Case 175
Flat Shark
A monk asked Java master Kaimu:
What is the “single-responsibility principle”?
Said Kaimu:
That a class shall do exactly one thing,
and do it completely.
The monk asked:
How shall I decide this “one thing”?
Said Kaimu:
The Fisherman does not build ships,
or we would call him a Shipwright.
The monk asked:
Is there no room in your philosophy for interfaces?
What if my class can serve as a Fisherman,
and a Shipwright and Sailor besides?
Said Kaimu:
What would you name such a three-headed monster?
The monk replied:
Shísho, after my uncle. He lives by the sea and does all these things.
Said Kaimu:
I would give your class Shísho three instance variables:
a Fisherman, a Shipwright, and a Sailor.
Then Shísho may implement those interfaces by delegation.
The monk replied:
I speak of inheritance, yet you answer in composition.
All of my uncle is a Fisherman, not just his left foot.
What use are objects, if we do not faithfully model the world?
Said Kaimu:
If I paint a fine shark upon this page, will you say, “Fine shark!”
or will you complain that it is flat and does not eat you?
The monk asked:
But how are we to know when the flat shark is shark enough?
Or when our uncle should fish with his left foot?
Said Kaimu:
Learning how is our “single reponsibility”.
Qi’s Commentary
Every class does exactly one thing—
but sometimes that one thing
is “doing too many things”.
Qi’s Poem
One morning Kaimu took up his brush.
In two strokes he painted an exquisite shark.
A passerby laughed. “It is a poor shark that can’t open its mouth!”
Kaimu replied, “Even so, it has told me that you are a fool.”
