Choosing the right collection

Scenarios to choose the collection

Q. Which is faster to iterate LinkedHashSet or LinkedList ?

LinkedList

Q. Arrange in the order of speed - HashMap, HashTable, Collections.synchronizedMap, concurrentHashmap 

HashMap is fastest, then - ConcurrentHashMap, Collections.synchronizedMap, HashTable

Q. Scenario : You need to insert huge amount of objects and randomly delete them one by one. Which Collection data structure is best ?

LinkedList

Choosing right collection to use

Best general purpose implementations are : ArrayList, LinkedHashMap, and LinkedHashSet (marked below as " * ")

Their overall performance is better, and you should use them unless you need a special feature like, ordering or sorting.

"Ordering" refers to the order of items returned by an Iterator.

"Sorting" refers to sorting items according to Comparable or Comparator.

Set (No duplicates)

  HashSet    |   LinkedHashSet *   |    TreeSet

List (Duplicate allowed)

  ArrayList *  |    LinkedList

  Vector          |    Stack

Map (No duplicate keys)

  HashMap   |   LinkedHashMap *   |   TreeMap

  Hashtable  |   Properties

Principal features of special implementations

• HashMap has slightly better performance than LinkedHashMap, but its iteration order is undefined.
• HashSet has slightly better performance than LinkedHashSet, but its iteration order is undefined.
• TreeSet and TreeMap are ordered and sorted, but slow.
• LinkedList has fast adding to the start of the list, and fast deletion from the interior via iteration.

Iteration order

• HashSet - undefined
• HashMap - undefined
• LinkedHashSet - insertion order
• LinkedHashMap - insertion order of keys (by default), or 'access order'
• ArrayList - insertion order
• LinkedList - insertion order
• TreeSet - ascending order, according to Comparable / Comparator
• TreeMap - ascending order of keys, according to Comparable / Comparator

Some more

• For LinkedHashSet and LinkedHashMap, the re-insertion of an item does not affect insertion order.
• For LinkedHashMap, 'access order' is from the least recent access to the most recent access. In this context, only calls to get,put, and putAll constitute an access, and only calls to these methods affect access order.
• While being used in a Map or Set, these items must not change state (hence, it is recommended that these items be immutable objects) : 

          1. keys of a Map

          2. items in a Set