JDK 5 Notes (Collection / Thread)

• ConcurrentHashMap -- High performance thread-aware map. A simple test which inserts 10000 Integer for each threads shows that:
  

  Type	No of threads	Average Time (sec)
  ConcurrentHashMap	2	0.06
  Synchronized HashMap	2	0.06
  ConcurrentHashMap	10	0.18
  Synchronized HashMap	10	0.59
  ConcurrentHashMap	20	0.25
  Synchronized HashMap	20	1.15

  
  

  more thread, more performance gain

• CopyOnWriteArrayList, CopyOnWriteArraySet -- A thread-aware List/Set optimize for read. DO NOT use these for a lots of write operations !
• Choose the right collection:
  
  

  Type	Eliminate Duplication	Auto-Sorted	Random write	Random read	Maintain Insertion Order	Note
  ArrayList	X	X	OK for < 1000	O	O	choose this first
  PriorityQueue	X	O	X	X	X	Not allow null
  LinkedList	X	X	O (fast sort)	poor	O	(Queue) allow null
  HashMap	O (key)	X	O	O	X	WeakHashMap for cache
  TreeMap	O (key)	O	O	O	X	with Comparator
  LinkedHashMap	O (key)	X	O	O	O
  HashSet	O	X	X	X	X
  TreeSet	O	O	X	X	X	with Comparator
  LinkedHashSet	O	X	X	X	O

  
  
• Never use Vector, Stack, and Hashtable.

To be continue...