COMP20608S General Programming 2

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COMP206-08SGeneral Programming 2
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Lectures

• Today
• Collections Continued
• Sets
• Trees
• Hashing

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So far

• Arrays
• Arraylists
• LinkedList
• Stacks
• Queues
• Items are all placed at a position in the data
  structure
• LinkedList has an iterator with an add method

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Sets

• A set is a collection of items that does not
  permit duplicates
• Adding an existing item has no effect
• Removing an item not in the set has no effect
• Java has a Set interface with two implementations
• HashSet (uses hashing to store items)
• TreeSet (uses a tree to store items Sorted)

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Example

• import java.util.HashSet
• import java.util.Iterator
• import java.util.Set
• public class SetTest
• public static void main(String args)
• Set names new HashSet ()
• names.add(Harry) // add lots more

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Continued

• names.add("Jane")
• names.add("Tom")
• names.add("Richard")
• names.add("Tom")
• names.add("Barbara")
• print(names)

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Printing

• private static void print(Set s)
• System.out.print(" ")
• for (String x s)
• System.out.print(x)
• System.out.print(" ")
• System.out.println("")

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TreeSet

• Changing only two lines of code lets us have a
  Tree behind the scenes
• Note the order of items!

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Maps

• Similar idea can be developed to make
  associations
• Two sets are kept in association
• Set of keys - Set of Values
• Two keys may refer to same value
• Names and phone numbers
• Java has a Map Interface
• Two maps are implemented
• HashMap
• TreeMap

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Test program for maps

• import java.util.HashMap
• import java.util.Iterator
• import java.util.Set
• import java.util.Map
• public class maps
• public static void main(String args)

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Continued

• Map yellowpages new
  HashMap ()
• yellowpages.put("Harry", "123456")
• yellowpages.put("Jane", "012345")
• yellowpages.put("Tom", "123458")
• yellowpages.put("Richard", "123457")
• yellowpages.put("Barbara", "123454")
• yellowpages.put("Barbara", "987654") //
  assignment

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Printing out maps

• Set keySet yellowpages.keySet()
• for (String key keySet)
• String value yellowpages.get(key) // value
  assoc with key
• System.out.println(key "-" value)

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What is hashing?

• Technique for finding elements without making a
  linear search through all elements
• Uses a hash function is a function that computes
  an integer value (called a hash code) from an
  object (different objects should have different
  hash codes)
• Object class has a hashCode method
• int h x.hashCode()

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Hash functions

• Should avoid collisions (two or more different
  objects with the same hash code)
• If you have your own objects you write
• public int hashCode( )
• When adding x.equals(y) x.hashCode()
  y.hashCode (avoid duplicates)
• Eg for a circle so that circles of different
  radii are stored separately.
• Forgetting to implement hashCode means the one
  assoc with Object is used not a good idea.

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Example

• Consider
• Set circles new HashSet()
• circles.add(new Circle(5))
• if (circles.contains(new Circle(5))
• System.out.println(Circle of radius 5 exists)

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Binary Search Trees

• Finding things is O(log N) with binary search
• With arrays insertion and deletion are O(N)
• Binary search trees are fast at everything
• Nodes of a binary search tree have two children
• Data values of all descendants to the left of any
  node are less than the data value stored at that
  node (similarly right and greater)

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Tree of names

• For the list Adam, Eve, Harry, Jane, Richard,
  Tom
• Jane (root) children Eve and Richard
• Eve children Adam and Harry
• Richard children Tom and null
• Adam, Harry and Tom are leaf nodes (both children
  null)

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Implementation overview

• public class BinarySearchTree
• // need reference to root (like first in LL)
• private node root
• // inner class for node
• private class Node
• public void addNode(Node newNode)
• public Comparable data
• public Node left
• public Node right

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Tree Operations

• Tree Traversal (sorted order)
• Print left subtree
• Print data
• Print right subtree
• Called Inorder traversal
• Preorder traversal (root, left, right)
• Postorder traversal (left, right, root) (345)

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Trees and Recursion

• // Note this is a method of the Node inner class
• public void printNodes( )
• if (left ! null) left.printNodes( )
• System.out.println(data)
• if (right ! null) right.printNodes( )
• // Method of BinarySearchTree
• public void print( )
• if (root ! null) root.printNodes( )

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HashSet or TreeSet

• If you have a good hash function for your objects
  then performance will be better than trees
• Hash sets are at the mercy of the hash function
• Iterators visit TreeSet data in sorted order
  (hash order is random)
• TreeSet requires objects to implement Comparable
  (or Comparator)

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How to choose ask questions

• How are elements accessed?
• Doesnt matter
• By key (eg a/c ) (MAP)
• Integer index (ARRAYLIST)
• Does data order matter?
• Doesnt matter
• Must be sorted (TREESET)
• Store in insert order (LL, ARRAYLIST)

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More questions

• What operations need to be fast?
• Doesnt matter (small dataset)
• Add/remove (LL)
• Search (SET)
• (DM ARRAYLIST)
• Hash or Tree?
• Strings should be hashed
• Own class need to check hashCode and equals
• Trees may need a Comparator