Sizing JPanels?

Question

Here's some code...
Can anyone tell me why the JPanel size doesn't seem to work like I want it to? (ie, in this case it should fill the container window)
import javax.swing.*;
import java.awt.*;
public class WindowTest extends JFrame {
void buildConstraints(GridBagConstraints gbc, int gx, int gy, int gw, int gh, int wx, int wy)
  gbc.gridx = gx;
  gbc.gridy = gy;
  gbc.gridwidth = gw;
  gbc.gridheight = gh;
  gbc.weightx = wx;
  gbc.weighty = wy;
public WindowTest()
  super("Test Window");
  Container c = getContentPane();
  c.setBackground(Color.black);
  setSize(600, 480);
  JPanel firstPanel = new JPanel();
  firstPanel.setSize(600, 480);
  GridBagLayout gridbag = new GridBagLayout();
  GridBagConstraints constraints = new GridBagConstraints();
  c.setLayout(gridbag);
  constraints.fill = GridBagConstraints.CENTER;
  firstPanel.setLayout(gridbag);
  firstPanel.setBackground(Color.red);
  //'Enter Text Below' Label
  buildConstraints(constraints, 1, 1, 1, 1, 0, 0);
  JLabel label1 = new JLabel("Enter text below");
  gridbag.setConstraints(label1, constraints);
  firstPanel.add(label1);
  //Text Entry Field
  buildConstraints(constraints, 1, 2, 1, 1, 0, 0);
  TextField textfield1 = new TextField("", 50);
  gridbag.setConstraints(textfield1, constraints);
  firstPanel.add(textfield1);
  c.add(firstPanel);
  show();
public static void main(String args[])
  WindowTest wt = new WindowTest();
}

Answer 1

Use setPreferredSize() instead of setSize().