Java Virtual Machine (JVM)

Java provides both compiler and a software machine called JVM for each computer machine.  The Java Compiler translates the Java Source Code (Java program) into an intermediate code known as byte code, which executes on special type of machine.  This machine is called Java Virtual Machine and exists only inside the computer memory.  The byte code is machine independent code and can run in any system.

 

The byte code is not a machine specific code.  Java Interpreter takes the byte code and translates it into its own system machine language and runs the results.

 

Just In Time (JIT): The Java Virtual Machine is an interpreter that translates and runs each byte code instruction separately whenever it is needed by the computer program.  In some cases it is very slow.

            As an alternative, Java also provides local compiler for each system that will compile byte code file into executable code for faster running.  Java calls these compilers Just In Time compilers.

Java Standard Library(JSL): The Java API (Application Programming Interface) is a collection of classes and methods grouped in the form of packages.  JDK 1.2 has 58 packages, it is also known as Java Standard Library.

            The important Packages in Java are

a) java.lang: It contains the main language support class. It contains wrappers, strings and basic features of Java

b) java.util: It provides classes that support date, time, basic event processing etc.,

c) java.io: It provides reading and writing data in the form of streams.

d) java.awt: It provides classes for creating GUI programs.

e) java.applet: It includes set of classes to support for applet programming.

f) javax.swing: It provides classes for swing operation programming.