04/20/23 IT 179 1

Queue

a

FIFO (First In First Out)

e fb h

front rear

poll,remove

offeradd

peek,element

capacity

size

Java Doc

java.utilInterface Queue<E>

04/20/23 IT 179 2

/*********************************************************//* API for Queue *//*********************************************************/

public interface Queue<T> {boolean offer(T item); // return false if failedT remove(); // throw NoSuchElementException if emptyT poll(); // return null if emptyT peek(); // return null if emptyT element();// throw NoSuchElementException if emptyint size(); // return the number of element in the queueint capacity(); // return the capacity of the queue

// -1 if unlimited}

Queue interface

04/20/23 IT 179 3

Queue interface

2 3 5

public interface Queuek<T> { boolean offer(T item); T remove(); T poll(); T peek(); T element(); int size(); int capacity();}

2

3

5

front

rear

size=3capacity=5

04/20/23 IT 179 4

Queue interface

2 3 5

2

3

5

front

rear

public interface Queuek<T> { boolean offer(T item); T remove(); T poll(); T peek(); T element(); int size(); int capacity();}

size=3

04/20/23 IT 179 5

Using arrays to implement queues

frontrear rear

offer: rear++poll: front++

offer

front

rear

front

offer

rear

offer

rear

front

front

poll

rearfront

poll

04/20/23 IT 1796

Using arrays to implement queues

front

rear

rear

offer

front

poll

rear

front

poll

offer

rear

offer

rear

front front

offer

rear

front

rear

offer: rear= rear+1)%capacity poll: front=(front+1)%capacity

04/20/23 IT 179 7

1 2 3 4

front rear

Data structures

Operations

Using linked lists to implement queues

Boolean offer(T item); T remove(); T poll(); T peek(); T element()int size();int capacity();

package myUtil;import java.util.NoSuchElementException;

/* This LQueue uses a linked list to implement interface Queue. */public class LQueue<T> implements Queue<T> {

/* This is an inner class for internal nodes */private class Node<E> { private E data; private Node<E> next; private Node(E data, Node<E> next) {

this.data = data; this.next = next; }}private Node<T> front, rear;private int size, capacity;

/* Default constructor initializes an empty queue. */public LQueue() { front = rear = null; size=0; capacity=-1; // -1 : no limit }

/* Set capacity. */public LQueue(int n) { front = rear = null; size=0; capacity=n; }

04/20/23IT 179 8

Queue using single linked lists

04/20/23 IT 179 9

Implementation using linked lists

public boolean offer(T item) { if (size == capacity) return false; if (rear == null)

front = rear = new Node<T>(item, null); else

rear = rear.next = new Node<T>(item, null); size++; return true; }

public T peek() { if (front==null) return null; return front.data;}

public T element() { if (front==null) throw new NoSuchElementException(); return front.data;}

04/20/23 IT 179 10

Implementation using linked lists

public T remove() { if (front==null) throw new NoSuchElementException(); T item = front.data; if (front == rear)

front = rear = null; else

front = front.next; size--; return item;}

public T poll() { if (front==null) return null; T item = front.data; if (front == rear) front = rear = null; else

front = front.next; size--; return item;}

package myUtil;import java.util.NoSuchElementException;

/* This AQueue uses a linked list to implement interface Queue. */public class AQueue<T> implements Queue<T> {

04/20/23 IT 179 11

Queue using single arrays

private Object[] queue;private int front, rear, size, capacity;

public AQueue() { front = rear = size = 0; capacity = -1; // -1: unlimited queue = new Object[10];

}

public AQueue(int capacity) {queue = new Object[capacity];this.capacity = capacity;front = rear = size = 0;

}

04/20/23 IT 179 12

Queue using single arrays

/** * A private method that doubles the size of the array, queue, * if its size is too small for a new item. */private void doubleCapacity(){

Object[] newQ = new Object[queue.length*2];for (int i=0; i<size; i++) {

newQ[i]=queue[front];front = (front+1)%size;

}front = 0;rear = size-1;queue = newQ;

}

04/20/23 IT 179 13

Queue using single arrays

public boolean offer(T item) {if (size==queue.length && capacity==-1) doubleCapacity();if (size==queue.length) return false;if (size != 0)

rear = (rear+1)%queue.length;queue[rear] = item;size++;return true;

}

public T remove() {if (size==0) throw new NoSuchElementException();size--;T item =(T) queue[front];if (size != 0) front = (front+1)%queue.length;return item;

}

04/20/23 IT 179 14

Double Ended Queue

a e fb h

front rearpollFirst,removeFirst

offerLastaddLast

peekFirst,getFirst

size

Java Doc

java.utilInterface Deque<E>

Deque<E>

peekLast,getLast

pollLastremoveLast

offerFirstaddFirst

capacity