SоrtedChаrSet is аn ADT (Abstrаct Data Type) that represents a sоrted set оf characters (no duplicates). Below is an implementation of SortedCharSet. // Specification: SortedCharSet represents a sorted set of characters (no duplicates) public class SortedCharSet { private StringBuilder str; // representation // Specification: toString() returns a String describing what the client expects to see public String toString() {…} … } If the rep-invariant is not empty and it states that str should have no duplicates, meanwhile str took on the value “aab”, what does that mean?