How do you model a Java for each loop on a UML sequence diagram?

Question

for (Item i : collection) {
i.foo();
} 

In some abstract sequence diagrams you have a loop combined fragment with a condition along the lines of e.g. [for each item].

On a sequence diagram featuring Java implementation; what would you use as a loop guard?

If all lifelines must represent an object instance, how would you show that i: Item is not a single instance, but on each iteration a different object from the collection?

Answer 1

You don't show each single bit in a SD but just an overview: it's an abstraction.

This quite clearly shows the intention of your loop.

You would show different Item instances only if you want to show different behavior during the loop.

Answer 2

For the second part of your question, there is a notation to draw multiple instances in the same lifeline using stacked boxes, for example:

from A Quick Introduction to UML Sequence Diagrams

The stacked boxes used for the figures variable indicate it refers to multiple instances, which can be interpreted as being a different instance around the loop. (As usual with UML, the semantics a somewhat in the eye of the beholder)

P. 571 of the UML 2.5 spec states:

The Lifeline head has a shape that is based on the classifier for the part that this lifeline represents. Often the head is a white rectangle containing the name.

The word "Often" can be interpreted as "take what is needed" as it does not obtrude the use of a single specific form.