Section 1.6
The Modern User Interface
WHEN COMPUTERS WERE FIRST INTRODUCED,
ordinary people -- including most programmers -- couldn't get
near them. They were locked up in rooms with white-coated
attendants who would take your programs and data, feed them
to the computer, and return the computer's response some time
later. When timesharing -- where the computer switches its
attention rapidly from one person to another -- was
invented in the 1960s, it became possible for several people
to interact directly with the computer at the same time.
On a timesharing system, users sit at "terminals" where they
type commands to the computer, and the computer types back its
response. Early personal computers also used typed commands and responses,
except that there was only one person involved at a time. This type of
interaction between a user and a computer is called a
command-line
interface.
Today, of course, most people interact with computers in a
completely different way. They use a Graphical
User Interface, or GUI. The computer draws interface
components on the screen. The components include things like
windows, scroll bars, menus, buttons, and icons. Usually,
a mouse is used to manipulate such
components. Assuming that you are reading these notes on a
computer, you are no doubt already familiar with the basics of graphical
user interfaces!
A lot of GUI interface components have become fairly standard.
That is, they have similar appearance and behavior on many
different computer platforms including Macintosh, Windows,
and various UNIX window systems. Java programs,
which are supposed to run on many different platforms without
modification to the program, can use all the standard GUI components.
They might vary in appearance from platform to platform, but
their functionality should be identical on any computer on which
the program runs.
Below is a very simple Java program -- actually an
"applet,"
since it is running right here in the middle of a page --
that shows a few standard GUI interface components.
There are four components that you can interact with:
a button, a checkbox, a text field, and a pop-up menu.
These components are
labeled. There are a few other components in the applet.
The labels themselves are components (even though you
can't interact with them). The right half of the applet is
a text area component, which can display multiple lines
of text. In fact, in Java terminology, the whole applet is itself
considered to be a "component." Try clicking
on the button and on the checkbox, and try selecting an item from the pop-up
menu. You will see a message in the text area about each action
that you perform. You can type in the text field, but you might have to
click on it first to activate it. When you press return while
typing in the text field, you will see a message in the text area:
Now, Java actually has two complete sets of GUI components. One of these,
the AWT or Abstract Windowing Toolkit, was
available in the original version of Java. The other, which is known as
Swing, is included in Java version 1.2 or later.
Here is a version of the applet that uses Swing instead of the AWT. If you
see the above applet but
just see a blank area here, it means that you are using a Web browser that
uses an older version of Java:
As you interact with the GUI components in these applets,
you generate "events." For example, clicking a push button
generates an event. Each time an event is generated, a message is sent to the
applet telling it that the event has occurred, and the applet
responds according to its program. In fact, the program consists
mainly of "event handlers" that tell the applet how
to respond to various types of events. In this example, the applets
have been programmed to respond to each event by displaying a message
in the text area.
The use of the term "message" here is deliberate.
Messages, as you saw in the previous section, are sent to
objects. In fact, Java GUI components are implemented as
objects. Java includes many predefined classes that represent
various types of GUI components. Some of these classes are subclasses
of others. Here is a diagram showing some of the GUI classes in the AWT
and their relationships:
Don't worry about the details for now, but try to get some feel
about how object-oriented programming and inheritance are used here.
Note that all the GUI classes are subclasses, directly or indirectly, of
a class called Component. Two of the direct subclasses of
Component themselves have subclasses. The classes TextArea and
TextField, which have certain behaviors in common, are grouped
together as subclasses of TextComponent. The class
named Container refers to components that can contain other
components. The Applet class is, indirectly, a subclass of
Container since applets can contain components such as buttons
and text fields.
Just from this brief discussion, perhaps you can see
how GUI programming can make effective use of object-oriented
design. In fact, GUI's, with their "visible
objects," are probably a major factor contributing to
the popularity of OOP.
Programming with GUI components and events is one of the
most interesting aspects of Java. However, we will spend
several chapters on the basics before returning to this topic
in Chapter 6.
