Frequently used Java class

1. TreeMap
1. Red-black tree based
2. Ordered for key
2. LinkedHashMap
1. Hashtable and linked list based
2. Insertion order preserved
3. HashMap
1. Hashtable based map
2. Allow null value and null key
3. Unsynchronized 
4. No order of key or value
4. ConcurrentHashMap
1. More modern than hashtable
2. Use more buckets therefore less likely to be blocked
5. Hashtable
1. Synchronized
6. Blocking queue