20 Sets

A set keeps distinct elements and answers membership queries.

main() {
  Set<String> names = Set<>()
  names.add("Ada")
  names.add("Ada")
  names.add("Lin")
  printLine(names.size())
  printLine(names.contains("Lin"))
  printLine(names.contains("Ada"))
  printLine(names.contains("Jo"))
}

Expected output:

2
true
true
false

Adding Ada twice leaves only one Ada entry. contains asks whether Lin is present; the set does not promise an index for either name.

Try it: Remove one repeated add and check that the output stays the same.

Precise rules: Reference.