doubly-linked lists cmput 115 - lecture 16 department of computing science university of alberta...

Doubly-Linked ListsDoubly-Linked Lists

Cmput 115 - Lecture 16

Department of Computing Science

University of Alberta©Duane Szafron 2000

Some code in this lecture is based on code from the book:Java Structures by Duane A. Bailey or the companion structure package

Revised 2/24/00

©Duane Szafron 2000

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About This LectureAbout This Lecture

In this lecture we will learn about an implementation of the List Interface called Doubly-Linked List.


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OutlineOutline

Drawing Doubly Linked Lists

DoublyLinkedListElement class

DoublyLinkedList class


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Doubly-Linked List DiagramsDoubly-Linked List Diagrams A doubly-linked list node has two links, one forward

and one backward.

The doubly-linked listdoubly-linked list has references to its head and tail nodes.

This symmetry makes the implementation simpler.

“Fred” “Wilma” “Barney”

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Constructing a NodeConstructing a Node When a DoublyLinkedListElement (node) is

constructed, four links may need to be set.

“Fred”

“Wilma”

“Barney”

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If one or both of the “neighboring” nodes is null then fewer links must be set.

“Wilma”

“Barney”

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null


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DoublyLinkedListElement 1DoublyLinkedListElement 1

public class DoublyLinkedListElement {protected Object data;protected DoublyLinkedListElement next;protected DoublyLinkedListElement previous;

public DoublyLinkedListElement (Object element, DoublyLinkedListElement nextNode, DoublyLinkedListElement previousNode) {//post: initializes the node to contain the given// object and link to the given next and previous nodes.

this.data = element;this.next = nextNode;this.previous = previousNode;

code based on Bailey pg. 118

element

previousNode nextNode


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DoublyLinkedListElement 2DoublyLinkedListElement 2

public DoublyLinkedListElement (Object element) {//post: initializes the receiver node to be the

// only node in a list.this(element, null, null);

}}


if (nextNode != null) nextNode.previous = this;if (previousNode != null) previousNode.next = this;}


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DoublyLinkedList - DoublyLinkedList - State & ConstructorState & Constructor

public class DoublyLinkedList implements List{

protected int count;protected DoublyLinkedListElement head;protected DoublyLinkedListElement tail;

public DoublyLinkedList() {//post: initializes the list to be empty.

this.head = null;this.tail = null;this.count = 0;

}



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DoublyLinkedList - DoublyLinkedList - Store InterfaceStore Interface

/* Interface Store Methods */public int size() {//post: returns the number of elements in the list.

return this.count; }

public boolean isEmpty() {// post: returns the true iff store is empty.

return this.size() == 0 }

public void clear() {// post: clears the list so that it contains no elements.

this.head = null;this.tail = null;this.count = 0; }



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DoublyLinkedList - Collection Interface 1DoublyLinkedList - Collection Interface 1

/* Interface Collection Methods */public boolean contains (Object anObject);// pre: anObject is non-null// post: returns true iff the collection contains the object

DoublyLinkedListElement cursor;

cursor = this.head; // partial traversalwhile ((cursor != null) &&

(!cursor.value().equals(anObject)) cursor = cursor.next();return cursor != null;

}



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DoublyLinkedList Collection Interface 2DoublyLinkedList Collection Interface 2

public void add (Object anObject) {// pre: anObject is non-null// post: the object is added to the collection. The // replacement policy is not specified

this.addToHead(anObject);}public Object remove (Object anObject);// pre: anObject is non-null// post: removes object “equal” to anObject and returns it,// otherwise returns nil

// Read the textbook after understanding this lecture.}public Iterator elements() {// post: return an iterator for traversing the collection

// Ignore this one until textbook Chapter 8! }code based on Bailey pg. 108


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head


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DoublyLinkedList -DoublyLinkedList - addToHead() vers 1addToHead() vers 1

/* Interface List Methods */public void addToHead(Object anObject) {// pre: anObject is non-null// post: the object is added to the beginning of the list

this.head = new DoublyLinkedListElement(anObject,this.head, null);

this.count++;}


“Pebbles”2

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DoublyLinkedList DoublyLinkedList - - addToHead() problemaddToHead() problem

/* Interface List Methods */public void addToHead (Object anObject) {// pre: anObject is non-null// post: the object is added to the beginning of the list

this.head = newDoublyLinkedListElement(anObject,this.head, null);

this.count++;}


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4 Check the boundaries!If the list is empty, the list tail must be bound to the new node.

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“Pebbles”

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DoublyLinkedList - DoublyLinkedList - addToHead()addToHead()

/* Interface List Methods */public void addToHead (Object anObject) {// pre: anObject is non-null// post: the object is added to the beginning of the list

this.head = new DoublyLinkedListElement(anObject, this.head, null);

// fix tail, if necessaryif (this.tail == null)

this.tail = this.head;this.count++;}



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DoublyLinkedListDoublyLinkedList - removeFromHead() vers 1- removeFromHead() vers 1

public Object removeFromHead() {// pre: list is not empty// post: removes and returns first object from the list

DoublyLinkedListElement temp;temp = this.head;this.head = this.head.next();if (this.head != null)

this.head.previous = null;this.count--;return temp.value();

}


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temp

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DoublyLinkedList -DoublyLinkedList - removeFromHead() problemremoveFromHead() problem


DoublyLinkedListElement temp;temp = this.head;this.head = this.head.next();if (this.head != null)

this.head.previous = null;this.count--;return temp.value();

}


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“Fred”

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1temp

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Check the boundaries!If the list has one node, the list tail must be bound to null.

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DoublyLinkedListDoublyLinkedList - - removeFromHead()removeFromHead()


DoublyLinkedListElement temp;

temp = this.head;this.head = this.head.next();if (this.head != null) this.head.previous = null;else // clean up tail if list is now empty this.tail = null;this.count--;return temp.value();}



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DoublyLinkedList DoublyLinkedList - - peek() and tailPeek()peek() and tailPeek()

public Object peek () {// pre: list is not empty// post: returns the first object in the list without // modifying the list

return this.head.value();}

public Object tailPeek () {// pre: list is not empty// post: returns the last object in the list without // modifying the list

return this.tail.value();}



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DoublyLinkedList - DoublyLinkedList - addToTail()addToTail()

public void addToTail (Object anObject) {// pre: anObject is non-null// post: the object is added at the end of the list

this.tail = new DoublyLinkedListElement(anObject, null, this.tail);

if (this.head == null)this.head = this.tail;

this.count++;}


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3 4 “Pebbles”Look at the symmetry with addToHead()!

Check the boundaries!If the list was empty, bind the head to the new node.

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DoublyLinkedList - DoublyLinkedList - removeFromTail()removeFromTail()public Object removeFromTail() {// pre: list is not empty// post: the last object in the list is removed and returned

DoublyLinkedListElement temp;

Assert.pre(!this.isEmpty(), “List is not empty”);temp = this.tail;this.tail = this.tail.previous();if (this.tail == null) this.head = null;else //must set next for the previously last node this.tail.setNext(null);this.count--;return temp.value();}


Just copy from removeFromHead() based on symmetry!


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Some Principles from the TextbookSome Principles from the Textbook

12. Question asymmetry

principles from Bailey ch. 6

doubly-linked lists cmput 115 - lecture 16 department of computing science university of alberta...

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