Collection classes in Java

Collections are very important on SCJP exam. It’s useful to remember the following…

List

• ordered (index)
• duplicates allowed

ArrayList

• fast iteration
• fast random access; as of Java 1.4 implements RandomAccess interface
• slower instertion and deletion

Vector

• the same as ArrayList but its metods are synchronized (therefore slower)

LinkedList

• elements are doubly linked to one another; linkage adds methods that allow adding/removing from beginning and end of the List (stacks, queues)
• fast insertion and deletion
• slower iteration
• as of Java 5 implements java.util.Queue

Set

• can be ordered
• can be sorted
• duplicates not allowed (eqauls())

HashSet

• unordered
• unsorted
• fast access
• uses hashCode() of the inserted object

LinkedHashSet

• ordered HashSet (order of insertion)
• unsorted
• elements are doubly linked to one another

TreeSet

• sorted (one of two sorted Collections) – elements are kept in ascending order according to natural order
• uses Red-Block tree structure
• attempt of adding an object which doesn’t implement compareTo() results in ClassCastException

Map

• can be ordered
• can be sorted
• duplicate keys not allowed (eqauls())
• any class used as a key for a Map must override hashCode() and eqauls()

HashMap

• unordered
• unsorted
• fast updates (key/value pairs)
• allow one null key and many null values

Hashtable

• synchronized HashMap (slower)
• old…
• can’t keep anything that is null

LinkedHashMap

• insertion order
• fast iteration
• slower addition and deletion
• allow one null key and many null values

TreeMap

• sorted
• can define custom order using Comparable or Comparator

Queue

• ordered by FIFO (first in – first out) or by priority

PriorityQueue

• you can create a priority-in and priority-out queue as opposed to typical FIFO
• ordered by natural order (elements sorted first will be accessed first) or according to Comparator
• size()
• peek() – returns the highest priority element, without removing
• poll() – returns the highest priority element and removes it
• offer() – adds an element