Center alignment of buttons in JPanel

Question

How do I align the buttons (S, S, D & D and A, U, D & d) to the middle of the panels (the blue and red)? The panels can grow (height not width), but the buttons should stay in the middle and keep the same size. So GridLayout is out.

    import javax.swing.JFrame;
    import javax.swing.JPanel;
    import javax.swing.JTable;

    import java.awt.Dimension;
    import java.awt.FlowLayout;
    import java.awt.GridLayout;

    import javax.swing.JButton;
    import java.awt.BorderLayout;
    import java.awt.Color;


    public class TestFrame extends JFrame {
        /**
         * 
         */
        private static final long serialVersionUID = 5671798241966272024L;

        public TestFrame() {

            FlowLayout layout = new FlowLayout();

            JButton moveUpButton = new JButton("U");
            JButton moveAllUpButton = new JButton("A");

            JButton moveDownButton = new JButton("D");
            JButton moveAllDownButton = new JButton("D");

            JButton selectButton = new JButton("S");
            JButton selectAllButton = new JButton("S");

            JButton deselectButton = new JButton("D");
            JButton deselectAllButton = new JButton("D");


            setSize(600, 400);

            getContentPane().setLayout(new GridLayout());


            JPanel selectButtonsPanel = new JPanel();
            selectButtonsPanel.setBackground(Color.BLUE);

            selectButtonsPanel.setLayout(layout);
            selectButtonsPanel.setPreferredSize(new Dimension(50, 0));
            selectButtonsPanel.add(selectAllButton);
            selectButtonsPanel.add(selectButton);
            selectButtonsPanel.add(deselectButton);
            selectButtonsPanel.add(deselectAllButton);

            JPanel moveButtonsPanel = new JPanel(layout);
            moveButtonsPanel.setBackground(Color.RED);
            moveButtonsPanel.setLayout(layout);
            moveButtonsPanel.setPreferredSize(new Dimension(50, 0));
            moveButtonsPanel.add(moveAllUpButton);
            moveButtonsPanel.add(moveUpButton);
            moveButtonsPanel.add(moveDownButton);
            moveButtonsPanel.add(moveAllDownButton);


            JPanel sourcePanel = new JPanel ();
            sourcePanel.setLayout(new BorderLayout());
            sourcePanel.add(new JTable(), BorderLayout.CENTER);
            sourcePanel.add(selectButtonsPanel, BorderLayout.EAST);


            JPanel destinationPanel = new JPanel ();
            destinationPanel.setLayout(new BorderLayout());
            destinationPanel.add(new JTable(), BorderLayout.CENTER);
            destinationPanel.add(moveButtonsPanel, BorderLayout.EAST);


            getContentPane().add (sourcePanel);
            getContentPane().add(destinationPanel);


        }

    }

Answer 1

Consider nesting JPanels each using their own layout. The BoxLayout could allow you to center your inner JPanel that holds the JButtons. The inner panel uses GridLayout and the BoxLayout using outer JPanel has glue added to the top and bottom:

    JPanel innerSelectPanel = new JPanel(new GridLayout(0, 1, 0, 5));
    // innerSelectPanel.setPreferredSize(new Dimension(50, 0));
    innerSelectPanel.add(selectAllButton);
    innerSelectPanel.add(selectButton);
    innerSelectPanel.add(deselectButton);
    innerSelectPanel.add(deselectAllButton);
    innerSelectPanel.setOpaque(false);
    innerSelectPanel.setBorder(BorderFactory.createEmptyBorder(0, 2, 0, 2));
    selectButtonsPanel.setLayout(new BoxLayout(selectButtonsPanel, BoxLayout.PAGE_AXIS));
    selectButtonsPanel.add(Box.createVerticalGlue());
    selectButtonsPanel.add(innerSelectPanel);
    selectButtonsPanel.add(Box.createVerticalGlue());

For example:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;

import javax.swing.*;
import javax.swing.table.DefaultTableModel;

public class TestFrame2 extends JPanel {
   public static final String[] COLS = { "One", "Two", "Three" };
   public static final String[][] BTN_LABELS = { { "S", "S", "D", "D" },
         { "A", "U", "D", "D" } };

   public TestFrame2() {
      setLayout(new GridLayout(1, 0));
      for (int i = 0; i < 2; i++) {
         add(createPanel(i));
      }
   }

   private JPanel createPanel(int row) {
      int gap = 3;
      JPanel btnPanel = new JPanel(new GridLayout(0, 1, 0, gap));
      btnPanel.setBorder(BorderFactory.createEmptyBorder(0, gap, 0, gap));
      btnPanel.setOpaque(false);
      for (int i = 0; i < BTN_LABELS[row].length; i++) {
         JButton btn = new JButton(BTN_LABELS[row][i]);
         btnPanel.add(btn);
      }
      btnPanel.setMaximumSize(btnPanel.getPreferredSize());

      JPanel rightPanel = new JPanel();
      rightPanel.setBackground(Color.red);
      rightPanel.setLayout(new BoxLayout(rightPanel, BoxLayout.PAGE_AXIS));
      rightPanel.add(Box.createVerticalGlue());
      rightPanel.add(btnPanel);
      rightPanel.add(Box.createVerticalGlue());

      JPanel panel = new JPanel(new BorderLayout());
      panel.add(new JScrollPane(new JTable(new DefaultTableModel(COLS, 5))),
            BorderLayout.CENTER);
      panel.add(rightPanel, BorderLayout.LINE_END);
      return panel;
   }

   private static void createAndShowGui() {
      TestFrame2 mainPanel = new TestFrame2();

      JFrame frame = new JFrame("TestFrame2");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}