HashSet, LinkedHashSet and TreeSet: Three Orderings, Three Costs

Three classes implementing Set, identical in what they promise and different in one thing the interface does not mention: what order you get things back in.

The three sets on iteration order, cost and what they require of elements
Class	Iterates in	Add and contains	Requires of elements
`HashSet`	No promised order at all	Constant	`hashCode` and `equals`
`LinkedHashSet`	The order you added them	Constant, plus two pointers each	`hashCode` and `equals`
`TreeSet`	Sorted	Proportional to the logarithm of the size	`Comparable`, or a `Comparator`

java

for (Set<String> s : List.of(new HashSet<String>(), new LinkedHashSet<String>(), new TreeSet<String>())) {
    s.addAll(List.of("pear", "apple", "fig"));
    System.out.println(s);
}

text

[apple, pear, fig]
[pear, apple, fig]
[apple, fig, pear]

The decision rule. Use HashSet unless you need an order. Use LinkedHashSet when output should be reproducible or should match input order. Use TreeSet when you need the elements sorted, or need the extra questions a sorted set can answer — first, last, headSet, ceiling.