Sunday, May 08, 2016

Graphical User Interface Nightmares in Java

Compared to Windows Forms, doing GUI work in Java is painful because you have to deal with a lot more detail. Recently, I was trying to customize tab drawing in JTabbedPane to make the font of the selected tab bold and its background color green. I ended up creating a class (MyTabbedPaneUI) that extends BasicTabbedPaneUI and overrides paintTabBackground().
public class MyFrame extends javax.swing.JFrame {
    public MyFrame() {
        javax.swing.JTabbedPane jtp = new javax.swing.JTabbedPane();
        jtp.setUI(new MyTabbedPaneUI());
        jtp.add("My Tab 1", new javax.swing.JPanel());
        javax.swing.JLabel jl1 = new javax.swing.JLabel(jtp.getTitleAt(0));
        jtp.setTabComponentAt(0, jl1);
        jtp.add("My Tab 2", new javax.swing.JPanel());
        javax.swing.JLabel jl2 = new javax.swing.JLabel(jtp.getTitleAt(1));
        jtp.setTabComponentAt(1, jl2);
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new MyFrame().setVisible(true);
public class MyTabbedPaneUI extends javax.swing.plaf.basic.BasicTabbedPaneUI {
     * NOTE: Do not perform lengthy operations (e.g. setting font to bold) 
     * inside this paint method because it causes high CPU load and has 
     * side effects like not being able to update java3D drawings.
    protected void paintTabBackground(Graphics g, int tabPlacement, 
                       int tabIndex, int x, int y, int w, int h, 
                       boolean isSelected) {
        for (int i = 0; i < tabPane.getTabCount(); i++) {
            Color bgColor = Color.YELLOW;
            javax.swing.JLabel jl = (javax.swing.JLabel) 
            if (jl != null) {
                if (i != tabIndex) {
                    bgColor = Color.GREEN;
                } else {
            Rectangle rect = rects[i];
            int pad = 2;
            g.fillRect(rect.x+pad, rect.y+pad, rect.width-2*pad,
Setting font to bold (by uncommenting the lines commented as "BAD") in BasicTabbedPaneUI.paintTabBackground causes more than 7x CPU usage:

I have witnessed cases where CPU usage shot up to 100% and my app could not update a java3D drawing on another window. You can imagine that it was not easy to find out why.

Other examples of why Java should be avoided for GUI work:
  • Changing color of a JTable cell is a lot of work.
  • For background color to have any effect, JLabel has to be opaque while JTextArea has to be not opaque (opposite of their defaults)!
  • Due to the unintuitive layout mechanism, what I see on design view is completely different from what I see when I run the application.
  • When I change the layout, sometimes all the components dissappear (their width and height becomes zero).
  • Setting the width/height to "Preferred" sometimes causes the component to shrink to zero size. From what I understand, the layout mechanism is there to ensure proper resizing, i.e. to have a similar look when screen resolutions, font size etc. change. As usual, when trying the solve the most general case, you make it more difficult to solve simple cases.
  • To properly set size of a JFrame or JDialog, calling setPrefferedSize() is not enough, you have to call pack() afterwards.
  • When, in design mode, you rearrange buttons, labels or when you just change the border of panels, NetBeans might convert them to local variables and you have to manually convert them back to private to be able to reference them.
My layout strategy:
  • Create a frame
  • Add a panel with null layout.
  • Add subpanels to group components
  • Set the layout of subpanels. The layouts I use most often:
    • null
      • Advantage: You can set the location and size of components.
      • Disadvantage: If your form is resizable, components won't resize.
    • GridLayout;
    • BoxLayout

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