1. CHAPTER 19 An Introduction to Data Structures

2. CHAPTER GOALS To learn how to use linked lists provided in the standard library To be able to use iterators to transverse linked lists To understand the implementation of linked lists To distinguish between abstract and concrete data types To know the efficiency of fundamental operations of lists and arrays To become familiar with the stack and queue types

3. Linked List A linked list consists of a number of links, each of which has a reference to the next link. Adding and removing elements in the middle of a linked list is efficient. Visiting the elements of a linked list in sequential order is efficient Random access is not efficient

4. Inserting an Element into a Linked List

5. Java's LinkedList class Easy access to first and last elements with methods o void addFirst(Object obj) o void addLast(Object obj) o Object getFirst() o Object getSecond() o Object removeFirst() o Object removeLast()

6. ListIterator List iterator gives access to elements inside a linked list ListIterator protects the linked list while giving access ListIterator encapsulates a position anywhere in the linked list

7. A List Iterator

8. Conceptual View of the ListIterator

9. List Iterator Think of an iterator as pointing between two links

10. List Iterator The listIterator method of the LinkedList class gets a list iterator LinkedList list =... ListIterator iterator = list.listIterator();

11. List Iterator The next method moves the iterator iterator.next(); next throws a NoSuchElementException if you are already past the end of the list

12. List Iterator hasNext returns true if there is a next element if (iterator.hasNext()) iterator.next();

13. List Iterator The next method returns the object of the link that it is passing while iterator.hasNext() { Object obj = iterator.next(); //do something with the object }

14. List Iterator To move the list position backwards, use: ohasPrevious oprevious

15. Adding and Removing from a LinkedList The add method: o Adds an object after the iterator o Moves the iterator position past the new element iterator.add("Juliet");

16. Adding and Removing from a LinkedList The remove method: o Removes and o Returns the object that was returned by the last call to next or previous

17. Adding and Removing from a LinkedList This loop removes all objects that fulfill a certain condition while (iterator.hasNext()) { Object obj = iterator.next(); if (obj fulfills condition) iterator.remove(); }

18. File ListTest.java ListTest is a sample program that o inserts elements into a list o iterates through the list, adding and removing elements o prints the list

19. File ListTest.java 01: import java.util.LinkedList; 02: import java.util.ListIterator; 03: 04: /** 05: A program that demonstrates the LinkedList class 06: */ 07: public class ListTest 08: { 09: public static void main(String[] args) 10: { 11: LinkedList staff = new LinkedList(); 12: staff.addLast("Dick"); 13: staff.addLast("Harry"); 14: staff.addLast("Romeo"); 15: staff.addLast("Tom"); 16: 17: // | in the comments indicates the iterator position

20. 18: 19: ListIterator iterator = staff.listIterator(); // |DHRT 20: iterator.next(); // D|HRT 21: iterator.next(); // DH|RT 22: 23: // add more elements after second element 24: 25: iterator.add("Juliet"); // DHJ|RT 26: iterator.add("Nina"); // DHJN|RT 27: 28: iterator.next(); // DHJNR|T 29: 30: // remove last traversed element 31: 32: iterator.remove(); // DHJN|T 33: 34: // print all elements 35: 36: iterator = staff.listIterator(); 37: while (iterator.hasNext())

21. 38: System.out.println(iterator.next()); 39: } 40: }

22. Implementing Linked Lists LinkedList class has a private inner class Link class LinkedList { private class Link { public Object data; public Link next; }

23. Implementing Linked Lists LinkedList class oHolds a reference first to the first link oHas a method to get the first element

24. Implementing Linked Lists class LinkedList { public LinkedList() { first = null; } public Object getFirst() { if (first == null) throw new NoSuchElementException(); return first.data; }... private Link first; }

25. Adding a New First Element When a new link is added to the list o It becomes the head of the list o The old first link becomes the next link

26. Adding a New First Element The addFirst method class LinkedList {... public void addFirst(Object obj) { Link newLink = new Link(); newLink.data = obj; newLink.next = first; first = newLink; }... }

27. Adding a Link to the Head of a Linked List

28. Removing the First Element When the first element is removed oThe data of the first link are saved and later returned as the method result oThe successor of the first link becomes the first link of the shorter list oThe link will be garbage collected when there are no further references to it

29. Removing the First Element The removeFirst method class LinkedList {... public Object removeFirst() { if (first == null) throw new NoSuchElementException(); Object obj = first.data; first = first.next; return obj; }... }

30. Removing the First Link from a Linked List

31. LinkedListIterator Private inner class of LinkedList Implements a simplified ListIterator interface Has access to the first field and private Link class

32. LinkedListIterator The LinkListIterator class class LinkedList {... public ListIterator listIterator() { return new LinkedListIterator(); } private class LinkedListIterator implements ListIterator { public LinkedListIterator() { position = null; previous = null; }... private Link position; private Link previous; }... }

33. LinkListIterator's next Method position reference is advances to position.next Old position is remembered as previous If the iterator points before the first element of the list, then the old position is null and position must be set to first

34. LinkListIterator's next Method private class LinkedListIterator implements ListIterator {... public Object next() { if (!hasNext()) throw new NoSuchElementException(); previous = position; // remember for remove if (position == null) position = first; else position = position.next; return position.data; }... }

35. LinkListIterator's hasnext Method The next method should only be called when the iterator is not at the end of the list The iterator is at the end o if the list is empty (first == null) o if there is no element after the current position (position.next == null)

36. LinkListIterator's hasnext Method private class LinkedListIterator implements ListIterator {... public boolean hasNext() { if (position == null) return first != null; else return position.next != null; }... }

37. LinkListIterator's remove Method If the element to be removed is the first element, call removeFirst Otherwise, the link proceeding the element to be removed needs to have its next reference updated to skip the removed element If the previous reference is null : o this call does not immediately follow a call to next o throw an IllegalArgumentException Set previous reference to null

38. LinkListIterator's remove Method private class LinkedListIterator implements ListIterator {... public void remove() { if (position == first) { removeFirst(); position = first; } else { if (previous == null) throw new IllegalStateException(); previous.next = position.next; position = previous; } previous = null; }... }

39. Removing a Link From the Middle of a Linked List

40. LinkListIterator's set Method Changes the data stored in the previously visited element The set method private class LinkedListIterator implements ListIterator {... public void set(Object obj) { if (position == null) throw new NoSuchElementException(); position.data = obj; }... }

41. LinkListIterator's add Method Inserts the new link after the current position Sets the successor of the new link to the successor of the current position

42. LinkListIterator's add Method private class LinkedListIterator implements ListIterator {... public void add(Object obj) { if (position == null) { addFirst(obj); position = first; } else { Link newLink = new Link(); newLink.data = obj; newLink.next = position.next; position.next = newLink; position = newLink; } previous = null; }... }

43. Adding a Link to the Middle of a Linked List

44. File LinkedList.java 001: import java.util.NoSuchElementException; 002: 003: /** 004: A linked list is a sequence of links with efficient 005: element insertion and removal. This class 006: contains a subset of the methods of the standard 007: java.util.LinkedList class. 008: */ 009: public class LinkedList 010: { 011: /** 012: Constructs an empty linked list. 013: */ 014: public LinkedList() 015: { 016: first = null; 017: }

45. 018: 019: /** 020: Returns the first element in the linked list. 021: @return the first element in the linked list 022: */ 023: public Object getFirst() 024: { 025: if (first == null) 026: throw new NoSuchElementException(); 027: return first.data; 028: } 029: 030: /** 031: Removes the first element in the linked list. 032: @return the removed element 033: */ 034: public Object removeFirst() 035: { 036: if (first == null) 037: throw new NoSuchElementException();

46. 038: Object obj = first.data; 039: first = first.next; 040: return obj; 041: } 042: 043: /** 044: Adds an element to the front of the linked list. 045: @param obj the object to add 046: */ 047: public void addFirst(Object obj) 048: { 049: Link newLink = new Link(); 050: newLink.data = obj; 051: newLink.next = first; 052: first = newLink; 053: } 054: 055: /** 056: Returns an iterator for iterating through this list. 057: @return an iterator for iterating through this list

47. 058: */ 059: public ListIterator listIterator() 060: { 061: return new LinkedListIterator(); 062: } 063: 064: private Link first; 065: 066: private class Link 067: { 068: public Object data; 069: public Link next; 070: } 071: 072: private class LinkedListIterator implements ListIterator 073: { 074: /** 075: Constructs an iterator that points to the front 076: of the linked list. 077: */

48. 078: public LinkedListIterator() 079: { 080: position = null; 081: previous = null; 082: } 083: 084: /** 085: Moves the iterator past the next element. 086: @return the traversed element 087: */ 088: public Object next() 089: { 090: if (!hasNext()) 091: throw new NoSuchElementException(); 092: previous = position; // remember for remove 093: 094: if (position == null) 095: position = first; 096: else 097: position = position.next;

49. 098: 099: return position.data; 100: } 101: 102: /** 103: Tests if there is an element after the iterator 104: position. 105: @return true if there is an element after the iterator 106: position 107: */ 108: public boolean hasNext() 109: { 110: if (position == null) 111: return first != null; 112: else 113: return position.next != null; 114: } 115: 116: /** 117: Adds an element before the iterator position

50. 118: and moves the iterator past the inserted element. 119: @param obj the object to add 120: */ 121: public void add(Object obj) 122: { 123: if (position == null) 124: { 125: addFirst(obj); 126: position = first; 127: } 128: else 129: { 130: Link newLink = new Link(); 131: newLink.data = obj; 132: newLink.next = position.next; 133: position.next = newLink; 134: position = newLink; 135: } 136: previous = null; 137: }

51. 138: 139: /** 140: Removes the last traversed element. This method may 141: only be called after a call to the next() method. 142: */ 143: public void remove() 144: { 145: if (position == first) 146: { 147: removeFirst(); 148: position = first; 149: } 150: else 151: { 152: if (previous == null) 153: throw new IllegalStateException(); 154: previous.next = position.next; 155: position = previous; 156: } 157: previous = null;

52. 158: } 159: 160: /** 161: Sets the last traversed element to a different 162: value. 163: @param obj the object to set 164: */ 165: public void set(Object obj) 166: { 167: if (position == null) 168: throw new NoSuchElementException(); 169: position.data = obj; 170: } 171: 172: private Link position; 173: private Link previous; 174: } 175: }

53. File ListIterator.java 01: import java.util.NoSuchElementException; 02: 03: /** 04: A list iterator allows access of a position in a linked list. This interface contains a subset of the methods of the 05: standard java.util.ListIterator interface. The methods for 06: backward traversal are not included. 07: */ 08: public interface ListIterator 09: { 10: /** 11: Moves the iterator past the next element. 12: @return the traversed element 13: */ 14: Object next(); 15: 16: /** 17: Tests if there is an element after the iterator

54. 18: position. 19: @return true if there is an element after the iterator 20: position 21: */ 22: boolean hasNext(); 23: 24: /** 25: Adds an element before the iterator position 26: and moves the iterator past the inserted element. 27: @param obj the object to add 28: */ 29: void add(Object obj); 30: 31: /** 32: Removes the last traversed element. This method may 33: only be called after a call to the next() method. 34: */ 35: void remove(); 36: 37: /**

55. 38: Sets the last traversed element to a different 39: value. 40: @param obj the object to set 41: */ 42: void set(Object obj); 43: }

56. Abstract Data Types Abstract data type defines the fundamental operations on the data Abstract data type does not specify an implementation

57. Abstract Data Types Abstract list o An ordered sequence of items that can be traversed sequentially o Allows for insertion and removal of elements at any position Abstract array o An ordered sequence of items o Allows for random access by specifying an integer index

58. An Abstract View of a Linked List

59. A Concrete View of a Linked List

60. An Abstract View of an Array List

61. A Concrete View of an Array List

62. Fundamental Operations on Array List public class ArrayList { public Object get(int index) {... } public void set(int index, Object value) {... } }

63. Fundamental Operations on Linked List public class LinkedList { public ListIteratior listIterator() {... }... } public interface ListIteratior { Object next(); boolean hasNext(); void add(Object value); void remove(); void set(Object value);... }

64. Efficiency of Linked List Adding or removing an element o A fixed number of link references need to be modified to add or remove a link, regardless of the size of the list o Thus, an element can be added or moved in constant time o In big-Oh notations: O(1)

65. Efficiency of Linked List Random access o On average n/2 elements need to be skipped o In big-Oh notation: O(n)

66. Efficiency of Array List Adding or moving an element o On average n/2 elements need to be moved o In big-Oh notations: O(n) Random access o In big-Oh notation: O(1)

67. Efficiency of Operations for Arrays and List

68. Abstract Data Type Stack Allows insertion and removal of elements only at one end oTraditionally called the top of the stack New items are added to the top of the stack Items are removed at the top of the stack Called last in, first out or LIFO order Think of a stack of books

69. A Stack of Books A stack can be visualized as a stack of books. You place books on top and remove from the top.

70. A Stack of Books

71. Abstract Data Type Stack The Stack class is a concrete implementation of a stack in the Java library The Stack class uses an Object[] to implement a stack

72. Abstract Data Type Stack Sample code to use the Stack class Stack s = new Stack(); s.push("A"); s.push("B"); s.push("C"); //the following loop prints C, B, A int i = s.size(); while (i > 0) { System.out.println(s.pop()); i--; }

73. Abstract Data Type Queue Items added to one end of the queue (the tail) Items removed from the other end (the head) Called first in, first out or FIFO order Think of a queue of people

74. A Queue A Queue can be visualized as a queue of people. People join the tail of the queue and wait until they reach the head.

75. A Queue

76. Abstract Data Type Queue No implementing class in the Java library Can be implemented using a linked list

77. A Queue Implementation public class Queue { /** Constructs an empty queue */ public Queue() { list = new LinkedList(); } /** Adds an item to the tail of the queue @param x the item to add */ public void add(Object x) { list.addLast(x);

78. } /** Removes an item from the head of the queue @return the removed item */ public Object remove() { return list.removeFirst(); } /** Gets the number of items in the queue @return the size */ public int size() { return list.size() } private LinkedList list; }