Laying Out Components within a Container

General Rules for Using Layout Managers

Unless you explicitly tell a Container not to use a layout manager, it will be associated with its very own instance of a layout manager. This layout manager is automatically consulted every time the Container might need to change its appearance. Most layout managers don't require programs to directly call the layout manager's methods.

How to Choose a Layout Manager

The AWT-provided layout managers have different strengths and weaknesses. This section discusses some common layout scenarios and which AWT layout managers might work for each scenario. If none of the AWT layout managers is right for your situation, you should feel free to use layout managers contributed to the net. such as [PackerLayout?].

Scenario: You need to display a component in as much space as it can get.

Consider using BorderLayout or GridBagLayout. If you use BorderLayout, you'll need to put the space-hungry component in the center. With GridBagLayout, you'll need to set the constraints for the component so that fill=GridBagConstraints.BOTH. Or, if you don't mind every other component in the same container being just as large as your space-hungry component, you can use a GridLayout.

Scenario: You need to display a few components in a compact row at their natural size.

Consider using a Panel to hold the components and using the Panel's default FlowLayout manager.

Scenario: You need to display a few same-sized components in rows and/or columns.

GridLayout is perfect for this. Use a Panel if necessary to contain the components.

How to Create a Layout Manager and Associate It with a Container

Every container has a default layout manager associated with it. All Panels (including Applets) are initialized to use a FlowLayout. All Windows (except special-purpose ones like FileDialog) are initialized to use a BorderLayout.

If you want to use a Container's default layout manager, you don't have to do a thing. The constructor for each Container creates a layout manager instance and initializes the Container to use it.

To use a non-default layout manager, you need to create an instance of the desired layout manager class and tell the Container to use it. Below is some typical code that does this. This code creates a CardLayout manager and sets it up as the layout manager for a Container.

aContainer.setLayout(new CardLayout());

Rules of Thumb for Using Layout Managers

The Container methods that result in calls to the Container's layout manager are add(), remove(), removeAll(), layout(), preferredSize(), and minimumSize(). The add(), remove(), and removeAll() methods add and remove Components from a Container; you can call them at any time. The layout() method, which is called as the result of any paint request to a Container, requests that the Container place and size itself and the Components it contains; you don't call it directly [CHECK]. The preferredSize() and minimumSize() methods return the Container's ideal size and minimum size, respectively. The values returned are just hints; they have no effect unless your program enforces these sizes [CHECK].

You must take special care when calling a Container's preferredSize() and minimumSize() methods. The values these methods return are meaningless unless the Container and its Components have valid peer objects. The peer object corresponding to a Component is created whenever the Component's addNotify() method is called, which happens in the following circumstances:

• When the Component is a peer-less Window, or is contained in a peer-less Window, and the Window's pack() or show() method is called
• When the Component is added to a Container that already has a peer
• When your program directly calls the Component's addNotify() method, or the addNotify() method of any Container above the Component [is this really OK?]

Laying Out Components within a Container