Question

From the JavaDocs of HashSet:

This class offers constant time performance for the basic operations
(add, remove, contains and size), assuming the hash function disperses
the elements properly among the buckets. Iterating over this set
requires time proportional to the sum of the HashSet instance’s size
(the number of elements) plus the “capacity” of the backing HashMap
instance (the number of buckets). Thus, it’s very important not to set
the initial capacity too high (or the load factor too low) if
iteration performance is important

Why does iteration takes time proportional to the sum(number of elements in set+ capacity of backing map) and not only to the number of elements in the set itself ?

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Answer 1

HashSet is imlemented using a HashMap where the elements are the map keys. Since a map has a defined number of buckets that can contain one or more elements, iteration needs to check each bucket, whether it contains elements or not.