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TheJPanel
class provides general-purpose containers for lightweight components. By default, panels don't paint anything except for their background; however, you can easily add borders to them and otherwise customize their painting. For information about using panels to perform custom painting, see Overview of Custom Painting
.
By default, panels are opaque. This makes them work well as content panes, and can help painting efficiency, as described in Painting
. You can make a panel transparent by invoking
setOpaque(false)
. A transparent panel draws no background, so that any components underneath show through.
The Converter application described in The Anatomy of a Swing-Based Programuses panels in several ways:
The following picture is the same as a picture in The Anatomy of a Swing-Based Program
- One
JPanel
instance serves as a content pane for the application's frame. This content pane uses a two-rowGridLayout
to lay out its contents, and an empty border to put 5 pixels of space around them. See Using Top-Level Containers for information about content panes.
- Two instances of a
JPanel
subclass, namedConversionPanel
, are used to both contain components and coordinate communication between components. The panels also have titled borders, which describe their contents and paint a line around them. The panels use left-to-rightBoxLayout
objects to lay out their contents.
- Two instances of an unnamed
JPanel
subclass are used to group components and restrict their size. The panels use top-to-bottomBoxLayout
objects to lay out their contents.
. It shows a colorized version of Converter, in which each panel has a background color.
As the figure shows, panels are useful for grouping components, simplifying component layout, and putting borders around groups of components. The rest of this section gives hints on grouping and laying out components. For information about using borders, see How to Use Borders
.
Like other containers, a panel uses a layout manager to position and size its components. By default, a panel's layout manager is an instance ofFlowLayout
, which places the panel's contents in a row. You can easily make a panel use any other layout manager by invoking the
setLayout
method or specifying a layout manager when creating the panel. Here is an example of the first approach:Here is an example of setting the layout manager at instantiation:JPanel aPanel = new JPanel(); aPanel.setLayout(new BorderLayout());JPanel aPanel = new JPanel(new BorderLayout());For more information about choosing and using layout managers, see Laying Out Components Within a Container
.
When you add components to a panel, you use theadd
method. Exactly which arguments you specify to theadd
method depend on which layout manager the panel uses. When the layout manager isFlowLayout
,BoxLayout
,GridLayout
, orGridBagLayout
, you'll typically use the one-argumentadd
method, like this:When the layout manager isaFlowPanel.add(aComponent); aFlowPanel.add(anotherComponent);BorderLayout
, you need to provide a second argument specifying the added component's position within the panel. For example:For information on the arguments to use withaBorderPanel.add(aComponent, BorderLayout.CENTER); aBorderPanel.add(anotherComponent, BorderLayout.SOUTH);BorderLayout
, see How to Use Borders.
The API in theJPanel
class itself is minimal. The methods you are most likely to invoke on aJPanel
object are those it inherits from its superclasses --JComponent
,
Container
, and
Component
. The following tables list the API you're most likely to use, with the exception of methods related to borders and layout hints. For more information about the API all
JComponent
s can use, see The JComponent Class.
Creating a JPanel
Constructor Purpose JPanel()
JPanel(LayoutManager)Create a panel. The LayoutManager
parameter provides a layout manager for the new panel. By default, a panel uses aFlowLayout
to lay out its components.
Managing a Container's Components Method Purpose void add(Component)
void add(Component, int)
void add(Component, Object)
void add(Component, Object, int)
void add(String, Component)Add the specified component to the panel. When present, the int
parameter is the index of the component within the container. By default, the first component added is at index 0, the second is at index 1, and so on. TheObject
parameter is layout manager dependent and typically provides information to the layout manager regarding positioning and other layout constraints for the added component. TheString
parameter is similar to theObject
parameter.int getComponentCount()
Get the number of components in this panel. Component getComponent(int)
Component getComponentAt(int, int)
Component getComponentAt(Point)
Component[] getComponents()Get the specified component or components. You can get a component based on its index or x, y position. void remove(Component)
void remove(int)
void removeAll()Remove the specified component(s).
Setting/Getting the Layout Manager Method Purpose void setLayout(LayoutManager)
LayoutManager getLayout()Set or get the layout manager for this panel. The layout manager is responsible for positioning the panel's components within the panel's bounds according to some philosophy.
Many examples contained in this lesson useJPanel
objects. The following table lists a few.
Example Where Described Notes Converter
and other filesThis section and The Anatomy of a Swing-Based Program Uses five panels, four of which use BoxLayout
and one of which usesGridLayout
. The panels use borders and, as necessary, size and alignment hints to affect layout.ListDemo
How to Use Lists Uses a panel, with its default FlowLayout
manager, to center three components in a row.ToolBarDemo
How to Use Toolbar Uses a panel as a content pane. The panel contains three components, laid out by BorderLayout
.BorderDemo
How to Use Borders Contains many panels that have various kinds of borders. Several panels use BoxLayout
.BoxLayoutDemo
How to Use BoxLayout Illustrates the use of a panel with Swing's BoxLayout
manager.
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