Question

can I consider File as a special case of InputStream and provide 1 API like

 test(InputStream stream)

or two overloaded APIs are better

test(File f)
test(InputStream stream)

Answer 1

A file is certainly not an input stream. It can be used to create an input stream, but it’s not one itself.

Note that the overloads you’ve given have a fundamentally different character – the method accepting File needn’t change any state, but the method accepting InputStream almost certainly would. Also, assuming they’re going to do something similar internally, the File method would want to close the stream opened in the method; the InputStream method wouldn’t want to close the stream, because it doesn’t “own” it.

That’s okay – it may well be fine to have these two methods anyway – but you need to be aware of the difference.

Another option is to use the Guava library and take an InputSupplier<InputStream> – then callers can use Files.newInputStreamSupplier(file) to specify the stream, but could also use a supplier of network-related input streams, etc.