16 queues_JAVA

8/10/2019 16 queues_JAVA

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Queues

Reading: Lewis and Loftus, JAVA:

Software Solutions, (3rded), Chapter 12.2Savitch Chapter 10.2


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Objectives

To introduce the definition of a queue

To learn how to implement a queue using a

linked list

To develop an implementation of a queue

using an array


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Queues


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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What is a queue?


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Definition of a queue

A list in which cells may be removed only

from the front of the list and new cells may

be added only to the end of the list

Called a FIFO list (first-in, first-out)


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Queue Operations

insertto add an element at the rear of thequeue

getto get the value and delete the elementat the front of the queue

isEmptyto return true if the queue has no

element isFullto return true if the queue is full

(array implementation)


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Class exercise: queues

int x;Queue q;

q.insert (8);q.insert (9);q.insert (3);x = q.get ();q.insert (18);

x = q.get ();q.insert (22);

while (! q.isEmpty())

System.out.println(q.get());

System.out.println(x);

What is output by the following code?


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Linked list implementation

JAVA declaration

class QueueNode {Object data;QueueNode next;QueueNode(Object _d) {

data = _d;next = null;

}... ...}

Variables are usedwith package access,

just for this teaching

session, normally

these variableswould have private

access


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public class Queue {private QueueNode front = null;

private QueueNode back = null;public void insert(Object new_data) {... ...}public Object get() {... ... }public boolean isEmpty() {... ...}

}


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As with StackNodeand Stack, QueueNode

may be defined as an inner class within the

class Queue.


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front

45 51 .. 13

back


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Linked list implementation - get

front

45 51 .. 13

backObject result = front.data

result

45


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front

45 51 .. 13

backfront = front.next

result

45


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front

51 .. 13

back

result

45


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Get from a singleton queue

front back

8


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front back

result = front.data

result8

8


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front / back

front = front .next

result8

8


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front / back

??

result8


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Special case

front = front.next;

if (front == null)back = null;


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Linked list implementation -

insert

.. 131851

front back


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insert

.. 131851

front back

7

temp


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insert

.. 131851

front back

7

temp

back.next = temp;


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insert

.. 131851

front back

7

temp

back = temp;


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Insert to an empty queue

/front /back


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/front /back15

temp


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/front /back15

temp

back.next = temp ??


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/front /back15

temp

front still null!!


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Special case

if (front == null) {front = temp;back = temp;

}else

back.next = temp;


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public void insert (Object new_data) {QueueNode temp = new QueueNode(new_data);

if (isEmpty())front = temp;else

back.next = temp;

back = temp;}

insert


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public Object get () {if (isEmpty() ) { error }

Object result = front.data;

front = front.next;if (front == null)

back = null;

return result;}

get


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Class exercise

public boolean isEmpty () {

.}

Write code for isEmpty


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Array implementation of a queue

0 1 2 3 4 5

front

back

0

3

q6 51 3


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Insertion

0 1 2 3 4 5

front

back

0

3

q

To insert a new cell:

q[back] = new_cell;

back++;

6 51 3


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Insertion

0 1 2 3 4 5

front

back

0

3

q


q[back] = new_cell;

back++;

6 51 3 9


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Insertion

0 1 2 3 4 5

front

back

0

4

q


q[back] = new_cell;

back++;

6 51 3 9


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Getting an item from the queue

0 1 2 3 4 5

front

back

0

4

q

result = q[front];

front++;

6 51 3 9


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0 1 2 3 4 5

front

back

0

4

q 6

result = q[front];

front++;

51 3 9

result 66


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0 1 2 3 4 5

front

back

1

4

q

result = q[front];

front++;

51 3 9


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Problem: insert 34 below

0 1 2 3 4 5

front

back

2

6

q 3 9 14 23


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Solution: wrap-around list

0 1 2 3 4 5

front

back

2

1

q 3 9 14 2334


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Solution: wrap-around list

0 1 2 3 4 5

front

back

2

1

q 3 9 14 2334

if(back == 6) back = 0;

q[back] = new_cell;back++;


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JAVA declaration & initialization

public class Queue {public final static int MAX_QUEUE_SIZE = 50;private Object[] storage;

private int front = -1;private int back = 0;

public Queue() {storage = new Object[MAX_QUEUE_SIZE]; }

public void insert(Object x) { ... ...}public Object get() {... ...}public boolean isEmpty() {... ...}public boolean isFull() {... ...}

}


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Queue operations

public void insert (Object x) {

if (isFull()) { ... queue full ... }storage[back] = x;

if(isEmpty())

front = back;

back = (back + 1) % MAX_QUEUE_SIZE;}

insert


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Queue operations

public Object get () {if (isEmpty()) { ... queue empty ... }

Object result = storage[front];front = (front + 1) % MAX_QUEUE_SIZE;

if (front == back) {front = -1;

back = 0;}return result;

}

get


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Queue operations

isEmpty

public boolean isEmpty () {

return front == -1;}


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Queue operations

isFull

public boolean isFull () {return back == front;

}


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Use of the Queue ADT

Simulations of real-life queues (e.g., banks,

supermarkets, traffic lights, etc)

Program design independent of

implementation

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16 queues_JAVA

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