Drawing 2D Graphics in the center of a JPanel

Question

Hi,
Below is my GUI. The basic idea is, I want this to create a neat UI on any machine with any monitor resolution settings. Because I dont really know what will be the resolution of the target machine, I do not perform a setPreferred size on any component. I have several problems here.
1. My DiagramPanel.paintComponent method relies on the Panel's preferred size to draw a rectangle in the center of the panel. When a panel is layed out using BorderLayout, it expands it to fit the space. So is there some method I can use to know, what space the component panel is occupying (its dimension) ?
2. Also, when the frame is maximized or resized, the DiagramPanel's dimension changes. So I want to be able to redraw the diagram in the new center position. How can I do this ?
4. I am using the Frame size to be the screen dimension and set it to be maximized. So when I try to restore the window, it takes up the whole screen even the space including the windows taskbar. Is there someway to get the screen dimension leaving out the space for the taskbar ?
3. I dont set the DividerLocation of the contentPane because, I want it to be set automatically based on the contents of the leftPanel. But when i print out the dividerLocation after creating the UI, it prints -1. How can I get the current divider location then ?
import java.awt.BorderLayout;
import java.awt.Button;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.LayoutManager;
import java.awt.Toolkit;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.JTabbedPane;
import javax.swing.JTextArea;
@SuppressWarnings("serial")
public class MainFrame extends JFrame {
public MainFrame() {
createAndShowGUI();
private void createAndShowGUI() {
initMainFrame();
initMenus();
initPanels();
initContentPane();
setVisible(true);
System.out.println("Divider location "+ contentPane.getDividerLocation());
private void initMainFrame() {
setTitle("Main Frame");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setState(JFrame.NORMAL);
//GETS SCREEN SIZE
Dimension screen_Dimension = Toolkit.getDefaultToolkit().getScreenSize();
setSize(screen_Dimension.width, screen_Dimension.height);
setExtendedState(MAXIMIZED_BOTH);
private void initContentPane() {
getContentPane().add(contentPane);
contentPane.setEnabled(false); / *prevent the divider location from being moved* /
contentPane.setLeftComponent(leftpanel);
contentPane.setRightComponent(rightpanel);
contentPane.setOpaque(true);
private void initMenus() {
exitMenu.setActionCommand(EXIT_COMMAND);
file.add(exitMenu);
menub.add(file);
setJMenuBar(menub);
private void initPanels() {
initLeftPanel();
initRightPanel();
private void initLeftPanel() {
leftpanel.setLayout(new FlowLayout(FlowLayout.CENTER));
leftpanel.add(new Button("Click"));
private void initRightPanel() {
LayoutManager border = new BorderLayout();
rightTab.setLayout(border);
scrollingDiagram = new JScrollPane(diagramPanel);
diagramDescriptionPanel.setLayout(new BorderLayout());
diagramDescriptionPanel.add(diagDescTextArea, BorderLayout.CENTER);
rightTab.add(scrollingDiagram, BorderLayout.CENTER);
rightTab.add(diagramDescriptionPanel, BorderLayout.SOUTH);
rightpanel.addTab("Right Tab", rightTab);
public static void main(String args[]) {
try {
new MainFrame();
} catch (Exception e) {
e.printStackTrace();
System.exit(0);
private JSplitPane contentPane = new JSplitPane();
private JMenuBar menub = new JMenuBar();
private JMenu file = new JMenu("File");
private JMenuItem exitMenu = new JMenuItem("Exit");
private JPanel leftpanel = new JPanel();
private JTabbedPane rightpanel = new JTabbedPane();
private JPanel rightTab = new JPanel();
private JScrollPane scrollingDiagram;
private DiagramPanel diagramPanel = new DiagramPanel();
private JPanel diagramDescriptionPanel = new JPanel();
private JTextArea diagDescTextArea = new JTextArea(18, 45);
public static final String EXIT_COMMAND = "Exit";
@SuppressWarnings("serial")
class DiagramPanel extends JPanel{
public static int RECT_WIDTH = 100;
public static int RECT_HEIGHT = 75;
public void paintComponent(Graphics g) {
super.paintComponents(g);
Coordinates centerOfPanel = getCenterCoordinates();
g.drawRoundRect(centerOfPanel.x, centerOfPanel.y, RECT_WIDTH, RECT_HEIGHT, 10, 10);
public Coordinates getCenterCoordinates() {
Dimension panelDimension = getPreferredSize();
int x = (panelDimension.width - RECT_WIDTH) / 2;
int y = (panelDimension.width - RECT_HEIGHT) / 2;
return new Coordinates(x,y);
class Coordinates {
int x;
int y;
Coordinates(int x, int y) {
this.x = x;
this.y = y;

Answer 1

Hi,
The getSize worked perfectly for me. But what I tried doing now is that, instead of just the simple centering of the rectangle, i did this
1. When the dimension of the rectangle is smaller than the Panel.getSize(), do centering as usual
2. else when the rectangle is bigger, say because its width was longer than Panel.getSize().getWidth(), I center the rectangle only on its Height, and set the preferred size of the Panel to (rectangle.getWidth(), panel.getSize().getHeight()). And I call revalidate, so that the scrollbars appear.
Problem im running into now is that, once I do a setPreferredSize, everytime I use a getSize later on, i always get the same size. But what I want to see is that, when the rectangle can fit in the panel, i want it centered without any scrollbars, but when it cannot fit on the width, i want it to appear centered on height, but have a horizontal scroll bar.
I'd be really grateful, if you could help me with this.
Here is the code
public void paintComponent(Graphics g) {
          super.paintComponents(g);
          Coordinates centerOfPanel = getCenterCoordinates();
          g.drawRoundRect(centerOfPanel.x, centerOfPanel.y, RECT_WIDTH, RECT_HEIGHT, 10, 10);
          //preferredSize might have changed, call revalidate
          revalidate();
public static final int PANEL_MARGIN = 50;
public Coordinates getCenterCoordinates() {
          setPreferredSize(null);
          Dimension panelDimension = getSize();
          Dimension myPreferredSize = new Dimension();
          if (panelDimension.width > RECT_WIDTH)
               myPreferredSize.width = panelDimension.width;
          else
               myPreferredSize.width = RECT_WIDTH + 2 * PANEL_MARGIN; // MARGIN on left end and right end
          if (panelDimension.height > RECT_HEIGHT)
               myPreferredSize.height = panelDimension.height;
          else
               myPreferredSize.height = RECT_HEIGHT + 2 * PANEL_MARGIN; // MARGIN on top and bottom
          System.out.println("Panel size is " + getSize());
          setPreferredSize(myPreferredSize);
          System.out.println("Preferred size is " + myPreferredSize);
          //center of the panel.
          int x = (myPreferredSize.width - RECT_WIDTH) / 2;
          int y = (myPreferredSize.height - RECT_HEIGHT) / 2;
          return new Coordinates(x,y);
     }