Why do I have to add a JSeparator twice?

Question

In my code, I am adding some JSeparators. Everything is working fine, except that in the beginning, I have to add two JSeparators for them to show, instead of one! Here's the code:

    setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
    int counter = 1;
    add(new JSeparator(SwingConstants.HORIZONTAL)); //Here is the
    add(new JSeparator(SwingConstants.HORIZONTAL)); //problem
    JPanel p2 = new JPanel();
    for (int i = 0; i < petsx; i ++) {
        for (int i2 = 0; i2 < petsy; i2 ++) {
            p2.add(new JLabel(new ImageIcon(pics[i][i2])));
            p2.add(new JLabel(names[i][i2]));
            if (counter % petsx == 0) {
                add(p2);
                add(new JSeparator(SwingConstants.HORIZONTAL));
                p2 = new JPanel();
                counter = 0;
            } else {
                JSeparator js = new JSeparator(SwingConstants.VERTICAL);
                js.setPreferredSize(new Dimension(1, newPetHeight));
                p2.add(js);
            }
            counter ++;
        }
    }

As you can see, I have to call add(new JSeparator(SwingConstants.HORIZONTAL)); twice for it to work. If I only call it once, the JSeparator does not show up. Why is this?

Here is the compilable code: http://pastebin.com/xbe47a29

no way to tell without an sscce. BTW: [never-ever call setXXSize](http://stackoverflow.com/questions/7229226/should-i-avoid-the-use-of-setpreferredmaximumminimumsize-methods-in-java-swi/7229519#7229519) on _anything_ (not even a separator), instead use a suitable LayoutManager. — kleopatra, Nov 06 '12 at 13:45

score 2 · Accepted Answer · answered Nov 06 '12 at 19:05

I have no problems...

enter image description here

I suspect that you're possibly doing one or more of the following

The first separator may be getting aligned to the very top of the container, right against the title bar, making it "appear" as if it's not being added..
You're not creating your UI in the EDT
You're showing the frame before you finish creating the pane
Something else you're not telling us

.

public class TestSeperator {

    public static void main(String[] args) {
        new TestSeperator();
    }

    public TestSeperator() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestSeperatorPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestSeperatorPane extends JPanel {

        public TestSeperatorPane() {
            setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
            int counter = 1;
            add(new JSeparator(SwingConstants.HORIZONTAL)); //Here is the
            add(new JSeparator(SwingConstants.HORIZONTAL)); //problem
            JPanel p2 = new JPanel();
            int petsx = 4;
            int petsy = 4;
            for (int i = 0; i < petsx; i++) {
                for (int i2 = 0; i2 < petsy; i2++) {
                    p2.add(new JLabel(":)"));
                    p2.add(new JLabel("A"));
                    if (counter % petsx == 0) {
                        add(p2);
                        add(new JSeparator(SwingConstants.HORIZONTAL));
                        p2 = new JPanel();
                        counter = 0;
                    } else {
                        JSeparator js = new JSeparator(SwingConstants.VERTICAL);
                        // This is a bad idea...
                        //js.setPreferredSize(new Dimension(1, newPetHeight));
                        js.setBorder(new EmptyBorder(6, 0, 6, 0));
                        p2.add(js);
                    }
                    counter++;
                }
            }
        }
    }
}

2. You're not creating your UI in the EDT ~ How would I go about doing this? — tckmn, Nov 06 '12 at 23:05
@PicklishDoorknob Take a look at the constructor from the example. The statement `EventQueue.invokeLater` is ensuring that all calls within the runnable are executed within the EDT — MadProgrammer, Nov 06 '12 at 23:35
Okay, I will try that. Edit: Problem. I have the statement `f.add(this);` in my class (`this` is the main JPanel), and when wrapped in the Runnable it assumes that the Runnable is `this`. — tckmn, Nov 06 '12 at 23:49
Something like `TestSeperatorPane.this` might suffice. If you're executing `invokeLater` inside a UI component's constructor then you're already to late. Simple rule, you want to interact with the UI (especially change it), you MUST do it from within the EDT. — MadProgrammer, Nov 07 '12 at 00:02

Why do I have to add a JSeparator twice?

1 Answers1