Five Requirements: Name the Collection for Each

Five short requirements. Name the collection for each and say what the requirement rules out. All five before the reveal.

The five requirements
	The requirement
1	Record which customer references have been seen, and ask often
2	The same, but the report must list them in the order they arrived
3	The same, but the report must list them alphabetically
4	Hold work items and always take the most urgent one next
5	Undo history: the last action taken is the first one undone

The five answers, and what each requirement rules out

Each requirement, the collection, and what it rules out
	Use	What the requirement ruled out
1	`HashSet`	A list, because `contains` on one is a walk
2	`LinkedHashSet`	`HashSet`, whose iteration order is not promised
3	`TreeSet`	Both hash sets — or sort a copy at the end, if adding is hot
4	`PriorityQueue`	`ArrayDeque`, which has no opinion about urgency
5	`ArrayDeque` used as a stack	`Stack`, which is synchronised and exposes `Vector`

The third has two defensible answers, which is the point of including it. A TreeSet keeps order on every add; sorting a HashSet once at the end does the work only once. Which wins depends on how many adds there are per report.