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In Java, the java.awt.Container class and its subclasses can
be used to display groups of components. For example, you might use a
JPanel to display a related set of buttons or add components
to the content pane of a JFrame. Layout managers are
classes used to control the size and location of each component that's
added to a container, and in most cases, a layout manager is also
responsible for determining the sizes returned from the container's
getMinimumSize(), getPreferredSize(), and
getMaximumSize() methods. Layout managers are important
because they simplify the task of positioning and sizing components and
because they allow you to create flexible user interfaces.
Java provides a number of different layout managers that you should be familiar with, and each one has advantages and disadvantages. Some are easy to use but provide limited flexibility, while others are very flexible but also much more difficult to use. When none of the layout managers provided with Java suits your needs, you can easily create your own, although it's not often necessary to do so if you're familiar with those that are already available. In this chapter we will see:
To assign a layout manager to a container, you must create an instance of
the manager and pass it to the setLayout() method defined in
Container. For example, the following code provides an example
of how to create an instance of BorderLayout and assign it to
a JPanel:
JPanel panel = new JPanel(); panel.setLayout(new BorderLayout());
The overloaded add() method defined in Container
is used to add a Component to a container, which then becomes
known as the component's parent container. Similarly, the component
added is referred to as a child component of the container.
Although Container defines a number of different implementations of add(), the two used most
often are:
add(Component comp)
add(Component comp, Object constraints)
In both cases, a reference to the child component is sent to the
Container. However, the second implementation also includes a
constraints parameter that provides information normally used by the
layout manager to determine where the component should be placed and/or
what its size should be. The specific subclass of Object used
for this parameter depends upon what type of layout manager is involved.
For example, if you're using a GridBagLayout, the constraints
parameter must be an instance of the
java.awt.GridBagConstraints class, while other layout managers
require you to pass a String value.
Some layout managers do not support constraints and use the order in which
components are added to their parent container to determine their
positions. When you're using a layout manager that does not accept
constraints, you should use the simpler add() method shown
above that takes only a single Component parameter. Doing so
is equivalent to passing a null value for the constraint
parameter, which means that the two lines of code shown below are
functionally identical to one another:
myContainer.add(someComponent); myContainer.add(someComponent, null);
On the other hand, using code like that shown above with a layout manager
that does support constraints will cause the layout manager to assign some
default constraint information to the component. Therefore, unless you're
certain that the default information will produce the results you want, you
should always explicitly specify a constraint parameter when
using a layout manager that supports constraints.
When add() is called, the container adds the component to a
list that it maintains and calls the layout manager's
addLayoutComponent() method. That method is passed references
to the component being added and to the constraints object specified, and
this allows the layout manager to save the constraint information and
associate it with the component for later use.
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