 Solution

Use parallel arrays for keys and their representative values, with only a prefix active:

override val capacity: Int = 1000
private val elements = new Array[Int](capacity)
private val representatives = new Array[Int](capacity)
private var used = 0

The chosen bound is an explicit implementation parameter of the abstract data type; another a-priori positive bound works identically. The initial map is empty because used == 0. Zero-filled unused cells are not members of the map, so zero and negative keys remain valid integers. Use DisjointSets consistently as the trait name; the source's prose variation DisjointSubsets does not change the declared interface.