How do I add a separator to a JComboBox in Java?

Question

I have a JComboBox and would like to have a separator in the list of elements. How do I do this in Java?

A sample scenario where this would come in handy is when making a combobox for font-family-selection; similar to the font-family-selection-control in Word and Excel. In this case I would like to show the most-used-fonts at the top, then a separator and finally all font-families below the separator in alphabetical order.

Can anyone help me with how to do this or is this not possible in Java?

Answer 1

Try adding this renderer. Just supply a list of index values that you want the separator to be above.

 private class SeperatorComboRenderer extends DefaultListCellRenderer
 {
      private final float SEPARATOR_THICKNESS = 1.0f;
      private final float SPACE_TOP = 2.0f;
      private final float SPACE_BOTTOM = 2.0f;
      private final Color SEPARATOR_COLOR = Color.DARK_GRAY;
    
      private final List<Integer> marks;
      private boolean mark;
      private boolean top;
    
      public SeperatorComboRenderer(List<Integer> marks)
      {
           this.marks = marks;
      }
    
      @Override
      public Component getListCellRendererComponent(JList list, Object object, int index, boolean isSelected, boolean hasFocus)
      {
           super.getListCellRendererComponent(list, object, index, isSelected, hasFocus);

           top = false;
           mark = false;
           marks.forEach((idx) ->
           {
               if(index - 1 == idx)
                    top = true;
               if(index == idx)
                    mark = true;
           });
           return this;
      }
    
      @Override
      protected void paintComponent(Graphics g)
      {
           if(mark)
                g.translate(0, (int)(SEPARATOR_THICKNESS + SPACE_BOTTOM));
           Graphics2D g2 = (Graphics2D)g;
           super.paintComponent(g);
           if(mark)
           {
                g2.setColor(SEPARATOR_COLOR);
                g2.setStroke(new BasicStroke(SEPARATOR_THICKNESS));
                g2.drawLine(0, 0, getWidth(), 0);
           }
      }
    
      @Override
      public Dimension getPreferredSize()
      {
           Dimension pf = super.getPreferredSize();
           double height = pf.getHeight();
           if(top) height += SPACE_TOP;
           else if(mark) height += SEPARATOR_THICKNESS + SPACE_BOTTOM;
           return new Dimension((int)pf.getWidth(), (int)height);
      }
  }

Answer 2

By the time I wrote and tested the code below, you probably got lot of better answers...
I don't mind as I enjoyed the experiment/learning (still a bit green on the Swing front).

[EDIT] Three years later, I am a bit less green, and I took in account the valid remarks of bobndrew. I have no problem with the key navigation that just works (perhaps it was a JVM version issue?). I improved the renderer to show highlight, though. And I use a better demo code. The accepted answer is probably better (more standard), mine is probably more flexible if you want a custom separator...

The base idea is to use a renderer for the items of the combo box. For most items, it is a simple JLabel with the text of the item. For the last recent/most used item, I decorate the JLabel with a custom border drawing a line on its bottom.

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


@SuppressWarnings("serial")
public class TwoPartsComboBox extends JComboBox
{
  private int m_lastFirstPartIndex;

  public TwoPartsComboBox(String[] itemsFirstPart, String[] itemsSecondPart)
  {
    super(itemsFirstPart);
    m_lastFirstPartIndex = itemsFirstPart.length - 1;
    for (int i = 0; i < itemsSecondPart.length; i++)
    {
      insertItemAt(itemsSecondPart[i], i);
    }

    setRenderer(new JLRenderer());
  }

  protected class JLRenderer extends JLabel implements ListCellRenderer
  {
    private JLabel m_lastFirstPart;

    public JLRenderer()
    {
      m_lastFirstPart = new JLabel();
      m_lastFirstPart.setBorder(new BottomLineBorder());
//      m_lastFirstPart.setBorder(new BottomLineBorder(10, Color.BLUE));
    }

    @Override
    public Component getListCellRendererComponent(
        JList list,
        Object value,
        int index,
        boolean isSelected,
        boolean cellHasFocus)
    {
      if (value == null)
      {
        value = "Select an option";
      }
      JLabel label = this;
      if (index == m_lastFirstPartIndex)
      {
        label = m_lastFirstPart;
      }
      label.setText(value.toString());
      label.setBackground(isSelected ? list.getSelectionBackground() : list.getBackground());
      label.setForeground(isSelected ? list.getSelectionForeground() : list.getForeground());
      label.setOpaque(true);

      return label;
    }
  }
}

Separator class, can be thick, with custom color, etc.

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

/**
 * Draws a line at the bottom only.
 * Useful for making a separator in combo box, for example.
 */
@SuppressWarnings("serial")
class BottomLineBorder extends AbstractBorder
{
  private int m_thickness;
  private Color m_color;

  BottomLineBorder()
  {
    this(1, Color.BLACK);
  }

  BottomLineBorder(Color color)
  {
    this(1, color);
  }

  BottomLineBorder(int thickness, Color color)
  {
    m_thickness = thickness;
    m_color = color;
  }

  @Override
  public void paintBorder(Component c, Graphics g,
      int x, int y, int width, int height)
  {
    Graphics copy = g.create();
    if (copy != null)
    {
      try
      {
        copy.translate(x, y);
        copy.setColor(m_color);
        copy.fillRect(0, height - m_thickness, width - 1, height - 1);
      }
      finally
      {
        copy.dispose();
      }
    }
  }

  @Override
  public boolean isBorderOpaque()
  {
    return true;
  }
  @Override
  public Insets getBorderInsets(Component c)
  {
    return new Insets(0, 0, m_thickness, 0);
  }
  @Override
  public Insets getBorderInsets(Component c, Insets i)
  {
    i.left = i.top = i.right = 0;
    i.bottom = m_thickness;
    return i;
  }
}

Test class:

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


@SuppressWarnings("serial")
public class TwoPartsComboBoxDemo extends JFrame
{
  private TwoPartsComboBox m_combo;

  public TwoPartsComboBoxDemo()
  {
    Container cont = getContentPane();
    cont.setLayout(new FlowLayout());

    cont.add(new JLabel("Data: ")) ;

    String[] itemsRecent = new String[] { "ichi", "ni", "san" };
    String[] itemsOther = new String[] { "one", "two", "three" };
    m_combo = new TwoPartsComboBox(itemsRecent, itemsOther);

    m_combo.setSelectedIndex(-1);
    cont.add(m_combo);
    m_combo.addActionListener(new ActionListener()
    {
      public void actionPerformed(ActionEvent ae)
      {
        String si = (String) m_combo.getSelectedItem();
        System.out.println(si == null ? "No item selected" : si.toString());
      }
    });

    // Reference, to check we have similar behavior to standard combo
    JComboBox combo = new JComboBox(itemsRecent);
    cont.add(combo);
  }

  /**
   * Start the demo.
   *
   * @param args   the command line arguments
   */
  public static void main(String[] args)
  {
    // turn bold fonts off in metal
    UIManager.put("swing.boldMetal", Boolean.FALSE);

    SwingUtilities.invokeLater(new Runnable()
    {
      public void run()
      {
        JFrame demoFrame = new TwoPartsComboBoxDemo();
        demoFrame.setTitle("Test GUI");
        demoFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        demoFrame.setSize(400, 100);
        demoFrame.setVisible(true);
      }
    });
  }
}

Answer 3

There is a pretty short tutorial with an example that shows how to use a custom ListCellRenderer on java2s http://www.java2s.com/Code/Java/Swing-Components/BlockComboBoxExample.htm

Basically it involves inserting a known placeholder in your list model and when you detect the placeholder in the ListCellRenderer you return an instance of 'new JSeparator(JSeparator.HORIZONTAL)'

Answer 4

You can use a custom ListCellRenderer which would draw the separator items differently. See docs and a small tutorial.