Question

While reading some Java source, I came across this line:

((Closeable) some_obj).close();

some_obj is obviously an instance of a class which implements the Closeable interface. My question is, why do they first cast some_obj to Closeable before invoking close().
Couldn’t I just do

some_obj.close();

Answer 1

Assuming the compile-time type of some_obj implements Closeable, then yes, you could.

You’d only need this if you had an object which you knew implemented Closeable, but where the compile-time type was something more general (the most obvious example being Object) or otherwise “different” (e.g. a different interface).

Just as a matter of interest, in C# a cast to an interface type can make a difference, even if the compile-time type is known to implement the interface, due to explicit interface implementation. I can give more details if anyone cares, but I just thought I’d throw it out there.