Queues. Data Structures and Design with Java and JUnit Rick Mercer 18-1

Transcription

1 Queues Data Structures and Design with Java and JUnit Rick Mercer 18-1

2 A Queue ADT First in first out access A Queue is another name for waiting line Example: Submit jobs to a network printer What gets printed first? Print the least recently submitted no priorities given Add new print jobs at the end of the queue Queues provide First In First Out (FIFO) access to elements could also say Last In Last Out (LILO) 18-2

3 The Java Queue class? Some languages have a Queue class or queue is part of a library that works with the language Java 1.4 used class LinkedList to offer FIFO functionality by adding methods addlast(object) and Object(getFirst) Java 1.5 added a Queue interface and several collection classes: ArrayBlockingQueue<E> and LinkedBlockingQueue<E> Outline of what we'll do Specify a Queue ADT as a Java interface Show a difficult to implement array based implementation 18-3

4 Designing a Queue Interface Queues typically provide these operations add adds an element at the end of the queue peek returns a reference to the element at the of the queue remove removes the element from the of the queue and returns a reference to the element isempty returns false if there is at least one element in the queue 18-4

5 Specify an interface We will use an interface to describe a queue ADT The interface specifies method names, return types, the type of elements to add, and hopefully comments interface OurQueue declares we must be able to add and remove any type of element Collection class must have <E> to make it a generic type 18-5

6 Interface to specify a FIFO Queue import java.util.nosuchelementexception; public interface OurQueue<E> { // Return true if this queue has 0 elements public boolean isempty(); // Store a reference to any object at the end public void add(e newel); // Return a reference to the object at the // of this queue public E peek() throws NoSuchElementException; } // Remove the reference to the element at the public E remove() throws NoSuchElementException; 18-6

7 Let SlowQueue implement the Queue interface We need to store an Object[] an array of Object objects avoids having queue of int and people and cars, and... public class SlowQueue<E> implements OurQueue<E> { private int ; private Object[] data; //... Now implement all methods of the OurQueue interface as they are written plus a constructor with the proper name 18-7

8 Bad array type queue Queue as an array could have the of the queue is always in data [0] public SlowQueue(int max) { data = new Object[max]; = -1; } data[0] data[1] data[2] data[3] == -1 So far so good. An empty queue 18-8

9 First version of add public void add(e element) { // This method will be changed later ++; data[] = element; } Send an add message aqueue.add("a"); data[0] data[1] data[2] data[3] "a" == 0 So far so good. A queue of size

10 add another public void add(e element) { // This method will be changed later ++; data[] = element; } Send two more add messages aqueue.add("b"); aqueue.add("c"); data[0] data[1] data[2] data[3] "a" "b" "c" == 2 So far so good. A Queue of size

11 Array Implementation of a Queue During remove, slide all elements left if size were 999, then 998 assignments would be necessary Before remove "a" "b" "c" After remove "b" "c" "c" A poor remove algorithm 18-11

12 Effect of queue operation using an array with a "floating" add("a") add("b") add("c") remove() "a" "b" "c"? "a" "b" "c"? add("d") "a" "b" "c" "d" remove() "a" "b" "c" "d" 18-12

13 What happens next when equals array.length? add("e") "a" "b" "c" "d" "e" indexes the last array index However, this queue is not full Where do you place "e"? data[0] is available What code would increment correctly even when is referencing the last available array index?? 18-13

14 The Circular Queue A "circular queue" implementation uses wraparound The queue has "c" "d" "e" either increase by 1 or set to 0 "d" "c" "e" "b" "a" data[0] add("e") now works in this "circular" queue. It reuses previously used array indexes 18-14

15 Implementing a Circular Queue Still have to work with arrays, not circles In order for the first and last indices to work in a circular manner: increase by one element at a time after largest index in the array, go to zero. = could contain code you just wrote on previous slide But what is an empty queue? What values should be given to and when the queue is constructed? 18-15

16 Problem: A full queue can not be distinguished from an empty queue One option is to have the constructor place one index before then increment during add a a d a empty 1 in q 3 in q full q c b c b What does == imply? An empty or full queue? 18-16

17 Corrected Circular Queue Use this trick to distinguish between full and empty queues The element referenced by never indexes the element the real is located at nextindex() private int nextindex(int index) { // Return an int to indicate next position return (index + 1) % data.length; } For example, use this during peek() return data[nextindex()]; 18-17

18 Correct Circular Queue Implementation Illustrated "a" "a" "a" empty 1 in q 2 in q full q "b" "c" "b" The index is always 1 behind the actual This wastes one array element but it's no big deal 18-18

19 Correct Circular remove Implementation Illustrated "a" full q 2 in q 1 in q empty q "c" "b" "c" "b" "c" remove three times to make this queue empty 18-19