AS COMPUTERS AND THEIR USER INTERFACES have become easier to use, they have also become more complex for programmers to deal with. You can write programs for a simple console-style user interface using just a few subroutines that write output to the console and read the user's typed replies. A modern graphical user interface, with windows, buttons, scroll bars, menus, text-input boxes, and so on, might make things easier for the user. But it forces the programmer to cope with a hugely expanded array of possibilities. The programmer sees this increased complexity in the form of great numbers of subroutines that are provided for managing the user interface, as well as for other purposes.
Someone who wants to program for Macintosh computers -- and to produce programs that look and behave the way users expect them to -- must deal with the Macintosh Toolbox, a collection of well over a thousand different subroutines. There are routines for opening and closing windows, for drawing geometric figures and text to windows, for adding buttons to windows, and for responding to mouse clicks on the window. There are other routines for creating menus and for reacting to user selections from menus. Aside from the user interface, there are routines for opening files and reading data from them, for communicating over a network, for sending output to a printer, for handling communication between programs, and in general for doing all the standard things that a computer has to do. Windows 98 and Windows 2000 provide their own sets of subroutines for programmers to use, and they are quite a bit different from the subroutines used on the Mac.
The analogy of a "toolbox" is a good one to keep in mind. Every programming project involves a mixture of innovation and reuse of existing tools. A programmer is given a set of tools to work with, starting with the set of basic tools that are built into the language: things like variables, assignment statements, if statements, and loops. To these, the programmer can add existing toolboxes full of routines that have already been written for performing certain tasks. These tools, if they are well-designed, can be used as true black boxes: They can be called to perform their assigned tasks without worrying about the particular steps they go through to accomplish those tasks. The innovative part of programming is to take all these tools and apply them to some particular project or problem (word-processing, keeping track of bank accounts, processing image data from a space probe, Web browsing, computer games,...). This is called applications programming.
A software toolbox is a kind of black box, and it presents a certain interface to the programmer. This interface is a specification of what routines are in the toolbox, what parameters they use, and what tasks they perform. This information constitutes the API, or Applications Programming Interface, associated with the toolbox. The Macintosh API is a specification of all the routines available in the Macintosh Toolbox. A company that makes some hardware device -- say a card for connecting a computer to a network -- might publish an API for that device consisting of a list of routines that programmers can call in order to communicate with and control the device. Scientists who write a set of routines for doing some kind of complex computation -- such as solving "differential equations", say -- would provide an API to allow others to use those routines without understanding the details of the computations they perform.
The Java programming language is supplemented by a large, standard API. You've seen part of this API already, in the form of mathematical subroutines such as Math.sqrt(), the String data type and its associated routines, and the System.out.print() routines. The standard Java API includes routines for working with graphical user interfaces, for network communication, for reading and writing files, and more. It's tempting to think of these routines as being built into the Java language, but they are technically subroutines that have been written and made available for use in Java programs.
Java is platform-independent. That is, the same program can run on platforms as diverse as Macintosh, Windows, UNIX, and others. The same Java API must work on all these platforms. But notice that it is the interface that is platform-independent; the implementation varies from one platform to another. A Java system on a particular computer includes implementations of all the standard API routines. A Java program includes only calls to those routines. When the Java interpreter executes a program and encounters a call to one of the standard routines, it will pull up and execute the implementation of that routine which is appropriate for the particular platform on which it is running. This is a very powerful idea. It means that you only need to learn one API to program for a wide variety of platforms.
Like all subroutines in Java, the routines in the standard API are grouped into classes. To provide larger-scale organization, classes in Java can be grouped into packages. You can have even higher levels of grouping, since packages can also contain other packages. In fact, the entire standard Java API is implemented in several packages. One of these, which is named "java", contains the non-GUI packages as well as the original AWT graphics user interface classes. Another package, "javax", was added in Java version 1.2 and contains the classes used by the Swing graphical user interface.
A package can contain both classes and other packages. A package that is contained in another package is sometimes called a "sub-package." Both the java package and the javax package contain sub-packages. One of the sub-packages of java, for example, is called "awt". Since awt is contained within java, its full name is actually java.awt. This is the package that contains classes related to the AWT graphical user interface, such as a Button class which represents push-buttons on the screen and the Graphics class which provides routines for drawing on the screen. Since these classes are contained in the package java.awt, their full names are actually java.awt.Button and java.awt.Graphics. (I hope that by now you've gotten the hang of how this naming thing works in Java.) Similarly, javax contains a sub-package named javax.swing, which includes such classes as javax.swing.JButton and javax.swing.JApplet.
The java package includes several other sub-packages, such as java.io, which provides facilities for input/output, java.net, which deals with network communication, and java.applet, which implements the basic functionality of applets. The most basic package is called java.lang. This package contains fundamental classes such as String and Math.
It might be helpful to look at a graphical representation of the levels of nesting in the java package, its sub-packages, the classes in those sub-packages, and the subroutines in those classes. This is not a complete picture, since it shows only a few of the many items in each element:
Let's say that you want to use the class java.awt.Color in a program that you are writing. One way to do this is to use the full name of the class. For example, you could say
to declare a variable named rectColor whose type is java.awt.Color. Of course, this can get tiresome, so Java makes it possible to avoid using the full names of classes. If you put
at the beginning of a Java source code file, then, in the rest of the file, you can abbreviate the full name java.awt.Color to just the name of the class, Color. This would allow you to say just
to declare the variable rectColor. (The only effect of the import statement is to allow you to use simple class names instead of full "package.class" names; you aren't really importing anything substantial. If you leave out the import statement, you can still access the class -- you just have to use its full name.) There is a shortcut for importing all the classes from a given package. You can import all the classes from java.awt by saying
and you can import all the classes from javax.swing with the line
In fact, any Java program that uses a graphical user interface is likely to begin with one or both of these lines. A program might also include lines such as "import java.net.*;" or "import java.io.*;" to get easy access to networking and input/output classes. (When you start importing lots of packages in this way, you have to be careful about one thing: It's possible for two classes that are in different packages to have the same name. For example, both the java.awt package and the java.util package contain classes named List. If you import both java.awt.* and java.util.*, the simple name List will be ambiguous. If you try to declare a variable of type List, you will get a compiler error message about an ambiguous class name. The solution is simple: use the full name of the class, either java.awt.List or java.util.List. Another solution is to use import to import the individual classes you need, instead of importing entire packages.)
Because the package java.lang is so fundamental, all the classes in java.lang are automatically imported into every program. It's as if every program began with the statement "import java.lang.*;". This is why we have been able to use the class name String instead of java.lang.String, and Math.sqrt() instead of java.lang.Math.sqrt(). It would still, however, be perfectly legal to use the longer forms of the names.
Programmers can create new packages. Suppose that you want some classes that you are writing to be in a package named utilities. Then the source code file that defines those classes must begin with the line "package utilities;". Any program that uses the classes should include the directive "import utilities.*;" to obtain access to all the classes in the utilities package. Unfortunately, things are a little more complicated than this. Remember that if a program uses a class, then the class must be "available" when the program is compiled and when it is executed. Exactly what this means depends on which Java environment you are using. Most commonly, classes in a package named utilities should be in a directory with the name utilities, and that directory should be located in the same place as the program that uses the classes.
In projects that define large numbers of classes, it makes sense to organize those classes into one or more packages. It also makes sense for programmers to create new packages as toolboxes that provide functionality and API's for dealing with areas not covered in the standard Java API. (And in fact such "toolmaking" programmers often have more prestige than the applications programmers who use their tools.)
However, I will not be creating any packages in this textbook. You need to know about packages mainly so that you will be able to import the standard packages. These packages are always available to the programs that you write. You might wonder where the standard classes are actually located. Again, that depends to some extent on the version of Java that you are using. But they are likely to be collected together into a large file named rt.jar or classes.zip, which is located in some place where the Java compiler and the Java interpreter will know to look for it.
Although we won't be creating packages explicitly, every class is actually part of a package. If a class is not specifically placed in a package, then it is put in something called the default package, which has no name. All the examples that you see in these notes are in the default package.
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