Layout Managers - The Definitive Guide to Java Swing - page 364

ViewportLayout Class
The ViewportLayout manager is used by the JViewport class, a container class (to be described
in Chapter 11). The JViewport is also used within the ScrollPaneLayout / JScrollPane combination.
Like ScrollPaneLayout , the ViewportLayout manager is closely tied to its component, JViewport
in this case, and isn't usable outside the component, except in a subclass. In addition, the
JViewport class is rarely used outside a JScrollPane . The ViewportLayout manager will be discussed
in the context of its container, JViewport , in Chapter 11.
SpringLayout Class
The newest addition to the Java layout manager front is the SpringLayout manager, added with
the J2SE 1.4 release. This allows you to attach “springs” to components so that they are laid out
relative to other components. For instance, with SpringLayout , you can say that a button
appears attached to the right border, no matter what size a user makes the screen.
The SpringLayout manager relies on SpringLayout.Constraints for the component
constraints. This works similarly to the GridBagConstraints class that complements the
GridBagLayout manager. Each component added to the container can have an attached
SpringLayout.Constraints . Therein lies the end to the similarities between these two types
of constraints.
You usually don't need to add the component with the constraints. Instead, you can add
the component, and then typically attach the constraints separately. There is nothing stopping
you from adding the constraints with the component, but SpringLayout.Constraints is not a
simple class. It is a collection of Spring objects, each a different constraint on the component.
You need to add each Spring constraint separately to SpringLayout.Constraints . You do this
by setting specific constraints on an edge of the component. Using the four SpringLayout
constants of EAST , WEST , NORTH , and SOUTH , you call the setContraints(String edge, Spring
spring) method of SpringLayout.Constraints , where the String is one of the constants.
For instance, if you want to add a component in the top left of a container, you can set up
two springs of a constant size, combine them together, and add the component to the
container with the combined set, as shown here:
Component left = ...;
SpringLayout layout = new SpringLayout();
JPanel panel = new JPanel(layout);
Spring xPad = Spring.constant(5);
Spring yPad = Spring.constant(25);
SpringLayout.Constraints constraint = new SpringLayout.Constraints();
constraint.setConstraint(SpringLayout.WEST, xPad);
constraint.setConstraint(SpringLayout.NORTH, yPad);
frame.add(left, constraint);