Iterators

1
Iterators
2
Iterator

• An iterator is any object that allows one to step
  through each element in a list (or, more
  generally, some collection).

3
Iterator

• Interface defined in java.util.Iterator
• For simplicity, we will discuss the
  non-parameterized version of java.util.Iterator.
• We will also implement our iterator as a public
  inner class of our linked list.

4
Iterator

• Methods required by java.util.iterator
• public void remove ( )
• public Object next ( )
• public boolean hasNext ( )

5
Iterator

• public void remove ( )
• optional
• removes from the collection the last element
  returned by next
• this method can be called only once per call to
  next()
• if the collection is changed in any way, other
  than by using remove, the behavior of the
  iterator is not specified (and thus should be
  considered unpredictable)
• throws IllegalStateException if the next() method
  has not yet been called, or if the remove()
  method has already been called after the last
  call to the next() method
• throws UnsupportedOperationException if the
  remove operation is not supported by this
  iterator

6
Iterator

• public Object next ( )
• returns the next element of the collection that
  produced the iterator
• throws a NoSuchElementException if there is no
  next element

7
Iterator

• public boolean hasNext ( )
• returns true if next() has not yet returned all
  the elements in the collection
• returns false otherwise

8
Our iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• public void remove ( ) //optional
• public Object next ( )
• public boolean hasNext ( )
• //--------------------------------------------
  ---------------------------

Inner class Node definition that we developed
previously.
9
Our iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• public void remove ( ) //optional
• public Object next ( )
• public boolean hasNext ( )
• //--------------------------------------------
  ---------------------------

Inner class Iterator definition with interface
methods defined.
10
Our iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• public void remove ( ) //optional
• public Object next ( )
• public boolean hasNext ( )
• //--------------------------------------------
  ---------------------------

The head of our linked list.
11
Our iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• public void remove ( ) //optional
• public Object next ( )
• public boolean hasNext ( )
• //--------------------------------------------
  ---------------------------

A method to add elements to our linked list (as
before).
12
Our iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• public void remove ( ) //optional
• public Object next ( )
• public boolean hasNext ( )
• //--------------------------------------------
  ---------------------------

New A public method that returns an iterator for
a specific linked list instance.
13
Our iterator

• public class Iterator implements
  java.util.Iterator
• private Node mNextElement mHead
• //----------------------------------------
  ---------------------------
• public void remove ( ) //optional
• throw new UnsupportedOperationExceptio
  n()
• //----------------------------------------
  ---------------------------
• public Object next ( )
• //----------------------------------------
  ---------------------------
• public boolean hasNext ( )

We need something to keep track of where we are
as we iterate through the list.
14
Our iterator

• public class Iterator implements
  java.util.Iterator
• private Node mNextElement mHead
• //----------------------------------------
  ---------------------------
• //removes from the collection the last
  element returned by next.
• //this method can be called only once per
  call to next(). If the
• // collection is changed in any way,
  other than by using remove, the
• // behavior of the iterator is not
  specified (and thus should be
• // considered unpredictable).
• //throws IllegalStateException if the
  next() method has not yet been
• // called, or if the remove() method has
  already been called after
• // the last call to the next() method.
• //throws UnsupportedOperationException if
  the remove operation is not
• // supported by this iterator.
• public void remove ( ) //optional
• throw new UnsupportedOperationExceptio
  n()
• //----------------------------------------
  ---------------------------
• public Object next ( )

Since remove() is optional, we will through the
indicated exception. It will be left as an
exercise to the reader.
15
Our iterator
Make a copy of the next string, move on to the
next item, and return the string. Privacy leak
possible?

• public class Iterator implements
  java.util.Iterator
• private Node mNextElement mHead
• //----------------------------------------
  ---------------------------
• public void remove ( ) //optional
• throw new UnsupportedOperationExceptio
  n()
• //----------------------------------------
  ---------------------------
• //returns the next element of the
  collection that produced the
• // iterator.
• //throws a NoSuchElementException if
  there is no next element.
• public Object next ( )
• if (mNextElementnull)
• throw new java.util.NoSuchElementE
  xception()
• String temp mNextElement.mData
• mNextElement mNextElement.mNext
• return temp
• //----------------------------------------
  ---------------------------
• public boolean hasNext ( )

16
Our iterator

• public class Iterator implements
  java.util.Iterator
• private Node mNextElement mHead
• public void remove ( ) //optional
• throw new UnsupportedOperationExceptio
  n()
• //----------------------------------------
  ---------------------------
• public Object next ( )
• if (mNextElementnull)
• throw new java.util.NoSuchElementE
  xception()
• String temp mNextElement.mData
• mNextElement mNextElement.mNext
• return temp
• //----------------------------------------
  ---------------------------
• //returns true if next() has not yet
  returned all the elements
• // in the collection returns false
  otherwise.
• public boolean hasNext ( )
• return (mNextElement ! null)

When next is null, there is nothing left.
17
Using our iterator

• public static void main ( String args )
• MyListWithIterator list new
  MyListWithIterator()
• list.add( "fred" )
• list.add( "sally" )
• MyListWithIterator.Iterator it
  list.iterator()
• while (it.hasNext())
• System.out.println( it.next() )

18
Our complete iterator

• public class MyListWithIterator
• private class Node
• String mData""
• Node mNextnull
• private Node ( String data )
• mData data
• //--------------------------------------------
  ---------------------------
• public class Iterator implements
  java.util.Iterator
• private Node mNextElement mHead
• //----------------------------------------
  ---------------------------
• //removes from the collection the last
  element returned by next.
• //this method can be called only once per
  call to next(). If the
• // collection is changed in any way,
  other than by using remove, the
• // behavior of the iterator is not
  specified (and thus should be
• // considered unpredictable).
• //throws IllegalStateException if the
  next() method has not yet been

This is too small to read but is provided in its
entirety so that you may cut and paste it into
your own code if you wish.
19
Our linked list and the for-each loop

• To use our linked list class and our iterator
  with the for-each loop, we must first implement
  the java.util.Collection interface (along with
  the methods that this interface requires).
• Then we may state
• for (Object s list)
• System.out.println( s )
• Note Object s above. String s will cause the
  compiler to issue an error.