1. Nov 22, 2010IAT 2651 Java Collections

2. Nov 22, 2010IAT 2652 Data Structures  With a collection of data, we often want to do many things –Organize –Iterate –Add new –Delete old –Search

3. Nov 22, 2010IAT 2653 Data Structures  Some are built to enable fast searching  Some enable fast insertion/deletion –WhatLnkListTreeHashTable –Store LightLess lightMedium –Iterate simplemoderatedifficult –AddO(1)O( lgN )O(1) –DeleteO(1)O( lgN + )O(1) –SearchO(n)O(lgN)O(1)

4. Nov 22, 2010IAT 2654  An array in which items are not stored consecutively - their place of storage is calculated using the key and a hash function  Hashed key: the result of applying a hash function to a key  Keys and entries are scattered throughout the array Hash Table keyentry Key hash function array index 4 10 123

5. Nov 22, 2010IAT 2655  insert: compute location, insert TableNode; O(1)  find: compute location, retrieve entry; O(1)  remove: compute location, set it to null; O(1) Hashing keyentry 4 10 123

6. Nov 22, 2010IAT 2656 Hashing example  10 stock details, 10 table positions keyentry 0 1 2 3 4 5 6 7 8 9  Stock numbers between 0 and 1000  Use hash function: stock no. / 100  What if we now insert stock no. 350? –Position 3 is occupied: there is a collision  Collision resolution strategy: insert in the next free position (linear probing) 85 85, apples 462 462, pears 912 912, papaya 323 323, guava 350 350, oranges  Given a stock number, we find stock by using the hash function again, and use the collision resolution strategy if necessary

7. Nov 22, 2010IAT 2657 Hashing performance  The hash function –Ideally, it should distribute keys and entries evenly throughout the table –It should minimize collisions, where the position given by the hash function is already occupied  The collision resolution strategy –Separate chaining: chain together several keys/entries in each position –Open addressing: store the key/entry in a different position  The size of the table –Too big will waste memory; too small will increase collisions and may eventually force rehashing (copying into a larger table) –Should be appropriate for the hash function used – and a prime number is best

8. Nov 22, 2010IAT 2658 Applications of Hashing  Compilers use hash tables to keep track of declared variables  A hash table can be used for on-line spelling checkers — if misspelling detection (rather than correction) is important, an entire dictionary can be hashed and words checked in constant time  Hash functions can be used to quickly check for inequality — if two elements hash to different values they must be different  Storing sparse data

9. Nov 22, 2010IAT 2659 When to use hashing?  Good if: –Need many searches in a reasonably stable table  Not So Good if –Many insertions and deletions, –If table traversals are needed –Need things in sorted order –More data than available memory Use a tree and store leaves on disk

10. Java Collections  A Java Collection class is a class that stores and organizes data –Uses one of the algorithms mentioned in previous lectures Nov 22, 2010IAT 26510

11. Java Collections Framework  A unified architecture for representing and manipulating collections: –Interfaces: The means by which you access a collection class Independent of implementation details –Implementations: The machinery behind the interface that does the work Concrete implementations –Algorithms: Algorithms that run on collection classes: Searching and sorting Reusable Functionality Nov 22, 2010IAT 26511

12. Java Collections Framework  Why?  Reduces programming effort  Increases program speed and quality  Interoperable: You and I can pass data in collections between our code Nov 22, 2010IAT 26512

13. Interfaces  Collection –A group of objects (elements) –This is a generic idea, there are more detailed Collection types that you actually use  Set –A collection with no duplicate elements  SortedSet –A Set in ascending order –(Typically alphabetical or numeric order) Nov 22, 2010IAT 26513

14. Interfaces  List –An ordered collection –May contain duplicates –Can access by index (like ArrayList)  Queue –A collection that holds items in some order –Typically FIFO (First in/First Out) Nov 22, 2010IAT 26514

15. Interfaces  Map –An collection that maps keys to values Think phone book –May contain duplicates –Can access by index (like ArrayList)  SortedMap –A Map in sorted order of keys Nov 22, 2010IAT 26515

16. Using a List Declare: ArrayList aList = new ArrayList(6);  Methods you use: SomeClass item, newItem, existingItem ; item = (SomeClass)aList.get( i ); aList.add( newItem ); aList.remove( existingItem ); Nov 22, 2010IAT 26516

17. Using a HashMap HashMap hm = new HashMap(); hm.put( "Ava", "555-8976" ); hm.put( "Mary", "555-1238" ); hm.put( "Joe", "555-2121" ); String phone = (String)hm.get( "Mary" ); Iterator I = hm.entrySet().iterator(); while( I.hasNext() ) { Map.Entry me = (Map.Entry)I.next(); print( me.getKey() + " is " ); println( me.getValue() ); } Nov 22, 2010IAT 26517

18. Using a TreeMap TreeMap tm = new TreeMap (); tm.put( "Ava", "555-8976" ); tm.put( "Mary", "555-1238" ); tm.put( "Joe", "555-2121" ); String phone = (String)tm.get( "Mary" ); Iterator I = tm.entrySet().iterator(); while( I.hasNext() ) { Map.Entry me = (Map.Entry)I.next(); print(me.getKey() + " is "); println(me.getValue()); } Nov 22, 2010IAT 26518