ch10

• Collection


  An object that groups multiple elements into a single unit
• Collections


  • Used to store, retrieve, manipulate, and communicate aggregate data
  • Typically represent data items that form a natural group
• Examples of collections


  • Poker hand (a collection of cards)
  • Mail folder (a collection of letters)
  • Telephone directory (a mapping from names to phone numbers)
• Collection interface


  Stores groups of Objects, with duplicates allowed
• Set interface


  Extends Collection but forbids duplicates
• List interface


  Extends Collection, allows duplicates, and introduces positional indexing
• Map


  A separate hierarchy from Collection
• Java does not provide any direct implementations of Collection. Rather, concrete implementations are based on other interfaces which extend Collection, such as Set, List, etc.

  
• The most general code will be written using Collection to type variables
• Map


  • An object that associates keys with values
  • A map cannot contain duplicate keys; each key can map to at most one value
• Map implementations


  • HashMap<K,V>
  • Hashtable<K,V>
  • TreeMap<K,V>
• Iterator


  An object that cycles through all the elements in a collection
• Iterator interface


  • Has methods: next(), hasNext(), remove()
• ListIterator


  • Extends the Iterator interface, designed to work with collections that satisfy the List interface
  • Can move in either direction along a list of elements
  • Has methods to modify elements
• Iterators for standard collection classes return references, not copies, so modifying the returned value will modify the element in the collection
• Differences between Iterator and ListIterator

  • Iterator can traverse elements in forward direction only, ListIterator can traverse in both forward and backward directions
  • Iterator can traverse Map, List and Set, ListIterator can only traverse List
  • Iterator cannot obtain indexes, ListIterator has methods to obtain indexes
  • Iterator cannot modify or replace elements, ListIterator can modify or replace elements
  • Iterator cannot add elements, ListIterator can add elements