Chapter11: java.util package

• Collections:A collection is simply an object that groups multiple elements into a single unit.
• Collections are used to store, retrieve and manipulate data,and to transmit data from one method to another.
• Collections typically represent data items that form a natural group, like a poker hand(collection of cards), a mail folder(collection of letters), or a telephone directory(a collection of name-to-phone number mappings)
• Collections define a set of core interfaces.These are:
• Collection
• Set
• List
• SortedSet
• Map
• SortedMap
• Collection Interface:The collection interface is the root of the new collections API Library.This interface defines the basic functions that all classes must implement.
• A class that implements Collection represents a group of Objects, known as that class's elements.
• All general purpose classes that implement collections should have a constructor that takes another collection.
• It has a couple of methods that will flatten all the elements of data structures into an array.This array is either one it allocates are one you give it as an argument.
• The JDK doesnot provide any direct implementations of this interface.There are two interfaces that extend Collection in different directions-
• Set
• List
• Set Interface:A set is a collection that cannot contain any duplicate elements.
• This interface models the mathematical set abstraction.
• Set Interface extends Collection Interface.
• A set contains no pair of elements e1 and e2 such that e1.equals(e2),and atmost one null element.
• an example of a set is courses making up a student's schedule or the processes running on a machine.
• The class HashSet implements the Set Interface.
• HashSet:This class implements the Set Interface, backed by a hash table.
• This class maintains a collection of individual objects and you can do intersection, set difference and iteration over the collection.The hash table makes this operations fast.
• This class permits the null element.
• HashSet makes no guarantees as to the iteration order of the set,in particular,it doesnot guarantee that the order will remain constant over time.
• TreeSet:This class implements Set interface, backed by a TreeMap instance.
• TreeSet guarantees that the sorted set will be in ascending element order,sorted according to the natural order of the elements,or by the comparator provided at set creation, depending on which constructor is used.
• List:a List is an ordered collection.
• The user has precise control over where in the list each element is inserted.
• The user can access the elements by their integer index and search for elements in the list.
• Unlike sets, lists typically allow duplicate elements.
• Array List,Vector and linked List are implementations of List Interface.
• Map Interface:A Map is a object that maps keys to values.
• A Map cannot contain duplicate keys, each key can map to utmost one value.
• Map doesnot implement collection interface.
• This interface provides three collection views, which allow viewing a map's contents as a set of keys,a collection of values or a set of key value pairs.
• Set and Map collections ensure uniqueness, List Collections donot ensure uniqueness but are sorted.
• SortedSet:A SortedSet is a set that maintains its elements in ascending order.
• The SortedSet Interface is used for things like ward lists and membership rolls.
• The class TreeSet is an implementation of interface SortedSet.
• SortedMap:A SortedMap is a map that maintains its mappings in ascending key order.
• It is the Map analogue of SortedSet.
• The SortedMap Interface is used for apps like dictionaries and telephone directories.

  Interface	class implementation
  Set	AbstractSet,HashSet
  List	AbstractList,LinkedList,Vector,Array List
  SortedSet	TreeSet
  Map	AbstactMap,HashMap,HashTable
  SortedMap	TreeMap

   

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