Introduction Of Java

1.1 History

Java is a programming language created by James Gosling from Sun Micro-systems in 1991. The first publicly available version of Java 1.0 was released in 1995.

Java programming language the Java platform evolved. The Java platform allows that the program code is written in other languages than the Java programming language and still runs on the Java virtual machine.

 

1.2 java Virtual Machine

The Java virtual machine (JVM) is a software implementation of a computer that executes programs.

The Java virtual machine is written specifically for a specific operating system, e.g. for Linux a special implementation is required as well as for Windows.

Java programs are compiled by the Java compiler into so-called bytecode. The Java virtual machine interprets this bytecode and executes the Java program.

 

1.3 Java Runtime Environment And Java Developement kit

The Java runtime environment (JRE) consists of the JVM and the Java class libraries and contains the necessary functionality to start Java programs.

The JDK contains in addition the development tools necessary to create Java programs. The JDK consists therefore of a Java compiler, the Java virtual machine, and the Java class libraries.

 

1.4 Characteristics Of Java

1 Java is Simple

No language is simple, but Java is a bit easier than the popular object-oriented programming language C++, which was the dominant software-development language before Java.
Java is partially modeled on C++, but greatly simplified and improved. For instance, pointers and multiple inheritance often make programming complicated. Java replaces the multiple inheritance in C++ with a simple language construct called an interface, and eliminates pointers.

2 Java Is Object-Oriented

Java is inherently object-oriented. Although many object oriented languages began strictly as procedural languages, Java was designed from the start to be object-oriented. Object-oriented programming (OOP) is a popular programming approach that is replacing traditional procedural programming techniques. 

3 Java Is Distributed

Distributed computing involves several computers working together on a network. Java is designed to make distributed computing easy. Since networking capability is inherently integrated into Java, writing network programs is like sending and receiving data to and from a file.

4 Java Is Interpreted

You need an interpreter to run Java programs. The programs are compiled into the Java Virtual Machine code called bytecode. The bytecode is machine-independent and can run on any machine that has a Java interpreter, which is part of the Java Virtual Machine (JVM).

5 Java Is Robust

Robust means reliable. No programming language can ensure complete reliability. Java puts a lot of emphasis on early checking for possible errors, because Java compilers can detect many problems that would first show up at execution time in other languages. Java has eliminated certain types of error-prone programming constructs found in other languages. It does not support pointers, for example, thereby eliminating the possibility of overwriting memory and corrupting data.  

6 Java Is Secure

As an Internet programming language, Java is used in a networked and distributed environment. If you download a Java applet (a special kind of program) and run it on your computer, it will not damage your system because Java implements several security mechanisms to protect your system against harm caused by stray programs. The security is based on the premise that nothing should be trusted. 

7 Java Is Architecture-Neutral

This feature enables Java to be architecture-neutral, or to use an alternative term, platform-independent. With a Java Virtual Machine (JVM), you can write one program that will run on any platform.

8 Java is Portable

Because Java is architecture neutral, Java programs are portable. They can be run on any platform without being recompiled.

9 Java's Performance

Java uses native code usage, and lightweight process called  threads. In the beginning interpretation of bytecode resulted the performance slow but the advance version of JVM uses the adaptive and just in time compilation technique that improves the performance. 

10 Java Is Multithreaded

Multithreading is a program’s capability to perform several tasks simultaneously. For example, downloading a video file while playing the video would be considered multithreading.  

11 Java Is Dynamic

Java programs carry with them substantial amounts of run-time type information that is used to verify and resolve accesses to objects at run time.

1.3 Development Process With Java

The programmer writes Java source code in a text editor which supports plain text. Normally the programmer uses an Integrated Development Environment (IDE) for programming. An IDE supports the programmer in the task of writing code, e.g. it provides auto-formating of the source code, highlighting of the important keywords, etc. At some point the programmer (or the IDE) calls the Java compiler (javac). The Java compiler creates the bytecode instructions. . These instructions are stored in .class files and can be executed by the Java Virtual Machine

Classpath

The classpath defines where the Java compiler and Java runtime look for .class files to load. This instructions can be used in the Java program.

For example if you want to use an external Java library you have to add this library to your classpath to use it in your program.

Your first Java Program.
 Write Source Code
The following java program which is developed under the Microsoft Windows.

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

Now save this program into bin directory of your java installation with .java extension.
eg.
C:\Program Files\Java\jdk1.6.0_23\bin\Hello.java
Now for compilation you open cmd and go to your bin directory and type command as like following.
 C:\Program Files\Java\jdk1.6.0_23\bin\javac Hello.java
after executing above command the Hello.class file is created which is bytes code file.
Now Executing your Hello file
C:\Program Files\Java\jdk1.6.0_23\bin\java  Hello