AWT Control Fundamentals

AWT Control Fundamentals

The AWT supports the following types of controls:

        Labels

        Push buttons

        Check boxes

        Choice lists

        Lists

        Scroll bars

        Text Editing

These controls are subclasses of Component. Although this is not a particularly rich set of controls, it is sufficient for simple applications. (Note that both Swing and JavaFX provide a substantially larger, more sophisticated set of controls.)

Adding and Removing Controls

To include a control in a window, you must add it to the window. To do this, you must first create an instance of the desired control and then add it to a window by calling add( ), which is defined by Container. The add( ) method has several forms. The following form is the one that is used for the first part of this chapter:

Component add(Component compRef)

Here, compRef is a reference to an instance of the control that you want to add. A reference to the object is returned. Once a control has been added, it will automatically be visible whenever its parent window is displayed.

Sometimes you will want to remove a control from a window when the control is no longer needed. To do this, call remove( ). This method is also defined by Container. Here is one of its forms:

void remove(Component compRef)

Here, compRef is a reference to the control you want to remove. You can remove all controls by calling removeAll( ).

Responding to Controls

Except for labels, which are passive, all other controls generate events when they are accessed by the user. For example, when the user clicks on a push button, an event is sent that identifies the push button. In general, your program simply implements the appropriate interface and then registers an event listener for each control that you need to monitor. As explained in Chapter 24, once a listener has been installed, events are automatically sent to it. In the sections that follow, the appropriate interface for each control is specified.

The HeadlessException

Most of the AWT controls described in this chapter have constructors that can throw a HeadlessException when an attempt is made to instantiate a GUI component in a non-interactive environment (such as one in which no display, mouse, or keyboard is present). You can use this exception to write code that can adapt to non-interactive environments. (Of course, this is not always possible.) This exception is not handled by the programs in this chapter because an interactive environment is required to demonstrate the AWT controls.