JPanel就是一个容器，对一个容器一般只有四个操作：设定layout（排列方式），add component，remove component，get component。
add(Componet,[int],[Object],[String]),其中int是component在panel中的index，默认index从0开始。Object和String表示一些有关layout的constraint信息，比如位置等。
panels do not add colors to anything except their own background; however, you can easily add borders to them and otherwise customize their painting.
Setting the Layout Manager
a panel uses a layout manager to position and size its components. By default, a panel's layout manager is an instance of FlowLayout, 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 by specifying a layout manager when creating the panel. The latter approach is preferable for performance reasons, since it avoids the unnecessary creation of a FlowLayout object.
JPanel p = new JPanel(new BorderLayout()); //PREFERRED!
This approach does not work with BoxLayout, since the BoxLayout constructor requires a pre-existing container. Here is an example that uses BoxLayout.
JPanel p = new JPanel(); p.setLayout(new BoxLayout(p, BoxLayout.PAGE_AXIS));
Adding Components
When the layout manager is FlowLayout, BoxLayout, GridLayout, or SpringLayout, you will typically use the one-argument add method, like this:
aFlowPanel.add(aComponent); aFlowPanel.add(anotherComponent);
When the layout manager is BorderLayout, you need to provide an argument specifying the added component's position within the panel. For example:
aBorderPanel.add(aComponent, BorderLayout.CENTER); aBorderPanel.add(anotherComponent, BorderLayout.PAGE_END);
With GridBagLayout you can use either add method, but you must somehow specify grid bag constraints for each component.
Reference: java api7