1 Iterators & the Collection Classes

2 » The Collection Framework classes provided in the JAVA API(Application Programmer Interface) contains many type of collections like an ArrayList » These classes implements the List interface

3 There are 35 methods in the Collections Interface. Several of the methods have to do with the iterator interface.

4 » A key operation of a Collections class is the necessity to traverse and process the data stored in the collection » Therefore, most Collection classes e.g the ArrayList class, implement a special Interface that provides for this – » the Iterator interface.

5 » In order to step through a collection in the past we used loops. » The question is : How can any Collection class allow users to loop through say an ArrayList without making sure they do not corrupt the data? » The solution is the use of iterators.

6 Iterator An iterator is an object that enables a user to loop through a collection.

7 » So associated with each class that implements the Collections interface there is an Iterator class. » Each iterator class must implement the iterator interface which consists of methods that we will examine.

8 The following methods are listed in the iterator interface; public interface iterator { T next(); boolean hasnext(); void remove() throws unsupportedException() }

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12  The ArrayList class ( and many others) have a nested class that implements the Iterator interface  The nested class -  The nested class - ArrayIterator implements the iterator interface..  Let’s look at the code:

13 import java.util.Iterator; // using the iterator in java. util public class ArrayList implements ListADT protected T[] list; public ArrayList() { rear = 0; list = (T[])(new Object[DEFAULT_CAPACITY]); } import java.util.Iterator; // using the iterator in java. util public class ArrayList implements ListADT { protected final int DEFAULT_CAPACITY = 100; private final int NOT_FOUND = -1; protected int rear; protected T[] list; // // Creates an empty list using the default capacity. // public ArrayList() { rear = 0; list = (T[])(new Object[DEFAULT_CAPACITY]); }

14 other Methods add and remove in Arraylist class etc. // method to create an object of ArrayIterator inner class public ArrayIterator iterator() new ArrayIterator iterator() { return new ArrayIterator ( list, rear ); } Note that we pass rear ( rear - stores the index of the last item in the array) ( rear - stores the index of the last item in the array) And list which is the array of T holding the data } And list which is the array of T holding the data }

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current COUNT

current COUNT

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19 Look at this code in the ArrayList class: // Pre: sends references to the arraylist object (1) ”list” and (2) the number of items (1) ”list” and (2) the number of items // Postcondition: an iterator to traverse the arraylist has been returned the main class /************************************************************************************************ /************************************************************************************************ public Iterator iterator( ) { return new ArrayInterator(list, count ); } // method iterator /****************************************** This method creates an object of the inner iterator class ArrayInterator( variables); and returns it to the main application and returns it to the main application

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23 public Iterator iterator( ) { return new ArrayInterator(list, count ); } // method iterator Inside the ArrayList class, we have a method to create the array iterator In DatingService when the find button is pressed we created an iterator: “List” is an object for the Arraylist of Clients “List” is an object for the Arraylist of Clients if (e.getActionCommand().equals("FIND")) { // code to get an iterator Iterator iter = List.iterator(); if (e.getActionCommand().equals("FIND")) { // code to get an iterator Iterator iter = List.iterator();

24 » For the Linked List class, the nested iterator class – » class ListIterator class - will have one field: ListNode cur; // used to traverse list

25 class LinkedIterator implements Iterator { private int count; // the number of elements in the collection private LinearNode curr; // the current position // // Sets up this iterator using the specified items. // first will be equal to head, size is # of items currently in list // public LinkedIterator (LinearNode head, int size) { curr = head; // where head points to first node count = size; }

26 // method hasNext() returns true as long as //next is not null - allows the iteration over //the collection to continue if there are more //items to process public boolean hasNext() { return cur != null; }

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29 raydoemenull head Data next cur Variable to traverse the list Pointer to next node in the list

30 raydoemenull head cur Data next Cur traverses the list

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class LinkedIterator implements Iterator private int count; // the number of elements in the collection private LinearNode cur; // the current position public LinkedIterator(LinerarNode first, size ) { cur = first; // sets up cut to point to the beginning of the list – first count -= size; } // constructor //************************************************************* // Precondition: this Iterator is positioned at an element in this Linked Collection. // Postcondition: The element this Iterator was (before this call) positioned at has been //returned, and this Iterator has advanced. //************************************************************* public T next() { T element = cur.data; // get the data cur = cur.getNext(); // advance to the next node return element; // return the element } // method next //************************************************************* // Postcondition: true has been returned if this Iterator is positioned // an element in this Collection. Otherwise, false has been returned. //************************************************************* public boolean hasNext() { return cur != null; // determines if there are more items to processs } // method hasNext //************************************************************* public void remove() { throws unsupported Exception

39 » There is also a ListIterator interface which extends Iterator. There is also a ListIterator interface which extends Iterator. » The description is at this link: The description is at this link: » » It is in java.util and has more methods than the parent interface Iterator

40 » Interface ListIterator, which allows a programmer to traverse a List in either direction and make modifications to the underlying List » -- Newer versions of the iterator:

41 » Some additional methods in ListIterator: » hasPrevious() returns true if there are more elements in a backward direction » previous() returns the previous element in the List » previousIndex() returns the index of the previous element in the list.

43 » New with JDK 1.5 is the Iterable interface. It resides in java.lang and has some enhancements » What are the differences between the two (class that implements Iterable and a class that implements Iterator)? » We can make an iterable object have different kinds of iterators?

44 » class IterableIterator implements Iterable » { » private Iterator iter; » public IterableIterator(Iterator iter) » { this.iter = iter; } » public Iterator iterator() » { return iter; » } » Allows different types of iterators to be created with different capabilities

45 class Tree // creates two iterators { IterableIterator depthFirstIterator(); IterableIterator breadthFirstIterator(); }

46 » But then, how does this work when we use the iterable object in a foreach statement? » How do we specfify which iterator we want to use? » for (Node node: aTree.depthFirstIterator()){}

47 » And by the way, the only benefit of the previous example is to be able to use a for loop on an iterable collection. »

48 » In addition, for now the Iterable interface is not backward compatible.