JAVA PPROGRAMMING

Elements of Programming :

Overview.

 Our goal in this chapter is to convince you that writing a computer program is easier than writing a piece of text such as a paragraph or an essay. Writing prose is difficult: we spend many years in school to learn how to do it. By contrast, just a few building blocks suffice to take us into a world where we can harness the computer to help us solve all sorts of fascinating problems that would be otherwise unapproachable. In this chapter, we take you through these building blocks, get you started on programming in Java, and study a variety of interesting programs.

Why java is called platform independent language ?

One of the major features of java includes that why java is called platform independent language.
Before understanding this feature we need to know about -

Javac – compiler that converts source code to byte code. 
JVM- interpreter that converts byte code to machine language code. 
As we know java is both compiler & interpreter based language. Once the java code also known as source code is compiled, it gets converted to native code known as BYTE CODE which is portable & can be easily executed on all operating systems. Byte code generated is basically represented in hexa decimal format. This format is same on every platform be it Solaris work station or Macintosh, windows or Linux. After compilation, the interpreter reads the generated byte code & translates it according to the host machine. . Byte code is interpreted by Java Virtual Machine which is available with all the operating systems we install. so to port Java programs to a new platform all that is required is to port the interpreter and some of the library routines.
Source code -> javac ->Universal byte code

In this section, our plan is to lead you into the world of Java programming by taking you through the three basic steps required to get a simple program running. The Java system is a collection of applications not unlike any of the other applications that you are accustomed to using (such as your word processor, e-mail program, or internet browser). As with any application, you need to be sure that Java is properly installed on your computer. You also need an editor and a terminal application. Here are system specific instructions for three popular home operating systems. [ Mac OS X · Windows · Linux ]

Programming in Java.

 We break the process of programming in Java into three steps:

1. Create the program by typing it into a text editor and saving it to a file named, say, MyProgram.java.
2. Compile it by typing "javac MyProgram.java" in the terminal window.
3. Run (or execute) it by typing "java MyProgram" in the terminal window.

The first step creates the program; the second translates it into a language more suitable for machine execution (and puts the result in a file named MyProgram.class); the third actually runs the program.

• Creating a Java program. A program is nothing more than a sequence of characters, like a sentence, a paragraph, or a poem. To create one, we need only define that sequence characters using a text editor in the same way as we do for e-mail. HelloWorld.java is an example program. Type these character into your text editor and save it into a file named HelloWorld.java.

  public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World"); } }

• Compiling a Java program. At first, it might seem to you as though the Java programming language is designed to be best understood by the computer. Actually, to the contrary, the language is designed to be best understood by the programmer (that's you). A compiler is an application that translates programs from the Java language to a language more suitable for executing on the computer. It takes a text file with the .java extension as input (your program) and produces a file with a.class extension (the computer-language version). To compile HelloWorld.java type the boldfaced text below at the terminal. (We use the % symbol to denote the command prompt, but it may appear different depending on your system.)

  % javac HelloWorld.java

  If you typed in the program correctly, you should see no error messages. Otherwise, go back and make sure you typed in the program exactly as it appears above.
• Executing a Java program. Once you compile your program, you can run it. This is the exciting part, where the computer follows your instructions. To run the HelloWorld program, type the following at the terminal:

  % java HelloWorld

  If all goes well, you should see the following response

  Hello, World

• Understanding a Java program. The key line with System.out.println() send the text "Hello, World". When we begin to write more complicated programs, we will discuss the meaning of public, class, main, String[], args, System.out, and so on.
• Creating your own Java program. For the time being, all of our programs will be just like HelloWorld.java, except with a different sequence of statements in main(). The easiest way to write such a program is to:

Below is the syntax  HelloWorld.java :

/*************************************************************************
 *  Compilation:  javac HelloWorld.java
 *  Execution:    java HelloWorld
 *
 *  Prints "Hello, World". By tradition, this is everyone's first program.
 *
 *  % java HelloWorld
 *  Hello, World
 *
 *  These 17 lines of text are comments. They are not part of the program;
 *  they serve to remind us about its properties. The first two lines tell
 *  us what to type to compile and test the program. The next line describes
 *  the purpose of the program. The next few lines give a sample execution
 *  of the program and the resulting output. We will always include such 
 *  lines in our programs and encourage you to do the same.
 *
 *************************************************************************/

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello, World");
    }

}