Set<T> stores unique elements; their order is generally undefined. null elements are unique as well: a Set can contain only one null. Two sets are equal if they have the same size, and for each element of a set there is an equal element in the other set.
```
val numbers = setOf(1, 2, 3, 4)
println("Number of elements: ${numbers.size}")
if (numbers.contains(1)) println("1 is in the set")
val numbersBackwards = setOf(4, 3, 2, 1)
println("The sets are equal: ${numbers == numbersBackwards}")
```
MutableSet is a Set with write operations from MutableCollection.
The default implementation of Set – LinkedHashSet – preserves the order of elements insertion. Hence, the functions that rely on the order, such as first() or last(), return predictable results on such sets.
```
val numbers = setOf(1, 2, 3, 4) // LinkedHashSet is the default implementation
val numbersBackwards = setOf(4, 3, 2, 1)
println(numbers.first() == numbersBackwards.first())
println(numbers.first() == numbersBackwards.last())
```
An alternative implementation – HashSet – says nothing about the elements order, so calling such functions on it returns unpredictable results. However, HashSet requires less memory to store the same number of elements.