Linked Lists Based on D.S. Malik, Java Programming: Program Design Including Data Structures

1. Linked ListsBased on D.S. Malik, Java Programming: Program Design Including Data Structures

2. Linked Lists • Linked list • List of items, called nodes • The order of the nodes is determined by the address, called the link, stored in each node • Every node (except the last node) contains the address of the next node • Components of a node • data/info: stores the relevant information • link: stores the address of the next node

3. Linked Lists • head or first • Holds the address of the first node in the list • The info part on the node can be either a value of a primitive type or a reference to an object • Class Node • Represents nodes on a list • It has two instance variables • info (of type int, but it can be any other type) • link (of type Node)

4. Linked Lists • Class Node public class Node { public int info; public Node link; } • Notice that instance variables of the classNode are declared as public

5. Linked List: Some Properties • Consider the following linked list

6. Linked List: Some Properties • Now consider the statement current = head;

7. Linked List: Some Properties • Now consider the statement current = current.link;

8. Linked List: Some Properties

9. Traversing a Linked List • Basic operations of a linked list that require the link to be traversed • Search the list for an item • Insert an item in the list • Delete an item from the list • You cannot use head to traverse the list • Why? You would lose the nodes of the list. • Use another reference variable of the same type as head: current

10. Traversing a Linked List • The following code traverses the list current = head; while (current != null){ //Do something - process current current = current.link; } • Example: The following code outputs the data /in each node current = head; while (current != null){ System.out.println(current.info + “ “); current = current.link; }

11. Item Insertion and Deletion • Consider the following definition of a node public class Node { public int info; public Node link; } • And the following variable declaration Node head, p, q, newNode;

12. Insertion • Consider the following linked list • We want to create a new node with info 50 and insert it after p

13. Insertion • The following statements create and store 50 in the info field of a new node newNode = new Node(); //create newNode newNode.info = 50; //store 50 in the new node

14. Insertion (continued) • The following statements insert the node in the linked list at the required place newNode.link = p.link; p.link = newNode; • The sequence of statements to insert the node is very important • If you reverse the sequence of the statements, you will not get the desired result

15. Insertion • newNode.link = p.link; • p.link = newNode; List after the statement newNode.link = p.link; executes List after the statement p.link = newNode; executes

16. Insertion • Using two reference variables, we can simplify the code somewhat • Consider the following List with reference variables p and q

17. Insertion • The following statements insert newNode between p and q newNode.link = q; p.link = newNode; or p.link = newNode; newNode.link = q; • The order in which these statements execute does not matter

18. Insertion • p.link = newNode; • newNode.link = q; List after the statement p.link = newNode; executes List after the statement newNode.link = q; executes

19. Deletion • Consider the following linked list • We want to delete node with info 34

20. Deletion • The following statement removes the node from the list p.link = p.link.link List after the statement p.link = p.link.link; executes

21. Deletion • Previous statement removed the node • However, the memory may still be occupied by this node • System’s automatic garbage collector reclaims memory occupied by unreferenced nodes • Could use System.gc(); to manually run the garbage collector

22. Deletion • Using two reference variables, you can simplify the code somewhat • Consider the following statements q = p.link; p.link = q.link; q = null; System.gc();

23. Deletion (continued) • q = p.link; • p.link = q.link; List after the statement q = p.link; executes List after the statement p.link = q.link; executes

24. Building a Linked List • You can build a list in two ways: forward or backward • Forward manner • A new node is always inserted at the end of the linked list • Backward manner • A new node is always inserted at the beginning of the linked list

25. Building a Linked List Forward • A new node is always inserted at the end of the linked list • You need three reference variables • One to point to the front of the list • Cannot be moved without destroying the list • One to point to the last node of the list • One to create the new node

26. Building a Linked List Forward Node buildListForward(){ Node first, newNode, last; int num; System.out.println(“Enter integers (999 to stop):”); num = input.nextInt(); //priming read first = null; while (num != 999) { newNode = new Node(); newNode.info = num; newNode.link = null; if (first == null) { //empty list first = newNode; last = newNode; } else { last.link = newNode; last = newNode; } num = input.nextInt(); //next read } return first; }

27. Building a Linked List Backward • A new node is always inserted at the beginning of the linked list • You only need two reference variables • One to point to the front of the list • Changes each time a new node is inserted • One to create the new node

28. Building a Linked List Backward Node buildListBackward(){ Node first, newNode; int num; System.out.println (“Enter integers (999 to stop):”); num = input.nextInt(); //priming read first = null; while (num != 999) { newNode = new Node(); //create a node newNode.info = num; //store the data in newNode newNode.link = first; //put newNode at the beginning first = newNode; //update the head of the list num = input.nextInt();//next read } return first; }

29. Linked List as an ADT UML class diagram of the interface LinkedListADT

30. Linked List as an ADT • There are two types of linked lists: sorted (ordered) and unsorted (unordered) • The algorithms to implement some of the operations differ for sorted and unsorted lists • Therefore, define the LinkedListClass as an abstract class • LinkedListClass has two derived classes • UnorderedLinkedList • OrderedLinkedList

31. Structure of Linked List Nodes • Each node of a linked list must keep track of the data as well as the next node in the list • The node has two instance variables • The class LinkedListNode is defined as an inner class of LinkedListClass • Simplify operations such as insert and delete • LinkedListNode is defined as protected and generic

32. Structure of Linked List Nodes UML class diagram of the class LinkedListNode and the outer-inner class relationship

33. Instance Variables of the Class LinkedListClass • Instance variables protected LinkedListNode<T> first; //variable to store the address of the first node protected LinkedListNode<T> last; //variable to store the address of the last node protected int count; //variable to store the number of nodes in the list

34. Linked List Iterators UML class diagram of the class LinkedListIterator and the outer-inner class relationship NOTE:An iteratoris an object that produces each element of a collection one element at a time

35. class LinkedListClass UML class diagram of the class LinkedListClass

36. class LinkedListClass • Definition of the class LinkedListClass public abstract class LinkedListClass<T> implements LinkedListADT<T> { //Place the definition of the class LinkedListNode<T> here. //Place the definition of the class LinkedListIterator<T> here. //Place instance variables here //Place the definition of the nonabstract methods here //Place the definition of the abstract methods here. }

37. Unordered Linked List UML class diagram of the class UnorderedLinkedList and the inheritance hierarchy

38. Ordered Linked Lists UML class diagram of the class OrderedLinkedList and the inheritance hierarchy

39. Double Linked Lists • Linked list in which every node has a next pointer and a back pointer • A double linked list can be traversed in either direction public class DoubleLinkedListNode<T> implements Cloneable{ T info; DoubleLinkedListNode<T> next; DoubleLinkedListNode<T> back; ... }

40. Circular Linked Lists • A linked list in which the last node points to the first node • It is convenient to make first point to the last node Circular linked list with more than one node