1. Programming II (CS300) Chapter 07: Linked Lists
Mouna KACEM
Spring 2019

2. Linked Lists Introduction Linked List Abstract Data Type
SinglyLinkedList Implementation of the ListADT
ArrayList Implementation of the ListADT
Keep in Mind

3. Introduction: General Linked List
This chapter introduces how to:
Declare the Abstract Data Type (ADT) Linked List
Define the most commonly Operations for General Linked Lists
Implement a General Linked List
The basic Linked List consists of a collection of connected, dynamically allocated nodes or items of the same type. 18 10 -5 78
linkedList
null
item next
item next
item next
item next

4. Introduction: General Linked List
A linked list contains a collection of data items of the same type that are stored in different objects referred as nodes
Each node consists of two fields:
A data item field : stores or refers to the data value
A reference to an object of the same type
This reference is used to connect to the next node in the list
node
linkedList 18 -5 78 10
null
item next
item next
item next
item next
data item
Reference to the next node in the list

5. LinkedNode
public class LinkedNode<T>{ // Fields private T item; // item data field of any type T private LinkedNode<T> next; // reference to the next node in the list // Constructors ... // Methods } // end generic class LinkedNode
node
linkedList 18 -5 78 10
null
item next
item next
item next
item next
data item
Reference to the next node in the list

6. Linked List Abstract Data Type
T refers to AnyType
public interface ListADT<T> {
// List of Operations
public void add(T newObject);
public void add(int index, T newObject);
public boolean contains(T findObject);
public boolean isEmpty();
public int size();
public int indexOf(T findObject);
public T get(int index);
public T remove(int index);
} // end ListADT generic interface

7. Implementation of the ListADT
/** * LinkedList class represents a reference-based implementation of ADT list. <author> */ public class LinkedList<T> implements ListADT<T> { private LinkedNode<T> head; // entry point reference to linked list of items of type<T> public LinkedList<T>() { head = null; } // end default constructor * TODO Implementation of the interface ListADT declared methods } // end LinkedList class

8. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
Create a NewNode that contains newObject as data field
If the list is empty, insert the newNode at the head of the list
Otherwise, insert the newNode at the end of the list

9. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
The list is empty 35
newNode: X
head:
item next

10. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
The list is empty
newNode:
head: 35
item next

11. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
The list is empty
(summary diagram)
newNode: 1 X 3 35
head:
head: 2
item next

12. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode:
newObject: 35
item next
runner: 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

13. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode: 35
item next
runner: 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

14. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode: 35
item next
runner: 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

15. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode: X 35
item next
runner: X 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

16. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode: X 35
item next
runner: X 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

17. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
List is not empty
newNode:
runner: 18 -5 78 10
head: 35
item next
runner: a pointer of type LinkedNode<T> used to traverse the list

18. Implementation of the ListADT
public void add(T newObject); // inserts newObject at the end of the linked list
The list is not empty
(summary diagram)
newNode: X 3 35
item next 1 2
runner:
Traverse the list 3 X 18 -5 78 10
head:
runner: a pointer of type LinkedNode<T> used to traverse the list

19. Implementation of the ListADT
public void add(int index, T newObject);
Precondition: 0 ≤ index ≤ list.size()
If index < 0 or index > size()  error (throw an exception or display an error message)
Otherwise, create a newNode (instance of LinkedNode) that contains the newObject as item data field
If index == 0  insert the newNode at the head of the list
If index == size()  insert the newNode at the end of the list
If index > 0 and index < size()  insert newNode in the middle of the list at the index position

20. Implementation of the ListADT
public void add(int index, T newObject);
Case3: 0 < index < list.size()
newNode:
Index: 3 -3
item next
runner:
inx = 0
inx = 1
indices: 18 -5 78 10
head: 35
item next
runner initialized to head : a pointer of type LinkedNode<T> used to traverse the list
inx an integer initialized to 0 : the index of the node pointed by runner in the list

21. Implementation of the ListADT
public void add(int index, T newObject);
Case3: 0 < index < list.size()
newNode:
Index: 3 -3
item next
runner:
inx = 1
inx = 2
indices: 18 -5 78 10
head: 35
item next
runner initialized to head : a pointer of type LinkedNode<T> used to traverse the list
inx an integer initialized to 0 : the index of the node pointed by runner in the list

22. Implementation of the ListADT
public void add(int index, T newObject);
Case3: 0 < index < list.size()
newNode:
Index: 3 X
inx == index -1 -3
item next
runner:
inx = 2 X
indices: 18 -5 78
head: X 10 35
item next
runner initialized to head : a pointer of type LinkedNode<T> used to traverse the list
inx an integer initialized to 0 : the index of the node pointed by runner in the list

23. Implementation of the ListADT
public void add(int index, T newObject);
Summary Diagram
Case3: 0 < index < list.size()
newNode:
Index: 3 1 4 X
Traverse the list until inx = index -1 -3 2
item next
runner: 4 3
inx = 2 X
indices: 18 -5 78 X 10
head: 35
item next
runner initialized to head : a pointer of type LinkedNode<T> used to traverse the list
inx an integer initialized to 0 : the index of the node pointed by runner in the list

24. Implementation of the ListADT
public void add(int index, T newObject);
Case3: 0 < index < list.size()
newNode:
runner:
Index: 3
indices: 18 -5 78 -3 10
head: 35
item next
runner initialized to head : a pointer of type LinkedNode<T> used to traverse the list
inx an integer initialized to 0 : the index of the node pointed by runner in the list

25. Implementation of the ListADT
If the list is empty return false
Otherwise (list is not empty)
define a runner (a reference to a LinkedNode object) to traverse the list and look for the first node whose data item matches with findObject
initialize runner to the head of the list
traverse the list looking for a much with findObject
while runner != null && there is no match go to the next node in the list
if the findObject is found return true
if the list is entirely traversed without finding a match, return false
public boolean contains(T findObject);
runner: a pointer of type LinkedNode<T> used to traverse the list

26. Implementation of the ListADT
List not empty
findObject: 10
is runner.getItem().equals(findObject)? No
runner: 18 -5 10 78
head: 10
item next
runner: a pointer of type LinkedNode<T> used to traverse the list

27. Implementation of the ListADT
List not empty
Example
findObject: 10
is runner.getItem().equals(findObject)?
Yes
runner:
return true 18 -5 10 78
head: 10
item next
runner: a pointer of type LinkedNode<T> used to traverse the list

28. Implementation of the ListADT
public int indexOf(T findObject);
Initialize index of type int to -1
if list is empty return index
Otherwise
define a runner (a reference to a LinkedNode object) to traverse the list and look for the first node whose data item matches with findObject
initialize runner to the head of the list and increment index
traverse the list looking for a much with findObject
while runner!= null && there is no match go to the next node in the list and increment index
if the findObject is found return index
if the list is entirely traversed without finding a match, return -1
runner: a pointer of type LinkedNode<T> used to traverse the list

29. Implementation of the ListADT
public T get(int index);
if index < 0 or index >= size()  error (throw an exception or return null)
Otherwise (list is not empty and index in the range of the list indices)
define a runner (a reference to a LinkedNode object) to traverse the list looking for the node of index “index”
initialize runner to the head of the list and inx of type int to 0
while inx < index go to the next node in the list (runner = runner.getNext()) and increment inx
return runner.getItem()
Precondition: 0 ≤ index < list.size()
runner: a pointer of type LinkedNode<T> used to traverse the list

30. Implementation of the ListADT
public T remove(int index);
if index < 0 or index >= size()  error (throw an exception or return null)
Otherwise (list is not empty and index in the range of the list indices)
We distinguish two cases
Case1: index == 0 : return the item at the head of the list, then remove the node at the head
Case2: Index > 0 and index <= size() -1 : remove the item stored at index position of the list and return it
Precondition: 0 ≤ index < list.size()

31. Implementation of the ListADT
public T remove(int index);
Case1: List not empty and index == 0 : remove the node at the head of the list
remove the node at the head of the list 1 2 3
item = head.getItem();
head = head.getnext();
return item 1
item: 18 18 -5 78
head: X 10 35 2 2
head:

32. Implementation of the ListADT
public T remove(int index);
Case3: List not empty and index > 0 and index <= size -1 : remove a node other than the head
To remove a node from the list (at a given index not zero), we need a pointer to its previous node
Use a runner to reach the node of index index - 1
Initialize runner to head and inx of type int to 0
While inx < index – 1, go to the next node (runner = runner.getNext()) and increment inx
item = runner.getNext().getItem();
remove the node at the position index the list: runner.setNext(runner.getNext().getNext());
return item 18 -5 78 10
head: 35
runner: a pointer of type LinkedNode<T> used to traverse the list

33. Implementation of the ListADT
public T remove(int index);
Case2: (Example) List not empty and index > 0 and index == size -1 : remove the node at the end (tail) of the list
Get the item at runner.getNext() and
Remove runner.getNext()
Return item 3
Index: 4
Traverse the list to reach the node of index index-1 2 1
item = runner.getNext().getItem();
item: 35
runner: 3
runner: 2
runner: 2
runner: 2
inx = 0
inx = 2
inx = 3
inx = 1
indices: 3 18 -5 78 10
head: X 35
runner: a pointer of type LinkedNode<T> used to traverse the list

34. Implementation of the ListADT
public T remove(int index);
Case2 (general case): List not empty and index > 0 and index <= size -1 : remove a node at the middle of the list
Get the item at runner.getNext()
// remove runner.getNext()
runner.setNext(runner.getNext().getNext()) return item 3
Index: 2
Traverse the list to reach the node of index index-1 2 1
runner:
item = runner.getNext().getItem();
item: 78
runner: 2 3
inx = 0
inx = 1 3
indices: 18 -5 X 78 10
head: 35 3
runner: a pointer of type LinkedNode<T> used to traverse the list

35. Practice Examples Declare the ListADT<T> interface
Implement the generic LinkedNode<T> class
Implement the linked list of Integers class
public class LinkedList implements ListADT<Integer>
Test the implementation
Implement the generic linked list SinglyLinkedList<T> class
public class SinglyLinkedList<T> implements ListADT<T>
Test the SinglyLinkedList class implementation considering different types T
Implement the generic class ArrayList<T> implements ListADT<T>

36. Doubly Linked Lists
The singly linked list does not efficiently support some important operations
For instance, it is time consuming to go to the end of the list
Singly linked lists cannot implement “retreat” method which allows one backward move at the time while traversing a list
A doubly linked list allows bidirectional traversal by storing two links per node
Each node now has two links (next and prev)
prev item next
node: -5

37. Doubly linked lists
Searching and traversing the list can easily be performed in both directions
Doubly linked lists allow
to move up just as easily as down in the list
to insert after as well as before a node at a given position (index)
Insertion (add) and removal (remove) operations involve twice as many link changes as for a singly linked list. 78 18 -5
: tail
head:

38. Circular Linked List
In a circularly linked list, the last node’s next link references the first one in the list (i.e. the head of the list)
The circular list is useful when we want searching to allow wraparound 18 -5 10
head:

39. Keep in Mind
A linked list represents a collection (that may be empty) of connected, dynamically allocated nodes (known as linked nodes)
A linked node of a singly linked list must contain a reference to a node of the same type. This reference is used to connect to the next node in the list
To add a node at the middle of a singly linked list, we need a reference to the previous node
We need a reference to the node at the prior position to the one that we want to insert
To remove a node from a singly linked list other than the head of the list, we need a reference to the node prior to the node that we want to remove
To remove an item or object that we don’t know the index, we can make use of the method indexOf which returns the position (index) of an item in the linked list

40. Keep in Mind
Doubly linked list allows bidirectional traversal of the list
A doubly linked node has two links: a reference to the next node and a reference to the previous node in the list
Insertion and deletion operations are more easy with doubly linked lists, but ensure more link changes compared to a singly linked list
Common error while implementing linked list operations: NullPointerException!
Methods should not be allowed to access fields via a null reference