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Professional Java Programming : Using Layout Managers

Layout Managers and GUI Construction - con't

When a layout manager's layoutContainer() method is called,  it is passed a reference to the container for which components should be arranged. The layout manager obtains the list of child components by calling the container's getComponents() method and sets the size and location for each visible child using Component methods such as setSize(), setLocation(), and setBounds(). If the layout manager supports constraints, it will use them to determine each component's size and location, but if it does not, it will arrange the components based on the order in which they occured in the list returned by getComponents().

To determine what a component's size should be, the layout manager usually also considers the container's size and may call each component's getPreferredSize(), getMinimumSize(), or getMaximumSize() methods. However, the layout manager is not required to respect the values returned by those methods, and in some cases, Java's layout managers do ignore them.

Each container has inset values that indicate the number of pixels around the container's edges that are reserved and cannot be used to display child components. Those values are encapsulated by an instance of java.awt.Insets, which defines four int values, each corresponding to one side of the container: top, left, bottom, and right. Those values usually describe the width of the border on the sides of the container, but in some cases, there may be additional space reserved. For example, JDialog and JFrame both include a title bar along their top edges, and that space is reserved by setting the top inset value appropriately.

When a layout manager calculates the amount of space available in a container, it subtracts the container's left and right insets from its width and the top and bottom insets from the height. In addition, when the layout manager arranges the child components, it will position them inside the container's inset area so that none of the components overlay the reserved portion of space around the container's edges.

Java's Layout Managers

It's possible to create your own layout manager class and this chapter describes how to do so, but the Java core classes include a number of layout managers that are flexible enough to meet the needs of most applications. The following list identifies the layout manager classes that are provided with Java, which are listed in what is arguably their order of complexity starting with the least complex and ending with the most complicated one:

• CardLayout
• FlowLayout
• GridLayout
• BorderLayout
• GridBagLayout
• BoxLayout (Java 2)

When you create an instance of a Container subclass that's provided with Java (for example, JPanel, JFrame, JDialog, etc.), that object will automatically be assigned a layout manager. The following table lists some of the classes that you might use and identifies the default layout manager type for each one:

This chapter examines the capabilities of the layout managers that are provided with Java, and specifically examines the following characteristics of each one:

• How a layout manager instance is constructed.
• The constraints that can be specified when adding a child component.
• How each child component's size is calculated.
• How each child component's position is calculated.
• What happens when the container has more or less space than it needs to display its child components. The emphasis here is on what happens when the container has less space than it needs, since that's usually when problems occur.
• How the values returned by a container's getMinimumSize(), getPreferredSize(), and getMaximumSize() methods are calculated by the layout manager.

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