Introduction of Java

Java is a simple and yet powerful object oriented programming language. Java is created by James Gosling at Sun Microsystems, Inc. in 1991. The first publicly available version of Java (Java 1.0) was released in 1995.

Java is defined by a specification and consists of a programming language, a compiler, core libraries and a run time machine (Java virtual machine). The Java platform is usually associated with the Java virtual machine and the Java core libraries.

Java Virtual Machine (JVM)

Java was designed with a concept of ‘write once and run everywhere’. Java Virtual Machine plays the central role in this concept. The JVM is the environment in which Java programs execute. It is a software that is implemented on top of real hardware and operating system. When the source code (.java files) is compiled, it is translated into byte codes and then placed into (.class) files. The JVM executes these bytecodes. So Java byte codes can be thought of as the machine language of the JVM. A JVM can either interpret the bytecode one instruction at a time or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler. The JVM must be implemented on a particular platform before compiled programs can run on that platform.

Java Runtime Environment (JRE) and Java Development Kit (JDK)

A Java distribution typically comes in two flavors, the JRE and the JDK. The JRE consists of the JVM and the Java class libraries. Those contain the necessary functionality to start Java Programs. The JDK additionally contains the development tools necessary to create Java programs. The JDK therefore consists of a Java compiler, the java virtual machine and the Java class libraries.