COSC 2 Dis

Gareth Beckham

AB-JAVA2-Chapter24.pdf

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Chapter 24: Implementing Lists, Stacks, Queues, and Priority Queues Dr. Adriana Badulescu

Objectives ▪ To design common features of lists in an interface and

provide skeleton implementation in an abstract class (§24.2).

▪ To design and implement a dynamic list using an array (§24.3).

▪ To design and implement a dynamic list using a linked structure (§24.4).

▪ To design and implement a stack class using an array list and a queue class using a linked list (§24.5).

▪ To design and implement a priority queue using a heap (§24.6).

Lists ▪ A list is a popular data structure to store data in

sequential order.

▪ For example, a list of students, a list of available rooms, a list of cities, and a list of books, etc. can be stored using lists.

▪ The common operations on a list are usually the following: ▪ Retrieve an element from this list.

▪ Insert a new element to this list.

▪ Delete an element from this list.

▪ Find how many elements are in this list.

▪ Find if an element is in this list.

▪ Find if this list is empty.

Two Ways to Implement Lists ▪ There are two ways to implement a list.

▪ Using arrays

▪ Use an array to store the elements.

▪ The array is dynamically created.

▪ If the capacity of the array is exceeded, create a new larger array and copy all the elements from the current array to the new array.

▪ Using linked list

▪ Use a linked structure.

▪ A linked structure consists of nodes.

▪ Each node is dynamically created to hold an element.

▪ All the nodes are linked together to form a list.

Design of ArrayList and LinkedList

▪ For convenience, let’s name these two classes: MyArrayList and MyLinkedList.

▪ These two classes have common operations, but different data fields.

▪ The common operations can be generalized in an interface or an abstract class.

▪ Prior to Java 8, a popular design strategy is to define common operations in an interface and provide an abstract class for partially implementing the interface.

Design of ArrayList and LinkedList

▪ So, the concrete class can simply extend the abstract class without implementing the full interface.

▪ Java 8 enables you to define default methods.

▪ You can provide default implementation for some of the methods in the interface rather than in an abstract class.

MyList

MyArrayList

MyLinkedList

java.util.Collection java.util.Iterable

MyList Interface

«interface» MyList<E>

+add(index: int, e: E) : void

+get(index: int) : E

+indexOf(e: Object) : int

+lastIndexOf(e: E) : int

+remove(index: int) : E

+set(index: int, e: E) : E

Override the add, isEmpty, remove, containsAll, addAll, removeAll, retainAll,

toArray(), and toArray(T[]) methods

defined in Collection using default methods.

Inserts a new element at the specified index in this list.

Returns the element from this list at the specified index.

Returns the index of the first matching element in this list.

Returns the index of the last matching element in this list.

Removes the element at the specified index and returns the removed element.

Sets the element at the specified index and returns the element being replaced.

«interface» java.util.Collection<E>

MyList

Array Lists ▪ Array is a fixed-size data structure.

▪ Once an array is created, its size cannot be changed.

▪ Nevertheless, you can still use array to implement dynamic data structures. The trick is to create a new larger array to replace the current array if the current array cannot hold new elements in the list.

▪ Initially, an array, say data of Object[] type, is created with a default size.

▪ When inserting a new element into the array, first ensure there is enough room in the array.

▪ If not, create a new array with the size as twice as the current one.

▪ Copy the elements from the current array to the new array.

▪ The new array now becomes the current array.

Array List Animation

https://liveexample.pearsoncmg.com/dsanimation/ArrayListeBook.html

Insertion Before inserting a new element at a specified index, shift all the elements after the index to the right and increase the list size by 1.

0 1 …

i i+1 k-1 Before inserting e at insertion point i

e1 … ei ei+1

…

… ek-1

data.length -1 Insertion point e

0 1 …

i i+1 After inserting e at insertion point i,

list size is

incremented by 1

e1 … e ei

…

… ek-1

data.length -1 e inserted here

ei-1

k+1 k

ei+1

i+2

…shift…

Deletion To remove an element at a specified index, shift all the elements after the index to the left by one position and decrease the list size by 1.

0 1 …

i i+1 k-1 Before deleting the element at index i

e1 … ei ei+1

…

… ek-1

data.length -1 Delete this element

0 1 …

i After deleting the element, list size is

decremented by 1 e1 …

…

… ek

data.length -1

ei-1

k-1

ei+1

k-2

ek-1

…shift…

Implementing MyArrayList

MyArrayList<E>

-data: E[]

-size: int

+MyArrayList()

+MyArrayList(objects: E[])

+trimToSize(): void

-ensureCapacity(): void

-checkIndex(index: int): void

«interface» MyList<E>

Creates a default array list.

Creates an array list from an array of objects.

Trims the capacity of this array list to the list’s current

size.

Doubles the current array size if needed.

Throws an exception if the index is out of bounds in

the list.

Array for storing elements in this array list.

The number of elements in the array list.

Implementing MyArrayList

Test MyArrayList

Linked Lists ▪ Since MyArrayList is implemented using an array, the

methods get(int index) and set(int index, Object o) for accessing and modifying an element through an index and the add(Object o) for adding an element at the end of the list are efficient.

▪ However, the methods add(int index, Object o) and remove(int index) are inefficient because it requires shifting potentially a large number of elements.

▪ You can use a linked structure to implement a list to improve efficiency for adding and removing an element anywhere in a list.

Linked List Animation

https://liveexample.pearsoncmg.com/dsanimation/LinkedListeBook.html

Nodes in Linked Lists A linked list consists of nodes. Each node contains an element, and each node is linked to its next neighbor. Thus a node can be defined as a class, as follows:

element

head

Node 1

element

Node 2

… element

null

Node n

tail

Adding Three Nodes

The variable head refers to the first node in the list, and the variable tail refers to the last node in the list. If the list is empty, both are null. For example, you can create three nodes to store three strings in a list, as follows:

Step 1: Declare head and tail:

The list is empty now Node<String> head = null;

Node<String> tail = null;

Adding Three Nodes, cont.

Step 2: Create the first node and insert it to the list:

head "Chicago"

next: null

tail

After the first node is inserted

inserted head = new Node<>("Chicago");

tail = head;

Adding Three Nodes, cont.

Step 3: Create the second node and insert it to the list:

tail.next = new Node<>("Denver");

head "Chicago"

"Denver"

next: null

tail

tail = tail.next;

head "Chicago"

"Denver"

next: null

tail

Adding Three Nodes, cont.

Step 4: Create the third node and insert it to the list:

head "Chicago"

"Dallas"

next: null

tail

"Denver"

tail.next =

new Node<>("Dallas");

head "Chicago"

"Dallas"

next: null

tail

"Denver"

tail = tail.next;

Traversing All Elements in the List ▪ Each node contains the element and a data field

named next that points to the next node. If the node is the last in the list, its pointer data field next contains the value null. You can use this property to detect the last node. For example, you may write the following loop to traverse all the nodes in the list. Node<E> current = head;

while (current != null)

{

System.out.println(current.element);

current = current.next;

}

MyLinkedList

MyLinkedList<E>

-head: Node<E>

-tail: Node<E>

-size: int

+MyLinkedList()

+MyLinkedList(elements: E[])

+addFirst(e: E): void

+addLast(e: E): void

+getFirst(): E

+getLast(): E

+removeFirst(): E

+removeLast(): E

m Node<E>

element: E next: Node<E>

Link

«interface» MyList<E>

Creates a default linked list.

Creates a linked list from an array of elements.

Adds an element to the head of the list.

Adds an element to the tail of the list.

Returns the first element in the list.

Returns the last element in the list.

Removes the first element from the list.

Removes the last element from the list.

The head of the list.

The tail of the list.

The number of elements in the list.

MyLinkedList

Test MyLinkedList

Implementing addFirst(E e) public void addFirst(E e) { Node<E> newNode = new Node<>(e); newNode.next = head; head = newNode; size++; if (tail == null)

tail = head; }

head

…

A new node

to be inserted

here

ei+1

tail

… ek

null

New node inserted here

(a) Before a new node is inserted.

(b) After a new node is inserted.

… ei

ei+1

tail

… ek

null

head

Implementing addLast(E e) public void addLast(E e) {

if (tail == null) {

head = tail = new Node<>(e); } else {

tail.next = new Node<>(e); tail = tail.next;

} size++;

}

head

… ei

ei+1

tail

… ek

null

New node inserted here

(a) Before a new node is inserted.

(b) After a new node is inserted.

head

… ei

ei+1

tail

… ek

A new node

to be inserted

here

null

Implementing add(int index, E e) public void add(int index, E e) {

if (index == 0) addFirst(e); else if (index >= size) addLast(e); else { Node<E> current = head; for (int i = 1; i < index; i++)

current = current.next; Node<E> temp = current.next; current.next = new Node<>(e); (current.next).next = temp; size++;

} }

current head

…

A new node

to be inserted

here

temp

ei+1

tail

… ek

null

(a) Before a new node is inserted.

current head

…

A new node

is inserted in

the list

temp

ei+1

tail

… ek

null

(b) After a new node is inserted.

Implementing removeFirst() public E removeFirst() {

if (size == 0) return null; else { Node<E> temp = head; head = head.next; size--; if (head == null)

tail = null; return temp.element;

} }

head

…

Delete this node

ei+1

tail

… ek

null

(a) Before the node is deleted.

(b) After the first node is deleted

… ei

ei+1

tail

… ek

null

head

Implementing removeLast()public E removeLast() {

if (size == 0) return null; else if (size == 1) {

Node<E> temp = head; head = tail = null; size = 0; return temp.element;

} else {

Node<E> current = head; for (int i = 0; i < size - 2; i++)

current = current.next; Node temp = tail; tail = current; tail.next = null; size--; return temp.element;

} }

head

…

Delete this node

ek-2

ek-1

tail

null

(a) Before the node is deleted.

(b) After the last node is deleted

current

head

… ek-2

ek-1

null

tail

Implementing remove(int index) public E remove(int index) {

if (index < 0 || index >= size) return null; else if (index == 0) return removeFirst(); else if (index == size - 1) return removeLast(); else { Node<E> previous = head; for (int i = 1; i < index; i++)

previous = previous.next; Node<E> current = previous.next; previous.next = current.next; size--; return current.element;

} }

previous head

element

…

Node to be deleted

element

tail

… element

null

element

(a) Before the node is deleted.

current

previous head

element

… element

element

tail

… element

null

(b) After the node is deleted.

current.next

Time Complexity for ArrayList and LinkedList

Methods MyArrayList/ArrayList MyLinkedList/LinkedList

add(e: E) )1(O )1(O

add(index: int, e: E) )(nO )(nO

clear() )1(O )1(O

contains(e: E) )(nO )(nO

get(index: int) )1(O )(nO

indexOf(e: E) )(nO )(nO

isEmpty() )1(O )1(O

lastIndexOf(e: E) )(nO )(nO

remove(e: E) )(nO )(nO

size() )1(O )1(O

remove(index: int) )(nO )(nO

set(index: int, e: E) )(nO )(nO

addFirst(e: E) )(nO )1(O

removeFirst() )(nO )1(O

Circular Linked Lists ▪ A circular, singly linked list is like a singly linked

list, except that the pointer of the last node points back to the first node.

element

head

Node 1

element

Node 2

… element

Node n

tail

Doubly Linked Lists ▪ A doubly linked list contains the nodes with two

pointers.

▪ One points to the next node and the other points to the previous node. These two pointers are conveniently called a forward pointer and a backward pointer.

▪ So, a doubly linked list can be traversed forward and backward.

element

head

Node 1

element

Node 2

… element

null

Node n

tail

null previous previous

Circular Doubly Linked Lists

▪ A circular, doubly linked list is doubly linked list, except that the forward pointer of the last node points to the first node and the backward pointer of the first pointer points to the last node.

element

head

Node 1

element

Node 2

… element

Node n

tail

previous previous previous

Stacks ▪ A stack can be viewed as a special type of list,

where the elements are accessed, inserted, and deleted only from the end, called the top, of the stack.

Data1

Data2 Data1 Data1

Data2

Data3

Data1 Data2 Data3

Data1

Data2

Data3

Data1

Data2 Data1

Stack Animation

https://liveexample.pearsoncmg.com/dsanimation/StackeBook.html

Implementing Stacks ▪ Using an array list to implement Stack

▪ Since the insertion and deletion operations on a stack are made only at the end of the stack, using an array list to implement a stack is more efficient than a linked list.

▪ This section implements a stack class using an array list .

Design of the Stack Class

▪ There are two ways to design the stack and queue classes: ▪ Using inheritance: You can define the stack class by

extending the array list class.

▪ Using composition: You can define an array list as a data field in the stack class.

▪ Both designs are fine, but using composition is better because it enables you to define a complete new stack class without inheriting the unnecessary methods from the array list

GenericStack<E>

-list: java.util.ArrayList<E>

+GenericStack()

+getSize(): int

+peek(): E

+pop(): E

+push(o: E): void

+isEmpty(): boolean

Creates an empty stack.

Returns the number of elements in this stack.

Returns the top element in this stack.

Returns and removes the top element in this stack.

Adds a new element to the top of this stack.

Returns true if the stack is empty.

An array list to store elements.

GenericStack

Test GenericStack

Queues ▪ A queue represents a waiting list.

▪ A queue can be viewed as a special type of list, where the elements are inserted into the end (tail) of the queue, and are accessed and deleted from the beginning (head) of the queue.

Data1

Data2 Data1 Data1

Data2

Data3

Data1 Data2 Data3

Data2

Data3

Data1

Data3

Data2 Data3

Queue Animation

https://liveexample.pearsoncmg.com/dsanimation/QueueeBook.html

Implementing Queues

▪ Use a linked list to implement Queue

▪ Since deletions are made at the beginning of the list, it is more efficient to implement a queue using a linked list than an array list.

▪ This section implements a queue using a linked list.

Design of the Queue Class

▪ There are two ways to design the queue class: ▪ Using inheritance: You can define the queue class by

extending the linked list class.

▪ Using composition: You can define a linked list as a data field in the queue class.

▪ Both designs are fine, but using composition is better because it enables you to define a complete new queue class without inheriting the unnecessary methods from the linked list

GenericQueue<E>

-list: LinkedList<E>

+enqueue(e: E): void

+dequeue(): E

+getSize(): int

Adds an element to this queue.

Removes an element from this queue.

Returns the number of elements from this queue.

GenericQueue

Test GenericQueue

Priority Queue ▪ A regular queue is a first-in and first-out data structure. ▪ Elements are appended to the end of the queue and are removed from the

beginning of the queue. ▪ In a priority queue, elements are assigned with priorities. ▪ When accessing elements, the element with the highest priority is removed

first. ▪ A priority queue has a largest-in, first-out behavior. ▪ For example, the emergency room in a hospital assigns patients with priority

numbers; the patient with the highest priority is treated first.

MyPriorityQueue

Test MyPriorityQueue