select or collect — I keep reaching for the wrong one

OCL and Business Logic
1

I understand what both do when I read the definition, and then I sit down to write an
expression and pick the wrong one about half the time. Is there a way to think about it that
sticks?

2

The framing that finally made it stick for me:

  • select changes how many. Same kind of thing, fewer of them.
    orders->select(total > 1000) is still orders.
  • collect changes what kind. Different kind of thing, same number of them.
    orders->collect(customer) is customers, not orders.

So ask yourself what the result is made of. If the answer is the same noun you started
with, you want select. If it is a different noun, you want collect.

The type checker catches it one line later either way, but knowing which you meant before you
type it is faster.

3

‘What is the result made of’ — that is the question I was missing. Thank you.

4

Borrowing this for teaching. The noun test is much better than the definition.