The Iterator Interface and Vectors

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The Iterator Interfaceand Vectors

• COMP114
• Tuesday February 19

2
Announcements

• Program 2
• Use encapsulation (getters/setters)
• By the way, program would not be done this way
• Start small (maybe begin with my callback
  example), build it up and keep it working
• Ok to use my code, with attribution
• I/O
• Lewis Loftus 8.2-8.5
• Weiss 2.6

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Design Patterns

• The idea is to catalog designs that have worked
  well.
• Giving a name to something also makes it easier
  to talk about and explain.
• The pattern is a description of the core problem
  and solution.
• The key is that these are recurring problems.

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The Iterator Pattern

• Very simple pattern (maybe simplest)
• Problem domain
• When you need to access a list
• One element after another
• Solution strives to
• Hide how it's implemented
• Hide how list is stored

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Functionality of Iterator

• Possible methods
• first ( )
• hasNext( )
• getNext( )
• Maybe also
• getCurrent( )

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Recall Java Interfaces

• An interface specifies a set of methods that must
  be supplied.
• Java Interface
• You can then use the name of the interface as the
  reference data type

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Java Iterator Enumeration

• In package java.util.Enumeration
• Methods
• hasMoreElements( )
• nextElement( )
• Exception
• NoSuchElementException

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Enumeration Interface

• public interface Enumeration
• boolean hasMoreElements()
• Object nextElement()

Show in J Help in java.util
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Example Use

• import java.util.
• public class Names implements Enumeration
• ...
• public boolean hasMoreElements()
• ...
• public Object nextElement()throws
  NoSuchElementException
• ...

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Code Example

• Project Enumeration
• Show simple use
• Cause exception by reading off end of list.
• Show that you must convert from Object to String

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Warning

• Notice that iterator is invalid if something has
  changed the list of items
• Be careful not to cause inconsistencies

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Java Iterator Interface

• In Java 1.2 (not in VJ)
• Very similar
• public interface Iterator
• boolean hasNext()
• Object next()
• void remove()

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Remove?

• What if you don't want remove( )?
• Like Program 1
• The specification says
• Throws UnsupportedOperationException if the
  remove operation is not supported by this
  Iterator.

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Why the Interface construct?

• More data encapsulation.
• It doesn't matter to the code calling the
  Enumeration what kind of object is sending the
  information.
• Could be a Vector (list of stored items)
• Could be List
• Or more advanced data structure

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Multiple Inheritance

• Back to abstract class vs. interface
• Some languages allow inheritance from multiple
  superclasses
• A dictionary (say)
• Also an iterator
• Problem it gets very complicated
• Interface is Java's way to give part of this.

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Vector

• The Java name for a class similar to arrays but
  it can grow in size
• You used fixed-length arrays on Program 1 (you
  knew there were at most 4 cities) and Program 2.
• Let's see how to do this right.
• See alternative ways later in semester

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First Try A Big Array

• String tokens
• new String10000000
• Problem is that you may not need anything that
  big.
• Wasteful and you still may run out!

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Next Get User to Tell You

• Make user input size as the first element.
• Very inelegant.
• You've just put the burden on someone else.
• Sometimes just not possible.

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Vector

• Just grow arrays as needed.
• In Java a Vector class provides a more flexible
  array
• Vector is documented in java.util
• Include import java.util in file

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Subset of Vector interface

• Constructor Vector( int capacity )
• addElement( Object obj ) to end
• elementAt( int index )
• setElementAt( Object obj,
• int index ) replaces Object at index
• removeElement( int )

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Vector Not Just Array That Grows!

• Inserts in the middle
• insertElementAt( Object obj,
• int index )
• Removes from the middle (shrinks array)
• removeElementAt( int index )
• Imagine an array implementation

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addElement( )

• May need to add space
• public void addElement(Object obj)
• ensureCapacity(elementCount1)
• elementDataelementCount obj
• elementCount

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Inserting Elements?

• To insert (not just replace) a new element need
  to shift entries down.

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insertElementAt( )

• public void insertElementAt(Object obj,
• int index)
• int i
• ensureCapacity(elementCount1)
• for (i elementCount i gt index i--)
• elementDatai elementDatai-1
• elementDataindex obj
• elementCount

Why Count Backwards?
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Forward Copy
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Backward Copy
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ensureCapacity( )

• public void ensureCapacity(
• int minCapacity)
• if (elementData.length lt minCapacity)
• int newLength elementData.length
• if (newLength 0)
• newLength 1
• while (newLength lt minCapacity)
• newLength 2

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Now Copy Elements

• Object newElementData
• new ObjectnewLength
• int i
• for (i 0 i lt elementCount i)
• newElementDatai elementDatai
• elementData newElementData

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Why Double Size?

• Could just increment by some fixed number
• Problem is copy cost

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Increment Size by One

• 1 2 3 4 5 6 (n 1) ? n2
• Ends up copying a lot.

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Doubling

• Doubling is a nice compromise
• Imagine Vector starts at size 1
• Increasing to 2 copies 1 element
• Increasing to 4 copies 2 elements
• Increasing to 8 copies 4 elements

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Eventually

• 1 2 4 8 n / 2 n 1
• So eventual capacity of n causes about n copies.
• Cost proportional to size.
• Not bad.

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Add Enumeration to Vector

• class VectorIterator implements Enumeration
• private Vector theVector
• private int current
• public VectorIterator(Vector v)
• theVector v current 0
• public boolean hasMoreElements()
• return current lt theVector.size()
• public Object nextElement()
• return theVector.elementAt(current)
  

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Clean Design

• Design Pattern like this makes code
• Easier to write
• Easier to understand
• Reusable

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References

• Design Patterns by Gamma, Helm, Johnson, and
  Vlissides. Addison Wesley.
• Java Design Patterns by Cooper. Addison Wesley.
• Patterns in Java, Volume 1, by Grand. Wiley.

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Tutorial

• Multi-part tutorial on building application
• First part discusses OOP
• Later GUIs
• On Sun Java web site
• http//developer.java.sun.com/developer/onlineTrai
  ning/new2java/divelog/