My question is that what is ".in" in system.in and i am not understandin the workin of System.in suppose

Scanner obj=new Scanner(System.in);

plz understand me the working of upper line.

in is a public constant in the System class. It refers to an InputStream object that gets the input from the console.

Here's part of the API doc for the System class:

public static final InputStream in

The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.