Unexpected space added to a JSplitPane

Question

Hello,
there is a strange problem with a JSplitPane that I cannot figure out. As seen below, a JSplitPane and another JPanel are added to a JPanel with a BoxLayout, creating a gray area above the JSplitPane. When commenting the code below (XXX), the space disappears. Any hint or suggestion of how to get rid of that unwanted area are more than welcome!
Thanks a lot,
Eph
package test;
import java.awt.Color;
import java.awt.Dimension;
import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSplitPane;
public class Bug extends JFrame
public Bug()
create_content();
setPreferredSize(new Dimension(600, 400));
pack();
private void create_content()
setTitle("Bug Tester");
// Main Panel
JPanel main_panel = new JPanel();
main_panel.setLayout(new BoxLayout(main_panel, BoxLayout.X_AXIS));
setContentPane(main_panel);
// Split Pane with two labels
JSplitPane test = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT);
test.setBackground(Color.YELLOW);
test.setLeftComponent(new JLabel("1"));
test.setRightComponent(new JLabel("2"));
main_panel.add(test);
// Another pane
// (XXX) Commenting the following lines makes the extra space disappear
JPanel test2 = new JPanel();
test2.setBackground(Color.BLUE);
test2.add(new JLabel("3"));
main_panel.add(test2);
public static void main(String[] args)
javax.swing.SwingUtilities.invokeLater(new Runnable()
public void run()
Bug b = new Bug();
b.setVisible(true);
}

Answer 1

Looks like the default Y-axis alignment of a split pane is 0.0f while that of a panel is 0.5f. BoxLayout is bit odd in that it gets the layout constraints directly from the JComponent instead of a separate constraint value (like most other layouts).
Add the following to fix your example:
test.setAlignmentY(0.5f);