Here is the answer,
The Runtime class provide system level functionality to Java applications.
The facilities of these classes are not necessarily portable across host operating system and may need to be recompiled when porting.
Some of the facilities provided are:
Reading from the keyboard as input
Writing to console output
Interaction with JVM process for event notification, or information such as free memory, etc
Reading/writing system properties/environment variables
executing other programs from within java apps.