Panel Appearing in Centre using GridBagLayout

[Deirdre Hackett]

Hi,

I'm trying to add panels dynamically to a base panel. I am trying to use GridBagLayout in order to do this.

I would like the first panel that is added to appear in the top left hand side of the frame. However it is appearing half way down the frame.

I'm sorry if this seems to be a very stupid question but am really a beginner have been trying to figure it out for a day. Any help appreciated.

The main class code is below. As is the class AssignmentPanel.

Regards, Deirdre

 
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class FlowChartFrame {
 
    private JFrame f;
    private MyCanvas myCanvas;
    private JPopupMenu popup;
    private JMenuItem assignmentMenuItem,  whileMenuItem;
    
    private GridBagConstraints gBC;
    private GridBagLayout gBL = new GridBagLayout();
    
    //private AssignmentPanel assignmentPanel; 
    //private WhilePanel whilePanel; 
    //private AssignmentPanel assignmentPanel3;
    public FlowChartFrame() {
 
        f = new JFrame("Draw");
 
        myCanvas = new MyCanvas();
 
        popup = new JPopupMenu();
        assignmentMenuItem = new JMenuItem("Assignment");
        whileMenuItem = new JMenuItem("While");
        
        gBC = new GridBagConstraints();
        
    }
 
    private void launchFrame() {
 
        System.out.println("Created GUI on EDT? " +
                SwingUtilities.isEventDispatchThread());
 
        //set up frame
        f.setLayout(new BorderLayout());
       
        
        myCanvas.setLayout(gBL);
 
        assignmentMenuItem.addActionListener(new AssignHandler());
        whileMenuItem.addActionListener(new WhileHandler());
 
        popup.add(assignmentMenuItem);
        popup.add(whileMenuItem);
 
        MouseListener popupListener = new PopupListener(popup);
        myCanvas.addMouseListener(popupListener);
 
        //myCanvas.add(assignmentPanel);
        //myCanvas.add(whilePanel);
        //myCanvas.add(assignmentPanel3);
        
        gBC.weightx = 1.0;
        gBC.anchor = GridBagConstraints.WEST;
        gBC.fill = GridBagConstraints.HORIZONTAL;
        gBC.insets = new Insets(5,5,5,5);
        gBC.gridx = 0;
        gBC.gridy = 0;
 
        f.add(myCanvas, BorderLayout.CENTER);
 
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(500, 500);
        f.setVisible(true);
 
    }
 
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
 
            public void run() {
                FlowChartFrame fc = new FlowChartFrame();
                fc.launchFrame();
            }
        });
    }
 
    class MyCanvas extends JPanel {
 
        public MyCanvas() {
 

            setBorder(BorderFactory.createLineBorder(Color.black));
 
        }
 
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(450, 450);
        }
 
        @Override
        public void paintComponent(Graphics g) {
 
            System.out.println("MyCanvas paintComponent");
 
            // Let UI Delegate paint first, which 
            // includes background filling since 
            // this component is opaque.
            super.paintComponent(g);
        //g.drawLine(20, 20, 20, 40);
        }
    }
 
    private class AssignHandler implements ActionListener {
 
        public void actionPerformed(ActionEvent e) {
            JMenuItem source = (JMenuItem) (e.getSource());
            System.out.println("Action Performed " + source.getText());
            myCanvas.add(new AssignmentPanel(), gBC);
            gBC.gridy++;
            myCanvas.revalidate();
            
        }
    }
 
    private class WhileHandler implements ActionListener {
 
        public void actionPerformed(ActionEvent e) {
            JMenuItem source = (JMenuItem) (e.getSource());
            System.out.println("Action Performed " + source.getText());
            myCanvas.add(new WhilePanel(), gBC);
            gBC.gridy++;
            myCanvas.revalidate();
        }
    }
 
    class PopupListener extends MouseAdapter {
 
        JPopupMenu popup;
 
        PopupListener(JPopupMenu popupMenu) {
            popup = popupMenu;
        }
 
        public void mousePressed(MouseEvent e) {
            maybeShowPopup(e);
        }
 
        public void mouseReleased(MouseEvent e) {
            maybeShowPopup(e);
        }
 
        private void maybeShowPopup(MouseEvent e) {
 
            if (e.isPopupTrigger()) {
                popup.show(e.getComponent(),
                        e.getX(), e.getY());
 
                System.out.println("PopupListener");
 
            }
        }
    }
}
 

 

   

  
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.BorderFactory;
import javax.swing.JPanel;
 
public class AssignmentPanel extends JPanel {
    
    private int panelWidth = 110;
    private int panelHeight = 100;
    
    public AssignmentPanel() {
 
        setBorder(BorderFactory.createLineBorder(Color.black));
 
    }
 
    @Override
    public Dimension getPreferredSize() {
        return new Dimension(panelWidth, panelHeight);
    }
 
    @Override
    public void paintComponent(Graphics g) {
 
        System.out.println("Assignment paintComponent");
 
        // Let UI Delegate paint first, which 
        // includes background filling since 
        // this component is opaque.
        super.paintComponent(g);
        g.drawRect(panelWidth/2-50, panelHeight/2-25, 100, 50);
        g.drawLine(panelWidth/2, panelHeight/2-45, panelWidth/2, panelHeight/2-25);
        g.drawLine(panelWidth/2, panelHeight/2+25, panelWidth/2, panelHeight/2+45);
    }
 
}
  

[Michael Dunn]

when using GridBagLayout you must read all the documentation, otherwise
you'll find it increasingly frustrating trying to get your GUI correct.

from this link
http://java.sun.com/javase/6/docs/api/java/awt/GridBagLayout.html

you'll find this snippet, which answers your 'centering' question

"GridBagConstraints.weightx, GridBagConstraints.weighty
Used to determine how to distribute space, which is important for specifying
resizing behavior. Unless you specify a weight for at least one component in
a row (weightx) and column (weighty), all the components clump together in the
center of their container. This is because when the weight is zero (the default),
the GridBagLayout object puts any extra space between its grid of cells and the
edges of the container."

[Deirdre Hackett]

Thanks Michael for that.
I included the line
gBC.weighty = 1;
but i still don't seeem to be getting what I want. It still places the first placed JPanel in the centre (vertically). I would like the first JPanel to appear in the top left hand corner, without there being anything else placed in the panel.

Or perhaps I am using the wrong Layout manager entirely. Any suggestions on what might work better?

[Michael Dunn]

> Any suggestions on what might work better?

just going by your
gBC.gridy++;

BoxLayout (Y_AXIS) might be a better LayoutManager

[Deirdre Hackett]

Thanks MD for your help but I have now abandoned that approach entirely based on other advice. I am going back to drawing the graphics directly onto a JPanel. Still none the wiser but apparently JPanels aren't the way.