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Chapter 4

Chapter 4. Queues. Chapter Objectives. Q ueue? Operations Insertion (offer and add) Removal (remove and poll) Accessing (peek and element) Queue implementation Single linked list Circular array Double linked list Deque ?. Queue. Stack—LIFO Queue--FIFO. Queue Abstract Data Type.

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Chapter 4

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  1. Chapter 4 Queues

  2. Chapter Objectives • Queue? • Operations • Insertion (offer and add) • Removal (remove and poll) • Accessing (peek and element) • Queue implementation • Single linked list • Circular array • Double linked list • Deque?

  3. Queue • Stack—LIFO • Queue--FIFO

  4. Queue Abstract Data Type Section 4.1

  5. Queue Abstract Data Type • Queue example • How a person is served? • Add a new element?

  6. Queue Examples • OS task/process queue • Print queue

  7. Print Stack? • Stack -- LIFO • What’s the problem with Print Stack?

  8. Another Example: Graph Traversal • Network graph (node, links) • Breadth-First traversal

  9. Queue Interface

  10. Queue LinkedList Implementation • LinkedList • Inserting and removing easily. • Java 5.0 Queue implementation Queue<String> names = new LinkedList<String>();

  11. Example-Maintaining a Queue of Customers Section 4.2

  12. Maintaining a Queue of Customers • Maintaining Methods • insert a new customer • display the customer • remove the customer • display the length • Special determination

  13. Designing a Queue of Customers • JOptionPane.showOptionDialog() menu • Use a queue the data structure • MaintainQueue class • a Queue<String> component customers

  14. Maintain a Queue of Customers Algorithm for processCustomers • while the user is not finished • Display the menu and get the selected operation • Perform the selected operation Algorithm for determining the position of a Customer • Get the customer name • Set the count of customers ahead of this one to 0 • for each customer in the queue • if the customer is not the one sought • increment the counter • else • display the count of customers and exit the loop • if all the customers were examined without success • display a message that the customer is not in the queue

  15. Implementing a Queue of Customers • Listing 4.1&4.2 MaintainQueue & ProcessCustomers

  16. Implementing the Queue Interface Section 4.3

  17. 1-Use Double-Linked List • Insertion and removal is O(1) • Problem: • More methods are available • Solution: • Create a new class with a LinkedList component • Only implement the methods for Queue

  18. 2-Use a Single-Linked List • Insertions –at the end of a queue • One reference to the last nodeO(1) performance • Removal-- from the front of a queue • Listing 4.3 ListQueue

  19. 3-Use a Circular Array • Time efficiency OK • a single- or double-linked list • Space efficiency– NOT OK • Due to refernces • Array Implementation • Insertion at rearO(1) • Removal from the frontO(n) • Removal from rear  O(1) • Insertion at the front  O(n)

  20. Using a Circular Array

  21. Using a Circular Array

  22. Initializing ArrayQueue q = new ArrayQueue(5); size = 0 front = 0 capacity = 5 public ArrayQueue(int initCapacity) { capacity = initCapacity; theData = (E[])new Object[capacity]; front = 0; rear = capacity – 1; size = 0; } rear = 4

  23. add (‘*’) q.offer('*'); size = 0 1 front = 0 * capacity = 5 public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } rear = 4 rear = 0

  24. add ‘+’ q.offer('+'); size = 1 2 front = 0 * capacity = 5 + public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } rear = 1 rear = 0

  25. add ‘/’ q.offer('/'); size = 2 3 front = 0 * capacity = 5 + / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } rear = 1 rear = 2

  26. add ‘-’ q.offer('-'); size = 3 4 front = 0 * capacity = 5 + / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } - rear = 3 rear = 2

  27. add ‘A’ q.offer('A'); size = 4 5 front = 0 * capacity = 5 + / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } - rear = 4 rear = 3 A

  28. poll next = q.poll(); size = 5 4 front = 0 front = 1 * capacity = 5 + / public E poll() { if (size == 0) { return null } E result = theData[front]; front = (front + 1) % capacity; size--; return result; } - rear = 4 A result = '*'

  29. Poll again next = q.poll(); size = 4 3 front = 1 front = 2 * capacity = 5 + / public E poll() { if (size == 0) { return null } E result = theData[front]; front = (front + 1) % capacity; size--; return result; } - rear = 4 A result = '+'

  30. add ‘B’ q.offer('B'); size = 3 4 front = 2 * B capacity = 5 + / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } - rear = 4 rear = 0 A

  31. add ‘C’ q.offer('C'); size = 4 5 front = 2 B capacity = 5 + C / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } - rear = 0 rear = 1 A

  32. add ‘D’ q.offer('D'); size = 5 front = 2 B capacity = 5 + C / public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } - rear = 1 A

  33. Reallocate theData B q.offer('D'); + C size = 5 front = 2 front = 2 B / capacity = 5 - + C A / private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } - rear = 1 rear = 1 A newCapacity = 10

  34. Reallocate newData theData i = 0 B q.offer('D'); + C size = 5 front = 2 j = 2 / capacity = 5 - A private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  35. Copy data newData theData i = 1 i = 0 B / q.offer('D'); + C size = 5 front = 2 j = 2 / / capacity = 5 j = 3 - A private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  36. Copy data newData theData i = 2 i = 1 / B q.offer('D'); + C - size = 5 front = 2 / capacity = 5 j = 3 - - A j = 4 private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  37. Copy data newData theData i = 3 i = 2 / j = 0 B q.offer('D'); + C - size = 5 front = 2 / A capacity = 5 - A A j = 4 private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  38. Copy data newData theData i = 4 i = 3 / B B j = 0 q.offer('D'); j = 1 + C - size = 5 front = 2 / A capacity = 5 - B A private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  39. Copy data newData theData i = 5 i = 4 / B q.offer('D'); j = 1 + C C - size = 5 front = 2 j = 2 / A capacity = 5 - B A C private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  40. Initialize new queue newData theData i = 5 / B q.offer('D'); + C C - size = 5 rear = 4 front = 0 front = 2 j = 2 / A 10 capacity = 5 - B A C private void reallocate() { int newCapacity = 2 * capacity; E[] newData = (E[])new Object[newCapacity]; int j = front; for (int i = 0; i < size; i++) { newData[i] = theData[j]; j = (j + 1) % capacity; } front = 0; rear = size – 1; capacity = newCapacity; theData = newData; } rear = 1 newCapacity = 10

  41. Add‘D’ theData / q.offer('D'); - size = 5 6 rear = 4 front = 0 rear = 5 A 10 capacity = 5 B C public boolean offer(E item) { if (size == capacity) { reallocate(); } size++; rear = (rear + 1) % capacity; theData[rear] = item; return true; } D

  42. Implementing a Queue Using a Circular Array -All Code • Listing 4.4 ArrayQueue

  43. Implementing Class ArrayQueue<E>.Iter private class Iter implements Iterator<E> { private int index; private int count = 0; public Iter() { index = front; } @Override public boolean hasNext() { return count < size; } .... • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface

  44. Implementing Class ArrayQueue<E>.Iter(cont.) private class Iter implements Iterator<E> { private int index; private int count = 0; public Iter() { index = front; } @Override public boolean hasNext() { return count < size; } .... • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface indexstores the subscript of the next element to be accessed

  45. Implementing Class ArrayQueue<E>.Iter(cont.) private class Iter implements Iterator<E> { private int index; private int count = 0; public Iter() { index = front; } @Override public boolean hasNext() { return count < size; } .... • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface The constructor initializes index to front when a new Iter object is created

  46. Implementing Class ArrayQueue<E>.Iter(cont.) private class Iter implements Iterator<E> { private int index; private int count = 0; public Iter() { index = front; } @Override public boolean hasNext() { return count < size; } .... • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface countkeeps track of the number of items accessed so far

  47. Implementing Class ArrayQueue<E>.Iter(cont.) private class Iter implements Iterator<E> { private int index; private int count = 0; public Iter() { index = front; } @Override public boolean hasNext() { return count < size; } .... • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface hasNext() returns true if count is less than size

  48. Implementing Class ArrayQueue<E>.Iter(cont.) @Override public E next() { if (!hasNext()) { throw newNoSuchElementException(); } E returnValue = theData[index]; index = (index + 1) % capacity; count+; return returnValue; } @Override public void remove { throw newUnsupportedOperationException(); } } • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface next() returns the element at position index and increments Iter's fields index and count

  49. Implementing Class ArrayQueue<E>.Iter(cont.) @Override public E next() { if (!hasNext()) { throw newNoSuchElementException(); } E returnValue = theData[index]; index = (index + 1) % capacity; count+; return returnValue; } @Override public void remove { throw newUnsupportedOperationException(); } } • Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queueinterface remove() throws an exception because removing an item other than the first item violates the queue's contract

  50. Time comparison • Computation time • All three implementations are comparable in terms of computation time • All operations are O(1) regardless of implementation • Although reallocating an array is O(n), it is amortized over n items, so the cost per item is O(1)

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