import java.awt.BorderLayout;

import java.awt.Color;

import java.awt.event.FocusAdapter;

import java.awt.event.FocusEvent;

import java.awt.event.FocusListener;

import javax.swing.JComboBox;

import javax.swing.JFrame;

import javax.swing.JTextField;

public class Main extends JFrame {

JComboBox box = new JComboBox(

new String[] { "1", "22", "33" });

public Main() {

box.setBackground(Color.RED);

box.addFocusListener(getFocusListener());

JTextField f = new JTextField();

add(box, BorderLayout.SOUTH);

add(f, BorderLayout.NORTH);

pack();

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

setVisible(true);

}

private FocusListener getFocusListener() {

return new FocusAdapter() {

@Override

public void focusGained(FocusEvent arg0) {

super.focusGained(arg0);

box.setBackground(Color.BLUE);

validate();

}

@Override

public void focusLost(FocusEvent arg0) {

super.focusLost(arg0);

box.setBackground(Color.red);

validate();

}

};

}

public static void main(String... s) {

new Main();

}

}