Abstract class InputStream
says that subclasses need to implement method read()
which reads one byte
and then turns it into an unsigned int
.
System.in
is an InputStream
and I can do:
int i = System.in.read();
My question is.. where is this method implemented? How come it works? Maybe an odd question but I'm trying to find out what's happening under the hood and since I'm using an object of class InputStream and not one of its subclasses, I'm wondering where the actual method is implemented and why it works...
InputStream
is the type of System.in
, and not it's class (since InputStream
cannot be directly instantiated as it is abstract).
Consider:
Object obj = "123";
The type of the variable obj
is Object
, but the instance referenced by obj
is an instance of String
. When toString()
is called on obj
the implementation in String
is used, and not the implementation in Object
.
The same goes for System.in
. The actual instance stored there will be some subclass of InputStream
, which will have its own implementation of any abstract methods. If you want to know the class of the instance stored in System.in
then you can call System.in.getClass()
.