JButton Layout Issue

I'm putting together the basic layout for a contacts book, and I want to know how I can make the 3 test buttons span from edge to edge just as the arrow buttons do.

 private static class ButtonHandler implements ActionListener {
  public void actionPerformed(ActionEvent e) {
   System.out.println("Code Placeholder");
  }
 }

 public static void main(String[] args) {

  //down button
  ImageIcon downArrow = new ImageIcon("down.png");
  JButton downButton = new JButton(downArrow);
  ButtonHandler downListener = new ButtonHandler();
  downButton.addActionListener(downListener);

  //up button
  ImageIcon upArrow = new ImageIcon("up.png");
  JButton upButton = new JButton(upArrow);
  ButtonHandler upListener = new ButtonHandler();
  upButton.addActionListener(u开发者_如何学编程pListener);

  //contacts
  JButton test1Button = new JButton("Code Placeholder");
  JButton test2Button = new JButton("Code Placeholder");
  JButton test3Button = new JButton("Code Placeholder");

  Box box = Box.createVerticalBox();
  box.add(test1Button);
  box.add(test2Button);
  box.add(test3Button);

  JPanel content = new JPanel();
  content.setLayout(new BorderLayout());
  content.add(box, BorderLayout.CENTER);
  content.add(downButton, BorderLayout.SOUTH);
  content.add(upButton, BorderLayout.NORTH);

  JFrame window = new JFrame("Contacts");
  window.setContentPane(content);
  window.setSize(400, 600);
  window.setLocation(100, 100);
  window.setVisible(true);

 }

Following up on @kloffy's suggestion:

package playground.tests;

import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

import junit.framework.TestCase;

public class ButtonTest extends TestCase {

    public void testThreeButtons() throws Exception {
          JPanel panel = new JPanel();
          panel.setLayout(new GridLayout());
          JButton button1 = new JButton("A");
          JButton button2 = new JButton("B");
          JButton button3 = new JButton("C");
          panel.add(button1);
          panel.add(button2);
          panel.add(button3);

          JFrame window = new JFrame("Contacts");
          window.setContentPane(panel);
          window.setSize(300, 600);
          window.pack();
          window.setVisible(true);
          int width = button1.getWidth();
          assertEquals(width, button2.getWidth());
          assertEquals(width, button3.getWidth());
    }
}