System.in - abstract type class variable

[Lorana Berd]

Hello Guys,

Can you please clarify to me how ot understand abstract class type (for example System.in is of type InputStream) as a class (or instance for that tmatter) variable.
What is behinnd "ataching" System.in to the keybpard?

Thanks a lot,
Lorana

[Ernest Friedman-Hill]

Hi,

Welcome to JavaRanch!

System.in points to an object that implements the InputStream interface. The name of that class is unimportant and implementation-dependent. It will likely either contain some native methods to hook up to standard input, or it will access other internal classes that do.

This sort of trick -- a variable of interface type, or method returning interface type, backed by an instance of some implementation-dependent class -- is very common in the JDK. The entire database access API JDBC (in the java.sql package, have a look!) is based on this idea, for example. You onl deal with interfaces; behind the scenes, the various database vendors can implement those interfaces with classes whose names you don't need to know.

[Jim Yingst]

A minor correction: InputStream is an abstract class, not an interface. But the general principle is the same. The variable System.in is declared to be of a general type, InputStream, and the actual object is of a more specific type, but you generally don't need to know what that type is. If you're curious, you can execute

System.out.println("System.in class: " + System.in.getClass());

to see what exact class you get. But for the most part, we don't need to know or care about those details.

[Lorana Berd]

Thank you so much Ernest and Jim, your answers actually cleared many things at once for me.
Thank you,
Loranja