Layout Managers - The Definitive Guide to Java Swing - page 337

Note that a layout manager is free to ignore some of its components; there is no requirement
that a layout manager display everything. For example, a Container using a BorderLayout might
include 30 or 40 components; however, the BorderLayout will display at most 5 of them (the last
component placed in each of its five named areas). Likewise, a CardLayout may manage many
components but displays exactly one at a time.
Besides ignoring components, a layout manager can do anything it wants with the compo-
nents' minimum, preferred, and maximum size. It is free to ignore any or all of these. It makes
sense that a layout manager can ignore a preferred size—after all, preferred means, “Give me
this size if it's available.” However, a layout manager can also ignore a minimum size. At times,
there is no reasonable alternative because the container may not have enough room to display
a component at its minimum size. How to handle this situation is left to the layout manager
designer's discretion.
LayoutManager Interface
The LayoutManager interface defines the responsibilities of the manager that lays out the
Component objects within a Container . As explained in the previous section, it is the duty of the
layout manager to determine the position and size of each component within the Container .
You will never call the methods of the LayoutManager interface directly; for the most part, layout
managers do their work behind the scenes. Once you have created a LayoutManager object and
told the container to use it (by calling setLayout(manager) ), you're finished with it. The system
calls the appropriate methods of the layout manager when necessary. Like any interface,
LayoutManager specifies the methods a layout manager must implement but says nothing
about how the LayoutManager does its job.
The LayoutManager interface itself is most important if you are writing a new layout manager.
I'll describe this interface first because it's the foundation on which all layout managers are
based. I'll also describe the LayoutManager2 interface, which is used by some layout managers.
Exploring the LayoutManager Interface
Five methods make up the LayoutManager interface:
public interface LayoutManager {
public void addLayoutComponent(String name, Component comp);
public void layoutContainer(Container parent);
public Dimension minimumLayoutSize(Container parent);
public Dimension preferredLayoutSize(Container parent);
public void removeLayoutComponent(Component comp);
}