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Choosing Java Collection

The following section summarizes the various Collection Classes as of Java 7 and their important attributes Including HashMap which is not in Collection Framework.

HashMap.
It provides constant time operation for operations like get and put and remove.
Iteration over the Collection values requires time proportional to the capacity of the Map.(values()).
Default load factor is .75.
HashMap does not maintain the order.
HashMap is not synchronized.

LinkedHashMap
LinkedHashMap orders are maintained
It provdes constant-time performance for operations like get, put and remove.
Performace is slightly slower then HashMap due to the overhead of maintaining Linked List.
Re-insertion does not effect the insertion order.
Iteration over the collection values is proportional to the size of the map  regardless capacity.


TreeMap
Red-Black tree based implementation.
Implement the SortedMap Interface.
Map is ascending Key order of the natural order.
It has log(n) time cost for  contansKey, put, get and remove operations.
operations lke get and put are slower then HashMap.

WeakHashMap
Keys of the map are stored as java.lang.ref.WeakReference.
Entry will be removed automatically when the Key is not in use.


Set
Set does not contain duplicate items.
It contains maximum one null item.
add method put the new item to the set if it does not present already and return true/false.

HashSet
It does not allow duplicate.
It does not guarantee the iteration order of the elements.
It provides constant time performance for operations like add,remove,contains, size.

LinkedHashSet
It maintains the insertion order.
Insertion order is not effected by reinsertion.
It provides constant time operations like  add, contains,remove.
Iteration over the collection values is proportional to the size of the map regardless capacity.


TreeSet
Set is sorted on ascending order of the elements.
It has log(n) time cost for  contansKey, put, get and remove operations.
It implements the SortedSet Interface.

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