IAT 265 Java Collections Jul 7, 2015 1
IAT 265
Jul 7, 2015
Java Collections
IAT 265 1
Data Structures
With a collection of data, we often want to do many things
– Organize
– Iterate
– Add new
– Delete old
– Search
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Data Structures
Some are built to enable fast searching
Some enable fast insertion/deletion
– What LnkList Tree HashTable
– Store Light
– Iterate simple
Less light Medium moderate difficult
– Add O(1)
– Delete O(1)
– Search O(n)
O( lgN ) O(1)
O( lgN + ) O(1)
O(lgN) O(1)
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Hash Table
An array in which items are hash function not stored consecutively - their place of storage is calculated using the key and a
Key hash function array index
Hashed key : the result of applying a hash function to a key
Keys and entries are scattered throughout the array
4
10
123 key
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4
Hashing
insert: compute location, insert TableNode; O(1)
find: compute location, retrieve entry; O(1)
remove: compute location, set it to null; O(1)
10
4 key
123 entry
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Hashing example
10 stock details, 10 table positions
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 )
Given a stock number, we find stock by using the hash function again, and use the collision resolution strategy if necessary
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5
2
3
0
1
6
7
8
9 key entry
85 85, apples
323 323, guava
462 462, pears
350 350, oranges
912 912, papaya
6
Hashing performance
The hash function
– Ideally, it should distribute keys and entries evenly throughout the table
– It should minimize collisions function is already occupied
, where the position given by the hash
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
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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
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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
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Java Collections
A Java
Collection
class is a class that stores and organizes data
– Uses one of the algorithms mentioned in previous lectures
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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
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Java Collections
Framework
Why?
Reduces programming effort
Increases program speed and quality
Interoperable: You and I can pass data in collections between our code
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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)
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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)
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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
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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 );
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Using a HashMap
HashMap<String,String> hm = new HashMap<String,String> (); hm .put( "Ava", "555-8976" ); hm .put( "Mary", "555-1238" ); hm .put( "Joe", "555-2121" );
String phone = 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() );
}
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Using a TreeMap
TreeMap <String,String> tm = new TreeMap <String,String> (); tm .put( "Ava", "555-8976" ); tm .put( "Mary", "555-1238" ); tm .put( "Joe", "555-2121" );
String phone = 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() );
}
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