Window Interfaces Using Swing Objects
"What is the use of a book," thought Alice, "without pictures or conversations?"
Lewis Carroll, Alice's Adventures in Wonderland
So far, almost all your programs have used the simplest form of input. The user enters simple text at the keyboard; and simple, unadorned text is sent to the screen as output. We have kept the input and output simple to concentrate on other basic features of programming and the Java language. But, modern programs do not use such simple input and output.
Modern programs use windowing interfaces with such features as menus and buttons that allow the user to make choices with a mouse. In this article, you will learn how to write Java programs that create such modern windowing interfaces for input and output using a special library of classes called Swing. Swing is a standard library that comes with all versions of Java 2 (also known as JDK 1.2 and as SDK 1.2). (Higher-numbered versions should also include the Swing library.) Entire books have been written on Swing, and so we will not have room to give you a complete description of Swing. However, in this article we will teach you enough to allow you to write simple windowing interfaces.
Swing is part of a larger collection of classes known as the Java Foundation Classes, or JFC. For what we are doing here, you do not need to be aware of anything about the JFC other than what we will tell you about Swing. However, if you hear about the JFC, you should be aware that you know something about the JFC, namely whatever you learn about Swing.
There is another, older library of classes for writing windowing interfaces. This older library is known as the Abstract Windows Toolkit, or AWT for short. Swing can be viewed as an improved version of the AWT. However, Swing did not replace the AWT. Swing added to the AWT to produce a richer collection of classes, and the AWT remains as a necessary complement to the Swing library. We will use classes from both Swing and the AWT. However, you will not go too far wrong if you think of them as all part of a single (larger) Swing library.
(If you are already familiar with the AWT, you will find that programming for Swing is very similar to programming with the AWT. In fact, in many situations the only difference is the spelling of some class names. If you are not familiar with the AWT, do not worry. We assume no knowledge of the AWT.)
event n. 1. An occurrence, incident, or experience, especially one of some significance.
The American Heritage Dictionary of the English Language, First Edition
Let's begin with some general background about the elements in any windowing interface, and about a programming technique known as event-driven programming, which is used when writing windowing interfaces.
GUIs: Graphical User Interfaces
Windowing systems that interact with the user are often called GUIs. GUI is pronounced "gooey" and stands for graphical user interface. The words are pretty much self-explanatory. It's called graphical because it uses graphical elements such as windows, buttons, and menus. It's called a user interface because it is the part of a program that interfaces with (that is, interacts with) the user. A GUI obtains information from the user and gives it to the program for processing. When the program is finished processing the information, the GUI gives the results to the user, usually in some sort of window.
Let's just briefly list the terms used for some basic elements that make up a GUI. Although you have undoubtedly used all these elements before, you may not have given them the same names we will use. A window is a portion of the user's screen that serves as a smaller screen within the screen. A window usually has a border defining its outside edges and a title of some sort giving the window a name. Inside a window you may have smaller window-like objects. Some of these smaller window-like objects are menus. A menu is a list of alternatives offered to the user, usually by offering a list of names. The user chooses one of these alternatives, usually by clicking it with a mouse. A button is very similar to an entry in a menu. A button is simply something that looks like a button to be pushed and that typically has a label. To "push" the button, you use your mouse to click on the button. These elements will have more precise definitions within Swing, but these are the basic properties they have within any windowing system.
Swing programs and most other graphical user interface (GUI) programs use events and event handlers. An event in a graphical user interface is an object that represents some action such as clicking a mouse, dragging the mouse, pressing a key on the keyboard, clicking the close-window button on a window, or any other action that is expected to elicit a response. Actually, events are more general than just the events of a graphical user interface. For example, a message from a printer to the operating system saying that the printer is ready to print another document can be considered an event. However, in this article, the only events that we will be concerned with are those generated within a graphical user interface.
When an object generates an event, it is called firing the event. In Swing, every object that can fire events, such as a button that might be clicked, can have one or more listener objects. You the programmer specify what objects are the listener objects for any given object that might fire an event. For example, if you click a button, that fires an event, and if the button has a listener object associated with it, then the event is automatically sent to this listener object. A listener object has methods that specify what will happen when events of various kinds are sent to the listener. These methods that handle events are called event handlers. You the programmer will define (or redefine) these event-handler methods.
Notice that event-driven programming is very different from the sort of programming we've seen before now. All our previous programs consisted of a list of statements executed in some order. There were some variations on this theme of performing a list of statements: Loops repeat statements, branches choose one of a list of statements to do next, and a method invocation brings in a different list of statements to be executed. However, at some level, all the programs we have seen so far were designed to be performed by one agent (the computer) following a simple set of instructions of the form "first do this, then do that, then do something else, and so forth."
Event-driven programming is a very different game. In event- driven programming, you create objects that can fire events and you create listener objects to react to the events. For the most part, your program does not determine the order in which things happen. The events determine that order. When an event-driven program is running, the next thing that happens depends on the next event.
Listener objects are almost like people sitting around a room waiting for phone calls. Each person has her or his own phone. When the phone rings, the person with that phone answers and does whatever the phone call says to do. Maybe the message says, "Joe this is your mother calling, I want you to close the window in your room." Then, Joe goes home and closes the window in her or his room. In a graphical user interface, the message is something like "close the window" or "The 'A' key has been pressed" or "The mouse was dragged" from someplace to someplace else. When an event is fired, it is automatically sent to the listener object(s) for the particular object that fired the event. The listener object then calls the appropriate event-handling method to handle the event.
If you have never done event-driven programming before, one aspect of event- driven programming may seem strange to you: You will be writing definitions for methods that you will never invoke in any program. This may seem strange, because a method is of no value unless it is invoked. So somebody or something other than you the programmer must be invoking these methods. That is exactly what does happen. The Swing system automatically invokes certain methods when an event signals that the method needs to be called.
The event-driven programming that we will be doing with the Swing library makes extensive use of inheritance. The classes that you define will be derived classes of some basic predefined classes that are in the Swing library. When you define these classes, they will inherit methods from their parent class. Some of these inherited methods will work fine just as they were written for the parent class (base class). However, often it will be necessary to override a method definition to provide a new definition that is appropriate to the derived class.