Best books about JAVA programming:

• Head First JAVA, Second Editon
• Effective Java Programming Language Guide
• Professional Java, JDK 5 Edition
• Ivor Horton's Beginning Java 2, JDK 5 Edition

JAVA JComboBox

This example shows how to extends JLabel and ListCellRenderer to add block(a seperate line) in jcombobox.

import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import javax.swing.plaf.metal.*;
import javax.swing.plaf.basic.*;

public class BlockComboBoxExample extends JFrame {
final String SEPARATOR = "SEPARATOR";

public BlockComboBoxExample() {
super("Block ComboBox Example");

String[][] str = {{"A" ,"B" ,"C" },
{"1" ,"2" ,"3" },
{"abc","def","ghi"}
};

JComboBox combo = new JComboBox(makeVectorData(str));
combo.setRenderer(new ComboBoxRenderer());
combo.addActionListener(new BlockComboListener(combo));

getContentPane().setLayout(new FlowLayout());
getContentPane().add(combo);
setSize(300, 100);
setVisible(true);
}

private Vector makeVectorData(String[][] str) {
boolean needSeparator = false;
Vector data = new Vector();
for (int i=0;i<str.length;i++) {
if (needSeparator) {
data.addElement(SEPARATOR);
}
for (int j=0;j<str[i].length;j++) {
data.addElement(str[i][j]);
needSeparator = true;
}
}
return data;
}

public static void main (String args[]) {
BlockComboBoxExample frame = new BlockComboBoxExample();
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}


class ComboBoxRenderer extends JLabel implements ListCellRenderer {
JSeparator separator;

public ComboBoxRenderer() {
setOpaque(true);
setBorder(new EmptyBorder(1, 1, 1, 1));
separator = new JSeparator(JSeparator.HORIZONTAL);
}

public Component getListCellRendererComponent( JList list,
Object value, int index, boolean isSelected, boolean cellHasFocus) {
String str = (value == null) ? "" : value.toString();
if (SEPARATOR.equals(str)) {
return separator;
}
if(isSelected) {
setBackground(list.getSelectionBackground());
setForeground(list.getSelectionForeground());
} else {
setBackground(list.getBackground());
setForeground(list.getForeground());
}
setFont(list.getFont());
setText(str);
return this;
}
}

class BlockComboListener implements ActionListener {
JComboBox combo;
Object currentItem;

BlockComboListener(JComboBox combo) {
this.combo = combo;
combo.setSelectedIndex(0);
currentItem = combo.getSelectedItem();
}

public void actionPerformed(ActionEvent e) {
String tempItem = (String)combo.getSelectedItem();
if (SEPARATOR.equals(tempItem)) {
combo.setSelectedItem(currentItem);
} else {
currentItem = tempItem;
}
}
}
}