It is a program that interprets the bytecodes and changes the states of a simulated virtual machine.

Java Virtual Machine:

It is a specification for an architecture independent state machine. This machine is implemented by the Java Interpreter.

Runtime Environment:

The support environment attached to the virtual machine. It connects the virtual machine to the underlying OS and add many basic functions, like I/O, networking, etc.

Portability:

Java bytecodes can run in any machine that has a Java Interpreter running. It is not necessary to recompile the Java program. Only the virtual machine and the runtime system need to be ported to each architecture.

java.lang

The collection of basic classes like Object, Class, String, etc.

java.io

I/O classes like Streams, IOStreams, etc.

java.util

Miscellaneous utility classes, like Dictionary, Stack, Date, Time, etc.

java.awt

Abstract Windowing Toolkit - Classes to deal with graphics (machine independently).

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