Java- An introduction

Java technology is both a programming language and a platform.

The Java Programming Language

The Java programming language is a high-level language that can be characterized by all of the following buzzwords:

Each of the preceding buzzwords is explained in The Java Language Environment , a white paper written by James Gosling and Henry McGilton.

In the Java programming language, all source code is first written in plain text files ending with the .java extension. Those source files are then compiled into .class files by the javac compiler. A .class file does not contain code that is native to your processor; it instead contains bytecodes — the machine language of the Java Virtual Machine¹ (Java VM). The javalauncher tool then runs your application with an instance of the Java Virtual Machine.

An overview of the software development process.

Because the Java VM is available on many different operating systems, the same .class files are capable of running on Microsoft Windows, the Solaris™ Operating System (Solaris OS), Linux, or Mac OS. Some virtual machines, such as the Java SE HotSpot at a Glance, perform additional steps at runtime to give your application a performance boost. This includes various tasks such as finding performance bottlenecks and recompiling (to native code) frequently used sections of code.

Through the Java VM, the same application is capable of running on multiple platforms.

The Java Platform

A platform is the hardware or software environment in which a program runs. We've already mentioned some of the most popular platforms like Microsoft Windows, Linux, Solaris OS, and Mac OS. Most platforms can be described as a combination of the operating system and underlying hardware. The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms.

The Java platform has two components:

• The Java Virtual Machine
• The Java Application Programming Interface (API)

You've already been introduced to the Java Virtual Machine; it's the base for the Java platform and is ported onto various hardware-based platforms.

The API is a large collection of ready-made software components that provide many useful capabilities. It is grouped into libraries of related classes and interfaces; these libraries are known aspackages. The next section, What Can Java Technology Do? highlights some of the functionality provided by the API.

The API and Java Virtual Machine insulate the program from the underlying hardware.

As a platform-independent environment, the Java platform can be a bit slower than native code. However, advances in compiler and virtual machine technologies are bringing performance close to that of native code without threatening portability.

The terms"Java Virtual Machine" and "JVM" mean a Virtual Machine for the Java platform.

What Can Java Technology Do?

The general-purpose, high-level Java programming language is a powerful software platform. Every full implementation of the Java platform gives you the following features:

• Development Tools: The development tools provide everything you'll need for compiling, running, monitoring, debugging, and documenting your applications. As a new developer, the main tools you'll be using are thejavac compiler, the java launcher, and the javadoc documentation tool.
• Application Programming Interface (API): The API provides the core functionality of the Java programming language. It offers a wide array of useful classes ready for use in your own applications. It spans everything from basic objects, to networking and security, to XML generation and database access, and more..
• Deployment Technologies: The JDK software provides standard mechanisms such as the Java Web Start software and Java Plug-In software for deploying your applications to end users.
• User Interface Toolkits: The JavaFX, Swing, and Java 2D toolkits make it possible to create sophisticated Graphical User Interfaces (GUIs).
• Integration Libraries: Integration libraries such as the Java IDL API, JDBC API, Java Naming and Directory Interface (JNDI) API, Java RMI, and Java Remote Method Invocation over Internet Inter-ORB Protocol Technology (Java RMI-IIOP Technology) enable database access and manipulation of remote objects.

How Will Java Technology Change My Life?

We can't promise you fame, fortune, or even a job if you learn the Java programming language. Still, it is likely to make your programs better and requires less effort than other languages. We believe that Java technology will help you do the following:

• Get started quickly: Although the Java programming language is a powerful object-oriented language, it's easy to learn, especially for programmers already familiar with C or C++.
• Write less code: Comparisons of program metrics (class counts, method counts, and so on) suggest that a program written in the Java programming language can be four times smaller than the same program written in C++.
• Write better code: The Java programming language encourages good coding practices, and automatic garbage collection helps you avoid memory leaks. Its object orientation, its JavaBeans™ component architecture, and its wide-ranging, easily extendible API let you reuse existing, tested code and introduce fewer bugs.
• Develop programs more quickly: The Java programming language is simpler than C++, and as such, your development time could be up to twice as fast when writing in it. Your programs will also require fewer lines of code.
• Avoid platform dependencies: You can keep your program portable by avoiding the use of libraries written in other languages.
• Write once, run anywhere: Because applications written in the Java programming language are compiled into machine-independent bytecodes, they run consistently on any Java platform.
• Distribute software more easily: With Java Web Start software, users will be able to launch your applications with a single click of the mouse. An automatic version check at startup ensures that users are always up to date with the latest version of your software. If an update is available, the Java Web Start software will automatically update their installation.

"Hello World!" for Java

To write your first program, you'll need:

1. The Java SE Development Kit 8 (JDK 8)

You can download the Windows version now. (Make sure you download the JDK, not the JRE.) Consult the installation instructions.

2. A text editor

In this example, we'll use Notepad, a simple editor included with the Windows platforms. You can easily adapt these instructions if you use a different text editor.

These two items are all you'll need to write your first application.

Creating Your First Application

Your first application, HelloWorldApp, will simply display the greeting "Hello world!". To create this program, you will: 

• Create a source file

A source file contains code, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and edit source files.

• Compile the source file into a .class file

The Java programming language compiler (javac) takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this file are known as bytecodes.

• Run the program

The Java application launcher tool (java) uses the Java virtual machine to run your application.

Create a Source File

To create a source file, you have two options:

• You can save the filejava on your computer and avoid a lot of typing. Then, you can go straight to Compile the Source File into a .class File.
• Or, you can use the following (longer) instructions.

First, start your editor. You can launch the Notepad editor from the Start menu by selecting Programs > Accessories > Notepad. In a new document, type in the following code:

/** * The HelloWorldApp class implements an application that * simply prints "Hello World!" to standard output. */class HelloWorldApp {    public static void main(String[] args) {        System.out.println("Hello World!"); // Display the string.    }}

Be Careful When You Type    

Note: Type all code, commands, and file names exactly as shown. Both the compiler (javac) and launcher (java) are case-sensitive, so you must capitalize consistently.

HelloWorldApp is not the same as helloworldapp.

Save the code in a file with the name HelloWorldApp.java. To do this in Notepad, first choose the File > Save As menu item. Then, in the Save As dialog box:

1. Using theSave in combo box, specify the folder (directory) where you'll save your file. In this example, the directory is myapplication on the C 
2. In theFile name text field, type "HelloWorldApp.java", including the quotation marks.
3. From theSave as type combo box, choose Text Documents (*.txt).
4. In theEncoding combo box, leave the encoding as ANSI.

When you're finished, the dialog box should look like this.

The Save As dialog just before you click Save.

Now click Save, and exit Notepad.

Compile the Source File into a .class File

Bring up a shell, or "command," window. You can do this from the Start menu by choosing Run... and then entering cmd. The shell window should look similar to the following figure.

A shell window.

The prompt shows your current directory. When you bring up the prompt, your current directory is usually your home directory for Windows XP (as shown in the preceding figure.

To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is myapplication on the C drive, type the following command at the prompt and press Enter:

cd C:\myapplication

Now the prompt should change to C:\myapplication>.

Note: 

To change to a directory on a different drive, you must type an extra command: the name of the drive. For example, to change to the myapplication directory on the D drive, you must enter D:, as follows:

C:\>D: D:\>cd myapplication D:\myapplication>

If you enter dir at the prompt, you should see your source file, as follows:

C:\>cd myapplication C:\myapplication>dir Volume in drive C is System Volume Serial Number is F2E8-C8CC  Directory of C:\myapplication 2014-04-24  01:34 PM   

Now you are ready to compile. At the prompt, type the following command and press Enter.

javac HelloWorldApp.java

The compiler has generated a bytecode file, HelloWorldApp.class. At the prompt, type dir to see the new file that was generated as follows:

C:\myapplication>javac HelloWorldApp.java C:\myapplication>dir Volume in drive C is System Volume Serial Number is F2E8-C8CC  Directory of C:\myapplication 2014-04-24  02:07 PM   

Now that you have a .class file, you can run your program.

If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

Run the Program

In the same directory, enter the following command at the prompt:

java -cp . HelloWorldApp

You should see the following on your screen:

C:\myapplication>java -cp . HelloWorldAppHello World! C:\myapplication>

Congratulations! Your program works!

If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

Lesson: A Closer Look at the "Hello World!" Application

Now that you've seen the "Hello World!" application (and perhaps even compiled and run it), you might be wondering how it works. Here again is its code:

 class HelloWorldApp {    public static void main(String[] args) {        System.out.println("Hello World!"); // Display the string.    }}

The "Hello World!" application consists of three primary components: source code comments, the HelloWorldApp class definition, and the main method. The following explanation will