How to use SpringLayout to arrange components vertically

Question

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?

Answer 1

You need to change

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

to

springLayout.putConstraint(NORTH, comp, padding, topConstraint, northComponent);

Answer 2

As Hitesh pointed out, you swapped the arguments to SpringLayout.putConstraints in your add method. The documentation states:

 public void putConstraint(String e1,
                           Component c1,
                           int pad,
                           String e2,
                           Component c2)

…

Parameters:
e1 - the edge of the dependent
c1 - the component of the dependent
pad - the fixed distance between dependent and anchor
e2 - the edge of the anchor
c2 - the component of the anchor

The first two arguments are the “dependent”—that is, the component you want to position. The last two arguments are the “anchor”—that is, the other component (or the container) on which the component being positioned will rely.

But that’s not the entire problem. To make all components the same width, you want to aggregate all of their heights using a Spring which is built using Spring.max.

A Spring based on a component is a “live” object: the exact minimum, preferred, and maximum values change whenever they change in the respective component. So the maximum of all those component-based Spring objects would be the width you want to apply to all of them.

Start by keeping the components’ width in a private field:

private Spring width = Spring.constant(0);

Then, in your add method, you only need one call to putConstraints. For the time being, don’t attach the horizontal dimensions of the component:

springLayout.putConstraint(NORTH, comp, padding, topConstraint, northComponent );
width = Spring.max(width, Spring.width(comp));

Finally, in finishedAdding, use that “intelligent” Spring as both the width of the container, and the width of each component.

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

  // Make every component's width fill this container.
  for (Component comp : componentList) {
    springLayout.putConstraint(WEST, comp, 0, WEST, this);
    springLayout.putConstraint(EAST, comp, width, WEST, this);
  }
}

First, we bind the container's width to the maximum width of all components (and its height to the last component). Then, to make sure each component stretches to fill the container’s width, we attach both its east and west sides to the container.

For what it’s worth, you don’t actually need SpringLayout for this. Box can do what you want, without the need for a specialized class:

JButton button1 = new JButton("ONE new button");
JButton button2 = new JButton("second b");
JRadioButton rButton = new JRadioButton("choose me");

int padding = 5;

Box panel = Box.createVerticalBox();
panel.add(button1);
panel.add(Box.createVerticalStrut(padding));
panel.add(button2);
panel.add(Box.createVerticalStrut(padding));
panel.add(rButton);

for (Component comp : panel.getComponents()) {
    Dimension size = comp.getMaximumSize();
    size.width = Integer.MAX_VALUE;
    comp.setMaximumSize(size);
}

Answer 3

Here's what I finally came up with:

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

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

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

public class OneWidthPanel extends JPanel
{
  private static final long serialVersionUID = 1L;
  
  private int padding = 5;
  SpringLayout springLayout = new SpringLayout();
  BoxLayout boxLayout = new BoxLayout(this, BoxLayout.PAGE_AXIS);
  
  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(NORTH, comp, padding, topConstraint, northComponent);
    springLayout.putConstraint(WEST, comp, padding, WEST, this);
//    springLayout.putConstraint(EAST, comp, padding, EAST, this);
    
    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);
    springLayout.putConstraint(WEST,   this, padding, WEST,  lastComponent);

    int maxComponentWidth = 0;
    int panelHeight = padding;
    
    // calculate overall height and maximum width
    for (Component component: componentList)
    {
      Dimension componentDimension = component.getPreferredSize();
      maxComponentWidth = Math.max(maxComponentWidth, componentDimension.width);
      panelHeight = panelHeight + componentDimension.height + padding;
    }
    
    // for each component, set the component width and preferred height,
    //    and set maximum width to MAX_VALUE; I was trying to get the components
    //    to stretch, but that doesn't work.
    for (Component component: componentList)
    {
      Dimension componentDimension = component.getPreferredSize();
      Dimension newD = new Dimension(maxComponentWidth, componentDimension.height);
//      Dimension maxD = new Dimension(Integer.MAX_VALUE, componentDimension.height);
      component.setPreferredSize(newD);
      component.setMinimumSize(newD);
      component.setMaximumSize(newD);
    }
    
    // set panel dimensions
    Dimension newD = new Dimension(maxComponentWidth + (padding*2), panelHeight);
    this.setPreferredSize(newD);
    this.setMinimumSize(newD);
  }
}

And the current tester:

import java.awt.BorderLayout;
import java.awt.Color;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JRadioButton;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

import rcutil.layout.OneWidthPanel;

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

  public static void main(String[] args)
  {
    OneWidthPanelTester tester = new OneWidthPanelTester();
    SwingUtilities.invokeLater(tester);
  }
  
  public void run()
  {
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    
    JButton button1 = new JButton("ONE new button");
    JButton button2 = new JButton("second b");
    JRadioButton rButton = new JRadioButton("choose me");
    JLabel label = new JLabel("I'm last");
    label.setBorder(BorderFactory.createLineBorder(Color.black));
    JTextField textField = new JTextField(25);
    
    OneWidthPanel panel = new OneWidthPanel();
    panel.add(button1);
    panel.add(button2);
    panel.add(rButton);
    panel.add(label);
    panel.add(textField);
    panel.finishedAdding();
    
    add(panel, BorderLayout.CENTER);
    pack();
    setVisible(true);
  }

}

These components are displayed at the width of the widest component (by preferred size) that was added to the panel. The buttons do not stretch with the panel, but the text field does - I had thought setting the max size of the buttons and adding and EAST-direction constraint would stretch them, but it doesn't seem to, and I don't need that for what I'm doing now.