if you want to call a methods which are specific to the 
platform or if you want to use the OS calls or if you want 
to use functionality which is developed in other language 
we go through native interface.

In a real world we need to execute statements which are 
specific to platform which java may or may not do, and 
other language can do much faster than java at that time we 
use to write the methods in that language and use that code.

And sometimes the code already developed in other language 
and it is very cost effective or time consuming to convert 
the code into java at that moments also we use java native 
interface.

native method is used to call the methods declared in a 
platform dependent environment.