Need to access each element of a list. Use index to access an element of the list. It s quadratic, while:

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1 ! " # $ $ % & ' () * + (, -. /* 0 * / ( ' 7 8 * / 9 : ; < 8 * /' 8 - /4 J K L M N O PQ M R S After studying this chapter you should understand the following: the role of iterators in container classes; various implementations of the Iterator interface; iterators as an abstraction of indexes in list operations; the notion of an internal iterator; S the basic structure of the java.util.collection hierarchy. Also, you should be able to: use iterators in algorithms dealing with lists; define a class that provides a new implement of the Iterator interface. >?@ABCDEFG HI = V N N M R R PW X Y PR O M YM Z M W O R S Need to access each element of a list. S Use index to access an element of the list. for array-based implementations: get(i) is constant. for linked implementations: get(i) is linear. for (int i = 0; i < list.size(); i = i+1) do something with list.get(i); S It s quadratic, while: int i = list.indexof(element); list.remove(i); S both statements linear for a linked implementation. S Need: Reduce cost of linked-list traversals. >?@ABCDEFG HI U >?@ABCDEFG HI [

2 It s object associated with container that sequentially accesses each element in the container. nhutilites.containers2 contains classes described. he standard package java.util includes a different interface also named Iterator<E>. public interface Iterator<Element> extends Cloneable Iterator for accessing and traversing elements of a container. public void reset () Initialize this Iterator to reference the first element. public void advance () Advance this Iterator to the next element. require:!this.done() public boolean done () No more elements to traverse in the container. public Element get () Element this Iterator currently references. require:!this.done() public boolean equals (Object obj) he specified Object is an Iterator of the same class as this, and references the same relative element of the same container. Assume container c, and i an iterator for it. Can access each element in container as follows: public boolean traverses (Object container) his Iterator traverses the specified container. public Object clone () A copy of this Iterator. public void setequalo (Iterator<Element> other) Set this Iterator to reference the same container element as the specified Iterator. Both Iterators must traverse the same container. require: ensure: this.traverses(container) implies other.traverses(container) this.equals(other) i.reset(); while (!i.done()) { do something with i.get(); i.advance(); Example: o see if element is in container: i.reset(); while (!i.done() &&!i.get().equals(element)) i.advance();

3 int ' ( ) * + (, * -. / 0). ) 1 ( + ( -, 1 2 An index int variable is a simple for of iterator: It is reset by setting variable to 0. Advanced by incrementing variable. It is done when variable equals the length of the list. Every iterator implementation is coupled to its container. «interface» Iterator<Element> ListIterator<Element> LinkedList<Element>.Iterator DoublyLinkedList<Element>.Iterator!"#$ %& : 8 ; : < = > 9? A B C D7 B 7 E A: E? : 8 ; : < = > 9? A B C D7 B 7 E A: E? public class ListIterator<Element> implements Iterator<Element> { private int current; // index into thelist private List<Element> thelist; // the List this Iterator references /** * Create a new iterator for the specified List. */ public ListIterator (List<Element> list) { thelist = list; current = 0;!"#$ %& 34 //Initialize this Iterator to reference the first public void reset () { current = 0; //Advance this Iterator to next element. public void advance () { current = current + 1; //No more elements to traverse in the container. public boolean done () { return current >= thelist.size(); //Container element this Iterator currently references. public Element get () { return thelist.get(current);!"#$ %& 33

4 -. / / 8 9 5:. : Define an iterator for LinkedList<Element> as a private inner class. It can then directly access implementation structure of the LinkedList and of component Nodes. It keeps a reference to Node containing current element. private class Iterator implements nhutilities.containers.iterator<element> { private Node current; //Create a new Iterator for this LinkedList public Iterator () { reset(); //Initialize this Iterator to reference the first element. public void reset () { current = LinkedList.this.first; //Advance this Iterator to next element. public void advance () { current = current.next;! "#$ %&'()* +, -. / / 8 9 5:. : = 0 / > 5. / : 3 2 0? / 0 : //No more elements to traverse in the container. public boolean done () { return current == List<Element> includes method for creating an iterator //Container element this Iterator currently references. public Element get () { return current.element; public Iterator<Element> iterator () Create a new Iterator for this List.! "#$ %&'()* +, ;! "#$ %&'()* +, <

5 ! " #$ % $ & ' #% ( ) ' #* " +, -. % " / #"! " $ * AbstractList<Element> creates ListIterator<Element> as a default iterator. public abstract class AbstractList<Element> implements List<Element> { public Iterator<Element> iterator () { return new ListIterator<Element>(this); his default implementation is only good for arraybased implementations. 0 he class LinkedList<Element>, 1 overrides iterator() method and returns an instance of AB C D E FG H G I C J K AC L M C G N O C P N Q O N O H G C G N AB C D E FG H G I C J K AC L M C G N O C P N Q O N O H G C G N B R We d like to be able to delete the element identified by an iterator, change it, insert an element in front of it, etc. R Overload List<Element> methods that take index arguments with methods taking iterator arguments :;<= >? 23 public Element get (Iterator<Element> iterator) he Element referenced by the specified Iterator. public Iterator<Element> iteratorat (Element element) An Iterator referencing the first occurrence of the specified Element in this List<Element>; Iterator<Element> is done if not found. public void add (Iterator<Element> iterator, Element element) Insert the specified Element at the specified position. he Iterator will reference the newly added Element. public void remove (Iterator<Element> iterator) Remove the Element at specified position. Iterator will reference Element following removed Element. If no Element follows the removed Element (i.e., the removed Element was last in this List), iterator.done() is true. public void set (Iterator<Element> iterator, Element element) Replace the element at the specified position with the specified Element. U VWX YZ[\]^ _` S

6 M H Q N P AI M C G N O C P N P AB C B M H Q N P AI M C G N O C P N P AB C B R LinkedList<Element>.Iterator references current node of a LinkedList<Element> R Sufficient to implement get and set. R o implement remove and add, need reference to Node preceding current Node. R Must traverse list to find needed Node. R o solve problem: keep a pair of references in the iterator, one to the current Node and one to the preceding Node. Or keep a reference to the Node preceding current one. In either case, it will simplify things if a LinkedList<Element> has a header U VWX YZ[\]^ _` U VWX YZ[\]^ _` S! " # $ % & # ' ( # ) * # ) " It is possible to move forward or backward through a doubly linked list as each node contains forward and backward references. Define an Iterator<Element> extension that permits forward and backward traversal of a list, and ensure that a DoublyLinkedList<Element> creates such an iterator., ) - # * ( # ) * # ) $ % & # ' public interface BiDirectionalIterator<Element> extends Iterator<Element> public boolean done () his Iterator has been advanced past the last element, backed up past the first element, or the container is empty. Equivalent to offright() offleft(). public boolean offright () his Iterator has been advanced past the last element, or the container is empty. public boolean offleft () his Iterator has been backed up past the first element, or the container is empty. public void advance () Move this Iterator forward to the next element. If this.offleft() and the container is not empty, move to the first element. public void backup () Move this Iterator back to the previous element. If this.offright() and the container is not empty, move to the last element. require!this.offleft() +

7 -. / : What is the state of an iterator when a node is deleted via the iterator? Example: iterator i = list.iterator(); while(!i.done()){ if condition(i.current()) list.remove(i); i.advance(); wo questions: What node (if any) should I reference after the deletion? How should other iterators on same list behave? ; What node (if any) should I reference after the deletion? It should not refer to the node containing the element deleted. It should not refer to the next element down the list, as that element would not be processed. ; hen to what? It should be set to null Methods current(), and advance() must be modified to handle this situation! "#$ %&'()* +, -. / : / : ; We assume that the list has a header and that iterator has a reference to current, and previous node. public void advance(){ if (current == null) current = previous.next; else current = current.next; ; We assume that the list has a header and that iterator has a reference to current, and previous node. public void remove(iterator i){ previous.next = current.next; current = null;! "#$ %&'()* +, <! "#$ %&'()* +, =

8 how an Iterator behaves if the List is modified by means of another Iterator, or by means of an index. For instance, suppose i and j are distinct Iterator instances referencing the same element List<Element> list a b c d e f Iterator i Iterator j Executing list.remove(j); or list.remove(2); what will the state of the i iterator be?. / / / : 8 ; 2 / / >. / / 3 1 4? : 3 2 ; A B 3 < If a List is modified via an index, all Iterators that reference the List become invalid. < Furthermore, if a List is modified by an Iterator, all other Iterators that reference the List become invalid. < client provides an operation and iterator itself applies operation to each container element. < hey support performing some specific operation on each item in the container. < We consider adding this feature to the List interface. < he operation to be performed should be provided as a method argument, encapsulated in an object. " #$% &'()*+,-! " #$% &'()*+,- =

9 We define an interface to model an operation: Using an Iterator method can be implemented in public void execute (Element element) he operation. AbstractList<Element> : public interface Operation<Element> An operation that can be preformed on an Element. Method foreachdo has an Operation parameter: /** * Perform the specified Operation on each element * of this List. */ public void foreachdo (Operation<Element> operation); public void foreachdo (Operation<Element> operation) { Iterator<Element> iterator = this.iterator(); iterator.reset(); while (!iterator.done()) { operator.execute(iterator.get()); iterator.advance();! "# " " For example, suppose roll is a List<Student> and want to add ten points to each Student s grade: roll.foreachdo( new Operation<Student> () { public void execute(student element) { element.setgrade(element.getgrade() + 10); ); foreachdo allow to perform an operation on each list element. It does not allow to replace elements. Nor to query list for a value that requires querying each element. Handling such problems requires more structure.

10 # "! " A B C B DE F GH I J K K L M N O J P Q R K L S L P N U VW X Y X Z U [ - $ (. / '$ & ' '( ) * + (, $ %!" # $ % \ Java package java.util defines a set of interfaces and classes rooted at interface Collection<Element>. \ his hierarchy contains members closely related to classes and interfaces described in this chapter. \ he library available since Java 1.2, and retrofitted for type parameters in Java 1.5. \ Collection ] models a generalized container. ] Some Collections allow duplicate elements, others do not. Some Collections impose an ordering on the elements; some are unordered :; <= 01 l m n o p q p rs t uv w x y y z { x ~ y z z ~ m un ƒ p ƒ m l m n o p q p rs t uv w x y y z { x ~ y z z ~ m un ƒ p ƒ m Collection methods include: public boolean contains (Element e) his collection contains the specified element. pubic boolean isempty() his collection contains no elements. public int size () he number of elements in this collection. public java.util.iterator iterator() An iterator over the elements in this collection. Operations add and remove elements are optional. An implementing class can choose to support them or not. public boolean add (Element element) throws UnsupportedOperationException, ClassCastException, IllegalArgumentException ` abcdefghi jk ^_ List extends Collection. his interface models a sequential collection in which elements can be accessed by index. A List can contain duplicate elements, but operations to add and remove elements are still specified as optional. java.util.set extends Collection and specifies a Collection that does not contain duplicate elements. Classes AbstractCollection, AbstractList, and AbstractSet. hese classes provide skeletal implementations of the interfaces. Concrete implementation classes extend these abstract classes. ` abcdefghi jk ˆ

11 ! " # $! % & & ' ( : ; < = >? B CD E F G G H I J K F L M N G H O H L J P Q RS U V Q W ) Array-based list implementations extending AbstractList * Java.util.ArrayList * java.util.vector. + Array-based implementations grow automatically. + java.util.linkedlist extends AbstractSequentialList, «interface» Collection<Element> «interface» List<Element> AbstractCollection<Element> AbstractList<Element> AbstracListElement> Vector<Element> which extends AbstractList. * It provides a doubly-linked list implementation. * Its iterator() method returns an iterator that allows bidirectional traversal of the linked list. AbstractSequentialList<Element> LinkedList<Element>. / ,- f g h i j g k i lm g h i n j o h ƒ ƒ ˆ p Interface Iterator<Element> specifies three methods: public boolean hasnext () he iteration has more elements.next public Element next () throws NoSuchElementException he next element in the iteration. hrows NoSuchElementException if hasnext() is false public void remove () throws UnsupportedOperationException, IllegalStateException Removes from the underlying collection the last element returned by the iterator (optional operation). his method can be called only once per call to next. Z [\]^_`abc de XY p Query hasnext is essentially the converse of our Iterator method done. p Note that method next is not a proper query since it changes the state of the iterator. p We could implement next by accessing the current element and then advancing the iterator: public Element next () { Element temp = this.get(); this.advance(); return temp; s tuvwxyz{ ~ qr

12 ƒ ƒ ˆ p ListIterator<Element> extends Iterator<Element>. p With a ListIterator, can traverse a list in either direction, can modify the list during iteration. It includes a method previous that serves as a companion to next, and optional methods for adding, removing, and setting list elements. s tuvwxyz{ ~ qq