There's clearly something I don't understand about SpringLayout. I'm trying to create a class, an extension of JPanel, that allows me to add components and have them appear vertically, all the same width.

My extension of JPanel would set its LayoutManager to use SpringLayout, and each time a component was added it would put in the SpringLayout constraints to attach it, to the panel for the first component, and then each component to the previous one.

First, here's an Oracle-written example of using SpringLayout that I altered to put components vertically instead of horizontally:

```
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import javax.swing.SpringLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import java.awt.Container;

public class SpringDemo3
{
  /**
   * Create the GUI and show it. For thread safety, this method should be
   * invoked from the event-dispatching thread.
   */
  private static void createAndShowGUI()
  {
    // Create and set up the window.
    JFrame frame = new JFrame("SpringDemo3");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // Set up the content pane.
    Container contentPane = frame.getContentPane();
    SpringLayout layout = new SpringLayout();
    contentPane.setLayout(layout);

    // Create and add the components.
    JLabel label = new JLabel("Label: ");
    JTextField textField = new JTextField("Text field", 15);
    contentPane.add(label);
    contentPane.add(textField);

    // Adjust constraints for the label so it's at (5,5).
    layout.putConstraint(SpringLayout.WEST, label, 5, SpringLayout.WEST, contentPane);
    layout.putConstraint(SpringLayout.NORTH, label, 5, SpringLayout.NORTH, contentPane);

    // Adjust constraints for the text field so it's at
    // (<label's right edge> + 5, 5).
//    layout.putConstraint(SpringLayout.WEST, textField, 5, SpringLayout.EAST, label);
//    layout.putConstraint(SpringLayout.EAST, textField, 5, SpringLayout.EAST, contentPane);
    layout.putConstraint(SpringLayout.NORTH, textField, 5, SpringLayout.SOUTH, label);

    // Adjust constraints for the content pane: Its right
    // edge should be 5 pixels beyond the text field's right
    // edge, and its bottom edge should be 5 pixels beyond
    // the bottom edge of the tallest component (which we'll
    // assume is textField).
    layout.putConstraint(SpringLayout.EAST, contentPane, 5, SpringLayout.EAST, textField);
    layout.putConstraint(SpringLayout.SOUTH, contentPane, 5, SpringLayout.SOUTH, textField);

    // Display the window.
    frame.pack();
    frame.setVisible(true);
  }

  public static void main(String[] args)
  {
    // Schedule a job for the event-dispatching thread:
    // creating and showing this application's GUI.
    javax.swing.SwingUtilities.invokeLater(new Runnable()
    {
      public void run()
      {
        createAndShowGUI();
      }
    });
  }
}

```

Based on what I understood SpringLayout requires, I wrote the following:

```
import java.awt.Component;
import java.awt.LayoutManager;
import java.util.ArrayList;

import javax.swing.JPanel;
import javax.swing.SpringLayout;

import static javax.swing.SpringLayout.NORTH;
import static javax.swing.SpringLayout.EAST;
import static javax.swing.SpringLayout.SOUTH;
import static javax.swing.SpringLayout.WEST;

public class OneWidthPanel extends JPanel
{
  private static final long serialVersionUID = 1L;

  private int padding = 5;
  SpringLayout springLayout = new SpringLayout();

  public OneWidthPanel() { super(); setLayout(springLayout); }
  public OneWidthPanel(boolean isDoubleBuffered) { super(isDoubleBuffered); }
  public OneWidthPanel(LayoutManager layout) { throw new IllegalArgumentException("Cannot set a layout manager on the OneWidthPanel class"); }
  public OneWidthPanel(LayoutManager l, boolean isDoubleBuffered) { throw new IllegalArgumentException("Cannot set a layout manager on the OneWidthPanel class"); }

  private ArrayList<Component> componentList = new ArrayList<>();

  @Override
  public Component add(Component comp)
  {
    super.add(comp);

    componentList.add(comp);
    int listSize = componentList.size();

    String topConstraint;
    Component northComponent;
    if (listSize == 1)
    {
      topConstraint = NORTH;
      northComponent = this;
    }
    else
    {
      topConstraint = SOUTH;
      northComponent = componentList.get(listSize - 2);
    }

    springLayout.putConstraint(topConstraint, northComponent, padding, SpringLayout.NORTH, comp);
    springLayout.putConstraint(WEST, this, padding, WEST, comp);
    springLayout.putConstraint(EAST, this, padding, EAST, comp);

    return comp;
  }

  public void finishedAdding()
  {
    Component lastComponent = componentList.get(componentList.size()-1);
    springLayout.putConstraint(EAST,   this, padding, EAST,  lastComponent);
    springLayout.putConstraint(SOUTH,  this, padding, SOUTH, lastComponent);
  }
}

```

Here's a little program to test it:

```
import java.awt.BorderLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JRadioButton;

import rcutil.layout.OneWidthPanel;

public class OneWidthPanelTester extends JFrame
{
  private static final long serialVersionUID = 1L;

  public static void main(String[] args)
  {
    OneWidthPanelTester tester = new OneWidthPanelTester();
    tester.go();
  }

  public void go()
  {
    OneWidthPanel panel = new OneWidthPanel();
    JButton button1 = new JButton("ONE new button");
    JButton button2 = new JButton("second b");
    JRadioButton rButton = new JRadioButton("choose me");

    panel.add(button1);
    panel.add(button2);
    panel.add(rButton);
    panel.finishedAdding();

    add(panel, BorderLayout.WEST);
    pack();
    setVisible(true);
  }

}

```

The components appear on top of each other at the top of the panel. I thought setting the constraints of each one, as I added it, to connect the north end of each component to the south edge of the previous component, would line them up vertically in the order added. I have the `finishedAdding()` method so that I can wrap up the last component's connection to its container, as told in the "How to use SpringLayout" tutorial at <https://docs.oracle.com/javase/tutorial/uiswing/layout/spring.html>, and as done in the demo program I copied.

I don't understand why my components overlay each other but the (two) demo components are next to each other vertically. And am I going to be able to satisfy my original desire, which is to have the vertical components stretch to be the same size in the panel?