JComboBox don't work here

Question

I have a code like below, it complile fine, but the JComboBox don't work. If I resize the GUI manaually to very small height, these JComboBox start work. I don't understand why this happens. Any idea is high appreciated. Please email me at [email protected] Many thanks.
import javax.swing.*;
import java.awt.HeadlessException;
import java.awt.*;
public class testComboBox extends JFrame {
String[] _tmp={"XY PLOT", "AREA", "PIE", "BAR", "CANDLE"};
JComboBox cb1= new JComboBox(tmp);
String[] _tmp1={"Top", "Bottom"};
JComboBox cb2=new JComboBox(tmp1);
String[] _tmp2={"SOUTH","EAST"};
JComboBox cb3=new JComboBox(tmp2);
public testComboBox() throws HeadlessException {
this.setSize(new Dimension(200, 180));
JPanel _jp=new JPanel();
_jp.setSize(new Dimension(200, 160));
_jp.setLayout(new GridBagLayout());
GridBagConstraints _myBag= new GridBagConstraints();
_myBag.insets=new Insets(2,2,2,2);
_myBag.weightx=1.0;
_myBag.weighty=1.0;
_myBag.gridx=0;
_myBag.gridy=0;
jp.add(cb1, _myBag);
_myBag.gridx=0;
_myBag.gridy=1;
jp.add(cb2, _myBag);
_myBag.gridx=0;
_myBag.gridy=2;
jp.add(cb3, _myBag);
this.getRootPane().setLayout(new GridBagLayout());
GridBagConstraints _myBag1= new GridBagConstraints();
_myBag1.insets=new Insets(2,2,2,2);
_myBag1.weightx=1.0;
_myBag1.weighty=1.0;
_myBag1.gridx=0;
_myBag1.gridy=0;
this.getRootPane().add(_jp, _myBag1);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);
public static void main(String[] args) throws HeadlessException {
testComboBox testComboBox1 = new testComboBox();
testComboBox1.show();

Answer 1

try adding testComboBox1.pack() before the show()
honestly, you'll get a better answer posing a link to this in the swing forum (do not X-post, just put in a link to this)