you want to program in Java? That's great, and you've come to the right place.
This column will give you a complete education on Java programming, starting with
the basics, and covering all of the core concepts you need to become productive
in the language. This column will be technical, with plenty of code examples to
help you along. I'll assume that you already have some programming experience.
I'll start off with a short overview of what Java is and how it works, move
on to basic coverage of some object-oriented programming concepts, and then
jump right into creating Java classes -- the heart of Java programming.
I'll try to give examples and instructions that are as platform-neutral as
possible, but I'll default to the Windows platform when necessary. Unix users
should have an easy time interpreting these examples for the Unix world. Mac
and other users will have to work a little harder (our apologies).
High-level overview of Java
Java is a general-purpose, object-oriented language that looks a lot like C
and C++. Its design, however, was built on those of its predecessors, making
it easier, safer, and more productive than C++. While Java started out as a
niche language for developing applets or small programs that run in Web browsers,
it has evolved to arguably become the most important programming language for
developing ecommerce and other Web-driven applications. Its area of use is growing
daily and includes dynamic Web-content generation with servlet technology, the
building of business components with Enterprise JavaBeans, the creation of cross-platform
user interfaces with Swing, and much more. Portable, distributed, multitier,
object-oriented programs driven by the Web are the order of the day, and there
is no language better than Java for writing these programs.
The Java Virtual Machine
Let's take a look at a central component of the Java architecture, the Java
Virtual Machine (JVM). The JVM is what gives Java its cross-platform functionality
and many of its security and safety capabilities. The JVM is basically an abstract
computer implemented in software. I'll focus mainly on its instruction set,
which is called bytecode. Bytecode is an intermediate language between
Java source and the target computer you want to run on. The following figure
demonstrates how it works at a very high level.
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From Java source to bytecode to host machine code
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- Programs are written in Java and stored in
.java files (for
example, MyClass.java)
- The
.java files are compiled by the Java compiler into bytecode
and stored in .class files (for example, MyClass.class)
- The JVM loads the bytecode (the
.class files), performs some
checks on it, and then converts it to the machine code of the target platform
that executes it
This is where Java gets its platform independence. The bytecode format is
the same on all platforms because it runs in the same abstract machine -- the
JVM. As long as there is a JVM on any given platform, you can run Java on it.
There's an old saying in computer programming, "You can solve any problem with
another level of indirection." The JVM and bytecode together is another level
of indirection.
That's about it on the JVM for now. If you want to know more, check out Sun's
JVM specification in the Resources section below.
Setting up Java on your system
The first thing you need is the Java 2 Software Development Kit or SDK (formerly
known as the Java Development Kit, or JDK). This is a set of software and software
tools supplied by Sun that includes all of the basic components needed to build
Java programs. If you don't have this, you'll need to download the Java 2 SDK
from Sun (see Resources below). Here's a brief description
of what you need to do to install version 1.2.x for the Windows platform:
- For Windows, the download will be a self-extracting archive, and the file
name follows the form:
jdk<version>-win.exe. For example,
for version 1.2.2, the file is named jdk1_2_2-win.exe.
- Execute the program to install the Java 2 SDK.
- You can execute it by typing its name at a command prompt.
- You can double-click it from Explorer.
- On Windows, the default installation directory is
C:\JDK1.2.x
- You must set up your environment correctly
- Your path must be set to include the
bin subdirectory
- For example, if you installed the Java 2 SDK in
C:\JDK1.2.x,
you must include C:\JDK1.2.x\bin in your path
- If you are using Windows 95/98, you can do this in
autoexec.bat
- If you are using Windows NT, go to Control Panel, System, Environment
- You can set environment variables in the dialog box there
- There are other parts of the environment that are important, especially
the
CLASSPATH environment variable. I'll discuss this in future
columns.
- It's also important to download the javadoc HTML documentation for the core
API classes. This is a separate download, in zip format, and the filename
is in the form of
jdk<version>-doc.zip (for example, jdk1_2_2-doc.zip).
- The javadoc HTML documentation should be unzipped into the Java 2 SDK installation
directory
- Note that the zip file has a directory structure within
it, so that unzipping it into
C:\ will place all of the files
at the root directory C:\JDK1.2.x\docs, which is a good place
for them.
The following tools will be used to create and run our first program:
javac: the Java compiler that converts Java source to bytecodejava: the Java Virtual Machine
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