This is another
principle developed by Robert Martin who I have had the great pleasure to see
speak several times at conferences. He
is a very lively speaker and you return to coding wanting to do your best work
after you see him speak. As for a
skilled software craftsman, he is at the top of his game.
ISP says that no
client should be forced to depend on methods it doesn't use. Martin came up with this when he was
consulting with Xerox. He found they had
a job class that was used for every single task in the system. A stapling job shouldn't have to know
anything about a print job and all the methods for printing.
OK people I work for
Xerox and there nothing called Staple
Job. Stapling is an attribute on a
job. Print jobs can have an attribute
that says staple me. But so can Copy
jobs. So I'm not sure where he saw this
but maybe it was a looong time ago.
In practice when
working with Microsoft Windows for example, there are what I call "Big
A!& Classes". They are huge
with a bazillion attributes and methods.
Often I don't care about 9/10ths of them. I search through the
documentation for what I need. This is
one of many interfaces I have used and dare say I have created that violate
this principle.
However, when I
create an interface, I literally fight people to the bone to keep it
simple. More functionality is desired
and interfaces get new methods and before you know it, you have a massive class
with more methods and data then any sane person can keep straight. I have to argue quite a bit to keep it simple
- deadlines are tight and people don't want to do the extra work for this
principle.
On the flip side, I
think this principle could be considered "too pure" for many software
engineers. And engineers I talk to about
this principle find it too difficult to practice in a real word setting. They understand it intellectually but don't
see the benefits. Testing is one huge
benefit. If you have a massive class for
a job you would have to implement the print functions even for a staple job
(which doesn't exist, just saying…).
Even if it was a no-op, you'd have to test it.
Testing takes up
over half of our development activities.
So if you can save on that, ISP is worth it!
