createLoweredBevelBorder();
	Border compoundBorder = BorderFactory.createCompoundBorder(raisedBorder, loweredBorder);
	drawingPanel.setBorder(compoundBorder);
	drawingPanel.setPreferredSize(new Dimension(1000,1000));
	drawingPanel.setMaximumSize(new Dimension(1000,1000));
	pane.add(drawingPanel);
 
	// Set up the main window
	pane.setPreferredSize(new Dimension(1050,1100));
 
	addMouseMotionListener(this);
        pack();
        setVisible(true);
	System.out.printf("XXX: %d, %d\n", drawingPanel.getY(),compoundBorder.getBorderInsets(drawingPanel).top);
    }
 
    public void paint(Graphics g) {
        paintComponents(g);
 
    }
 
    public void mouseMoved(MouseEvent me) {
	int x = me.getX();
	int y = me.getY();
	System.out.printf("(%d, %d)\n", x, y);
 
    }
 
    public void mouseDragged(MouseEvent me) {
    }
 
    public static void main(String[] args) {
        JFrame.setDefaultLookAndFeelDecorated(true);
        Example frame = new Example();
    }
}

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Your MouseListener is on the JFrame, so the coordinates that it prints out may or may not include the JFrame border, the top bar with the close/minimize buttons, etc. Your drawingPanel is added to your JPanel pane, so its coordinates do not include those things.

You might want to check out the SwingUtilities class for converting mouse locations to and from different coordinate systems.

The drawingPanel.getY() is in its parent's coordinate system- the JPanel you're calling pane. However, the MouseListener is on the JFrame, which is pane's parent. I don't really know why you're expecting anything different than what's happening.

Say I have panel1, which has a y coordinate (in its parent, let's say a JFrame) of zero. That panel1 contains two more JPanels, panelTop and panelBottom. The y-coord of panelTop (in panel1's coordinate system) is 0, and panelBottom's y-coord is 500. Now I add panel2 to panelBottom. What would you expect its y-coord (in its parent's, panelBottom's, coordinate system) to be? If your answer isn't zero, I recommend you put together a sample program that does exactly that.

Originally Posted by jnewcomer

I naively expected (pane.getY() == 0) which would imply that pane and the JFrame have the same y-coordinate system. If pane.getY() == 0, you would share my confusion, right? If so, you presumably expect pane.getY() > 0 (in fact, pane.getY() ~= 30 based on my numbers in my first post), right? I'll check that when I get home from work.

Thanks for your help!

Yeah, exactly. That's what I was trying to point out in my first post, sorry if I wasn't clear. I believe that ~30 is from the JFrame's title bar.