What a Set Refuses, and What a Queue Chooses

Five things this topic established, and where each one was.

What the topic established, and which card it was on
The thing	Where
A set decides duplication with `hashCode` and `equals` on the element	A Collection That Decides What Counts as a Duplicate
`add` returns whether the element was new, which saves a second lookup	Adding the Same Order Twice
`HashSet` has no order, `LinkedHashSet` keeps insertion order, `TreeSet` sorts	Three Orderings, Three Costs
A priority queue is a heap, so only the front element is in order	Why Printing a PriorityQueue Does Not Show Poll Order
Use `ArrayDeque` for both queues and stacks; `Stack` is a 1996 class	Why the Stack Class Still Exists

Both halves of this topic are collections that make a decision for you. A set decides what is the same; a queue decides what is next. Choosing one means choosing whose decision you want.

Java
Java Collections