Java: JPanel - Container

Description. A JPanel is an empty area that can be used either to

1. Layout other components, including other panels. See below.
2. Draw graphics on. See JPanel - Drawing Area.

Use a JPanel to layout other components

Declare and create the JPanel

For example,

JPanel p = new JPanel();

Set the layout and other characteristics

See Layouts. For example,

   p.setLayout(new BorderLayout());
   p.setPreferredSize(new Dimension(200, 100));
   p.setBackground(Color.blue);

You may want to set these characteristics in the constructor of a new class that extends JPanel. The default layout for a JPanel is FlowLayout, but it is a good idea to explicitly set it to avoid any problems with future changes in the default (as happened in the change from Java 1.1 to 1.2). To add to the confusion BorderLayout is the default layout for the JPanel that is the default content pane of a JFrame. It's really clearer to just set it each time.

Add components

Use add(...) to add components to a panel. For example,

   p.add(plusButton, BorderLayout.NORTH);
   p.add(minusButton, BorderLayout.SOUTH);

Constructors

The most common JPanel constructor has no parameters. This creates a panel with the default layout (FlowLayout). Call setLayout() to change the layout.

JPanel p = new JPanel();
p.setLayout(new FlowLayout());

You can optionally specify the layout in the JPanel constructor call.

JPanel p = new JPanel(new FlowLayout());

However, this doesn't work with BoxLayout, so calling setLayout(...) is still the preferred style of many programmers.

Common methods

p.setLayout(LayoutManager layout);
p.setPreferredSize(new Dimension(int width, int height));
p.setBackground(Color c);
p.add(some component);
w = p.getWidth();  // Used for graphics, not components
h = p.getHeight();  // Used for graphics, not components

Example

JPanel p = new JPanel();
p.setLayout(new GridLayout(4, 3));  // 4 rows and three columns
JButton bx = new JButton("Top left");
p.add(bx);   // bx goes into first grid element.
...          // add more elements to the panel.