java defination

About the java Technology

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 buzzword.

• Simple
• Object oriented
• Distributed
• Multithreaded
• Dynamic
• portable
• High performance
• Robust
• Secure

      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 by the java compiler.

 a .class file does not contain code that is native to your processor.it's contain the bytecode - the machine language of the java virtual Machine (java VM).  Because the java virtual machine is available on many different operating system,the same .class files are capable of running on microsoft Windows,Linux or MAC.

java Program

class hello

{

public static void main(string args[])

{

System.out.println("hello World ");

}

}

A plateform is the h/w and s/w environment in which a program runs.

The java plateform has two components:

• The java virtual Machine
• the java Application Programming Interface(API)

Java Program Structure

 A file containing Java source code is considered a compilation unit. Such a compilation unit contains a set of classes and, optionally, a package definition to group related classes together. Classes contain data and method members that specify the state and behavior of the objects in your program. Java programs come in two flavors: Standalone applications that have no initial context such as a pre-existing main window Applets for WWW programming The major differences between applications and applets are: Applets are not allowed to use file I/O and sockets (other than to the host platform). Applications do not have these restrictions. An applet must be a subclass of the Java Applet class. Aplications do not need to subclass any particular class. Unlike applets, applications can have menus. Unlike applications, applets need to respond to predefined lifecycle messages from the WWW browser in which they're running. 

Java Program Execution

 The Java byte-code compiler translates a Java source file into machineindependent byte code. The byte code for each publicly visible class is placed in a separate file, so that the Java runtime system can easily find it. If your program instantiates an object of class A, for example, the class loader searches the directories listed in your CLASSPATH environment variable for a file called A.class that contains the class definition and byte code for class A. There is no link phase for Java programs.all linking is done dynamically runtime.