Java - Introduction

JAVA was developed by Sun Microsystems Inc in 1991, later acquired by Oracle Corporation.

It was developed by James Gosling and Patrick Naughton.

It is a simple programming language.  Writing, compiling and debugging a program is easy in java.  It helps to create modular programs and reusable code.

Java Virtual Machine (JVM)
The phases of program execution are as follows:

Write the program, then compile the program and at last run the program.

1) Writing of the program is done by java programmer.

2) Compilation of program is done by javac compiler, javac is the primary java compiler included in java development kit (JDK). It takes java program as input and generates java bytecode as output.

3) In third phase, JVM executes the bytecode generated by compiler. This is called program run phase.

The primary function of JVM is to execute the bytecode produced by compiler. Each operating system has different JVM, however the output they produce after execution of bytecode is same across all operating systems. That is why java is called platform independent language.

Java Development Kit(JDK)
Java development kit includes JRE (Java Runtime Environment), compilers and various tools like JavaDoc, Java debugger etc. In order to create, compile and run Java program JDK has to be installed on your computer.

Java Runtime Environment(JRE)
JRE is a part of JDK which means that JDK includes JRE. When JRE is installed in a system, a java program can be executed however it cannot compile it. JRE includes JVM, browser plugins and applets support. To run a java program on a computer, JRE is needed.