Lists are the ordered sequence collection in Java.
Unlike arrays, a List
can grow in size to accomodate any number of items.
One standard implementation is the ArrayList
which is backed by a re-sizable array.
Another standard implementation is the LinkedList
class which is backed by a doubly-linked list.
List
s may be empty or hold any number of items (including duplicates).
List
s are a generic interface typed to indicate which type of objects they can contain.
For example:
List<String> emptyListOfStrings = List.of();
List<Integer> singleInteger = List.of(1);
List<Boolean> threeBooleans = List.of(true, false, true);
List<Object> listWithMulitipleTypes = List.of("hello", 1, true);
List
s have various helpful methods to add, remove, get, and check for an element to be present:
List<Character> vowels = new ArrayList<>(List.of('a', 'e', 'i', 'o', 'i', 'e', 'a'));
int startingSize = vowels.size(); // 7
vowels.add('u'); // vowels is now ['a', 'e', 'i', 'o', 'i', 'e', 'a', 'u']
char a = vowels.get(0); // 'a'
boolean hadI = vowels.remove('i'); // true and vowels is now ['a', 'e', 'o', 'i', 'e', 'a', 'u']
boolean hasI = vowels.contains('i'); // true (still have one more left)
The Collections
class also has some helpful utilities for dealing with collections like List
:
List<Integer> numbers = new ArrayList<>(List.of(1, 5, 3, 2, 4));
Collections.sort(numbers); // [1, 2, 3, 4, 5]
Collections.reverse(numbers); // [5, 4, 3, 2, 1]
Collections.fill(numbers, 42); // [42, 42, 42, 42, 42]
List<Integer> fiveNines = Collections.nCopies(5, 9); // [9, 9, 9, 9, 9]