Map implementations in java

A number of classes implement the Map interface, including HashMap, TreeMap, LinkedHashMap, WeakHashMap, ConcurrentHashMap, and Properties. The most generally useful class is HashMap.

• java.util.HashMap is implemented with a hash table. Access time is O(1). Entries are unsorted.
  – similar to HashTable but allows null keys and values
  – not thread safe
• java.util.LinkedHashMap is implemented with a hash table. Access time is O(1). Entries are sorted in either entry order or order of last access, which is useful for implementing a LRU (least recently used) caching policy.
• java.util.TreeMap is implemented as a balanced binary tree. Access time is O(log N). Entries are sorted. 
• java.util.HashTable(old classes)
  – updated class from earlier Java versions
  – does not allow null keys or values
  – thread safe
• java.util.WeakHashMap -
  – like HashMap
  – entry is automatically removed from HashMap if no more "ordinary" references to key
• java.util.IdentityHashMap
• java.util.EnumMap
• java.util.Properties