Java Collections Interface Set

The Set Interface

A Set is a Collection that cannot contain duplicate elements. To achieve this, Set applies a stronger contract on equals() and hashCode() method.

The Java platform contains three general-purpose Set implementations: HashSet, TreeSet, and LinkedHashSet.

• HashSet
  • Hash based, best performing implementation.
  • Not sorted.
  • Insertion order not kept.
• TreeSet
  • Substantially slower than HashSet
  • Sorted.
  • Insertion order not kept.
• LinkedHashSet
  • A little bit higher cost.
  • Not sorted.
  • Insertion order kept.

Set is often used to filter duplicate element in Collections:

• Using constructor

Collection<Type> noDups = new HashSet<Type>(c);

• Stream API

c.stream().collect(Collectors.toSet()); // no duplicates

Set Interface Basic Operations

• add()
  • return true if this set contains the specified element.
• remove()
  • return true if this set contains the specified element.

Set Interface Bulk Operations

• s1.containsAll(s2): Tell if s2 is subset of s1
• s1.addAll(s2): s1 union s2
• s1.retainAll(s2): s1 intersect s2
• s1.removeAll(s2): s1 difference s2

Set Interface Array Operations