Making a Java SplitButton

Question

The following code seems like it should fill the given space evenly with two associated JButtons, but instead it fills the FlowLayout's entire top row with one button--ignoring the button's maximum size--and puts the other button on a new row, only partially visible. How can I get the buttons to evenly fill the JPanel side-by-side?

import java.awt.*;
import javax.swing.*;

public class SplitButton extends JPanel {

    // The other half.
    private JButton button2;
    private JButton button3;

    public SplitButton(String left, String right, int width, int height) {
        super();
        //setBackground(Color.GREEN);
        //setBorder(BorderFactory.createLineBorder(Color.BLUE));
        setMinimumSize(new Dimension(width, height));
        FlowLayout flow = new FlowLayout();
        flow.setAlignment(FlowLayout.LEFT);
        flow.setHgap(0);
        flow.setVgap(0);

        setLayout(flow);

        button2 = new JButton(right);
        button2.setMinimumSize(new Dimension(width / 2, height));
        button2.setMaximumSize(new Dimension(width / 2, height));
        button2.setPreferredSize(new Dimension(width / 2, height));
        add(button2);

        button3 = new JButton(right);
        button3.setMinimumSize(new Dimension(width / 2, height));
        button2.setMaximumSize(new Dimension(width / 2, height));
        button2.setPreferredSize(new Dimension(width / 2, height));
        add(button3);
    }
}

Answer 1

After a while of struggling, I tried a GridLayout--which makes SplitButtons in Swing a very rational UI element.

import java.awt.Font;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JPanel;

public class SplitButton extends JPanel {

    // The buttons.
    private JButton[] buttons;

    // Self-explanatory.
    public Color prim, trim;
    public Font font;

    public SplitButton(String[] labels, Color prim, Color trim, Font font) {
        super();

        this.prim = prim;
        this.trim = trim;
        this.font = font;

        // Evenly lays out buttons evenly in a row.
        GridLayout grid = new GridLayout(1, labels.length);
        grid.setHgap(0);
        grid.setVgap(0);
        setLayout(grid);

        setOpaque(false);
        initButtons(labels);
    }

    /**
    * Constructs and styles the button array.
    * @param labels The button labels.
    */
    private void initButtons(String[] labels) {
        buttons = new JButton[labels.length];
        for(int i = 0; i < buttons.length; i++) {
            buttons[i] = new JButton(labels[i]);

            // Only outer borders are thick.
            int lb = (i == 0) ? 2 : 1;
            int rb = (i == buttons.length - 1) ? 2 : 0;

            buttons[i].setBorder(BorderFactory.createMatteBorder(2, lb, 2, rb, trim));
            buttons[i].setFocusPainted(false);
            buttons[i].setFont(font);
            buttons[i].setBackground(prim);
            buttons[i].setForeground(trim);
            add(buttons[i]);
        }
    }

    /**
    * Enable/disable the buttons.
    * @param enabled Is the button enabled?
    */
    public void setEnabled(boolean enabled) {
        if(buttons != null)
            for(int i = 0; i < buttons.length; i++)
                buttons[i].setEnabled(enabled);
    }

    /**
    * Listen to all the buttons.
    * @param listener The listening object.
    */
    public void addActionListener(ActionListener listener) {
        if(buttons != null)
            for(int i = 0; i < buttons.length; i++)
                buttons[i].addActionListener(listener);
    }
}