Skip to content

Set

Set is an interface in the java.util package.

The most important aspect of Set is: Sets do not contain duplicate elements. * No two elements in a Set can be equals() to each other. * At most, one null element may be placed in a Set. * Some implementations may disallow null.

Set is unordered. * Subinterfaces may guarantee an order, but Set does not.

Set<E> Is Generic

Like List, we tell Set what type of elements it will contain. 

Set<String> strSet;  // Set of String objects

Set Methods

Many methods of Set are the same as List. * int size() - get the number of elements in the Set. * boolean isEmpty() - returns true if this Set contains no elements. * boolean contains(Object o) - returns true if the object is in the Set (such that o.equals(element) is true). * boolean remove(Object o) - returns true and removes the object if it was in the Set.

Set vs. List

Set is different than List, both in its concept and its methods.

  • boolean add(E)

    This method returns true if adding to the set succeeded. * Since a Set does not contain duplicates, add only succeeds when the new element is not .equals() to an element already in the Set. * If a .equals() element was already in the Set, this returns false.

  • Set does not maintain an order, so there is no get(index) or set(index, E).

  • get() does not exist at all.

    • Reading objects from a Set requires an Iterator object to step through the Set's elements.
  • iterator() - returns an Iterator<E> reference for accessing a Set's elements.

Iterator<E>

Set provides an Iterator object, which allows us to step through the elements in the Set. * boolean hasNext() determines if there are more elements in a collection. * E next() gets the next element.

Iterator<String> it = strSet.iterator();

while (it.hasNext()) {
  String element = it.next();
  System.out.println(element);
}

Prev -- Up -- Next