Making BoxLayout move components to top while stacking left to right

Question

Hi,

I have a JPanel which uses a BoxLayout in the X_AXIS direction. The problem I have is best shown by an image:

As you can see the JPanel on the left is has been centered rather than aligned at the top. I would like them both to be aligned at the top and stacked left to right, how can I achieve this with this layout manager? The code I have written is as follows:

public GameSelectionPanel(){

    setLayout(new BoxLayout(this, BoxLayout.X_AXIS));

    setAlignmentY(TOP_ALIGNMENT);

    setBorder(BorderFactory.createLineBorder(Color.black));

    JPanel botSelectionPanel = new JPanel();

    botSelectionPanel.setLayout(new BoxLayout(botSelectionPanel, BoxLayout.Y_AXIS));

    botSelectionPanel.setBorder(BorderFactory.createLineBorder(Color.red));

    JLabel command = new JLabel("Please select your opponent:"); 

    ButtonGroup group = new ButtonGroup();

    JRadioButton button1 = new JRadioButton("hello world");
    JRadioButton button2 = new JRadioButton("hello world");
    JRadioButton button3 = new JRadioButton("hello world");
    JRadioButton button4 = new JRadioButton("hello world");

    group.add(button1);
    group.add(button2);
    group.add(button3);
    group.add(button4);

    botSelectionPanel.add(command);
    botSelectionPanel.add(button1);
    botSelectionPanel.add(button2);
    botSelectionPanel.add(button3);
    botSelectionPanel.add(button4);

    JPanel blindSelectionPanel = new JPanel();
    blindSelectionPanel.setBorder(BorderFactory.createLineBorder(Color.yellow));

    blindSelectionPanel.setLayout(new BoxLayout(blindSelectionPanel, BoxLayout.Y_AXIS));

    JRadioButton button5 = new JRadioButton("hello world");
    JRadioButton button6 = new JRadioButton("hello world");

    ButtonGroup group2 = new ButtonGroup();
    group2.add(button5);
    group2.add(button6);

    JLabel blindStructureQuestion = new JLabel("Please select the blind structure:");

    blindSelectionPanel.add(blindStructureQuestion);
    blindSelectionPanel.add(button5);
    blindSelectionPanel.add(button6);

    add(botSelectionPanel);
    add(blindSelectionPanel);

    setVisible(true);
}

Answer 1

Well, the setAlignmentY method has no effect here, since it acts on the panel considered as a component.

As you have guessed, the layout of contained panels is defined by the layout manager you use. Unfortunatly, BoxLayout do not provide the kind of feature you're looking at.

in standard JDK, obviously, the layout of choice for your problem is GridBagLayout. Although rather hard to understand at first, it will fast reveal to you its power in components arrangement.

using the useful GBC class, your components could be arranged as such :

setLayout(new GridBagLayout(this));

add(botSelectionPanel, new GBC(0,1).setAnchor(TOP));
add(blindSelectionPanel, new GBC(0,2).setAnchor(TOP));

or I think so ;-)

Answer 2

Riduidel is correct about setting setAlignmentY on the GameSelectionPanel itself, and GridBagLayout is an excellent alternative. If you prefer to stick with BoxLayout, the article Fixing Alignment Problems discusses the matter, suggesting "all the components controlled by a left-to-right Boxlayout should generally have the same Y alignment." In your example, add

botSelectionPanel.setAlignmentY(JPanel.TOP_ALIGNMENT);
blindSelectionPanel.setAlignmentY(JPanel.TOP_ALIGNMENT);