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Collections KeyPoints

>> Thursday, May 19, 2011

The Main Collections KeyPoints in java

Collection is a group of objects.  java.util package provides important types of collections. There are two fundamental types of collections they are Collection and Map. Collection types hold a group of objects, Eg. Lists and Sets where as Map types hold group of objects as key, value pairs.
Eg. Hashtable, HashMap, TreeMap, and LinkedHashMap (LinkedList+HashTable). Explained in detail in last post, how to iterate HashMap.
Here the collections of classes and interfaces having some their own properties.They are


1.Heterogenious Objects:
  • TreeSet and TreeMap will allow different types of objects as Integer,String,Long and user defined objects . etc.
2. Insertion Order:
  • Given Classes can follow the insertion order:
    • ArrayList, LinkedList, LinkedHashSet and LinkedHashMap -insertion order of keys (by default).
3. Duplication:
  • Only List and its Child Classes will allow the duplicate objects. Ex: ArrayList, LinkedList and Vector.
  • Only Map and its Child Classes, values will be allowed as duplicates and keys are not.
  • Map allows only one null key and many null values as duplicates.
  • If you given again the key,value it will be overriden in Map.
4. Sorting Order:
  • Only TreeMap and TreeSet Classes will follow the Sorted order.
  • These two classes implements SortedSet and SortedMap Interfaces.
5. Null Rejections:
  •    HashTable, TreeSet and TreeMap classes only rejects nulls.remaining will allow null.
6. Synchronized:
  • Vector, Stack, HashTable and Properties are Synchronized.
Breif Description on Collections and its Interfaces

Interface Brief Description Derived Classes
Collection It forms the root interface and comes with methods that define basic operations and makes all DS as one unit (called by a common name) Majority of all DS come under this interface
Set Subclasses of this interface have a common feature of not allowing duplicate elements HashSet, LinkedHashSet
SortedSet Subclasses of this interface come with a common feature of printing the elements in ascending order implicitly. Being sub interface of Set, it allows only unique elements TreeSet
List Subclasses allow duplicate elements LinkedList, ArrayList, Vector
Map Allows the subclasses to store key/value pairs. Does not allow duplicate keys. HashMap, LinkedHashMap, Hashtable
SortedMap Prints elements in ascending order of keys. Being a sub interface of Map, it inherits the property of not allowing the duplicate keys. TreeMap

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