What exactly is System.in?

[Ernest Lee]

The System.in is declared as
"public static final InputStream in"...
but InputStream is an abstract class so it cannot be instaniated... so.. what is System.in?

[Valentin Crettaz]

in fact System.in is initialized with the static member in of FileDescriptor (FileDescriptor.in).
In the System class, in is further initializedas a FileInputStream wrapped around that FileDescriptor

[...]
FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
[...]
setIn0(new BufferedInputStream(fdIn));
[...]

As you can see, System.in is a BufferedInputStream wrapped around a FileInputStream which in turn wraps a FileDescriptor...
You don't really need to know what type is System.in of, but it's good practice to look inside the sources...
HIH

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Valentin Crettaz
Sun Certified Programmer for Java 2 Platform