Map you can convert its keys or values into a Collection, transfer them into a new List, and then sort using Collections.sort. The same process applies to a Set. Obviously, this method is inefficient as it requires a complete copy of the content.A more efficient way is to store the data already sorted. For this purpose, the
SortedSet and SortedMap interfaces were introduced.SortedSet implementations provide a total order for the set. Elements are ordered in ascending order. This order is either natural (elements implement the Comparable interface), or determined by a Comparator constructor parameter.This interface adds methods to get subsets from a specified element (
tailSet), up to an element (headSet), and between two elements (subSet). The subset includes the lower bound but not the upper bound.SortedSet is extended by the NavigableSet interface which allows for iterating in order, and retrieving the closest element below (floor), above (ceiling), higher than, or lower than a given element.The same rules apply to the keys of a
SortedMap/NavigableMap.The primary implementations are
TreeSet and TreeMap. Under the hood these are self-balancing red-black trees. The structure and method of balancing of these trees merit a separate post. Another interesting implementation from java.util.concurrent is the ConcurrentSkipListMap.