1. COMPUTER 2430 Object Oriented Programming and Data Structures I

2. Queue Container Normally fixed size (could grow if needed)
FIFO: First In First Out
Example
Mongolian Grill
Printing jobs
Network routing
. . .

3. Queue Implementation An array Elements are added at the end
Elements are removed from the front

4. Queue Empty queue Add item1 Item1 Add item2 Item1 Item2 Add item3
Remove the first one
Item2
Item3
Add item4
Item2
Item3
Item4

5. First In First Out (FIFO) In at one end and Out at the other end
Queue
First In First Out (FIFO) In at one end and Out at the other end
Out In

6. Public Queue Methods add (at the end) remove (from the front) isEmpty
isFull

7. Queue Implementation I
Add is easy Remove has to move all remaining elements
Remove is always from slot 0
count also indicates where next element goes

8. Queue Implementation II
Add at the end and moves rear
Remove at the front and moves pointer front
Rear is not count any more!
front points to where the first element is
rear points to where next element goes

9. Queue Implementation II
Very efficient Need to keep count Trouble when hit the end of array
front points to where the first element is
rear points to where next element goes

10. Queue Implementation III
Make the array circular Need to keep count
front points to where the first element is
rear points to where next element goes

11. front
rear
count: 0
Add . . .
front
rear
count: 7
Remove
front
rear
count: 6
Add &&
Remove
front
rear
count: 8
Add
rear
front
count: 9

12. rear
front
count: 9
Add
rear
front
count: 10
Remove
rear
front
count: 9
Add &&
Remove
rear
front
count: 6
Remove
front
rear
count: 5

13. Full: count == items.length Empty: count == 0
Add &&
Remove
front
rear
count: 8
Add . . .
front
rear
count: 12
Add
rear
front
count: 13
Add
front
rear
count: 14
Full: count == items.length
Empty: count == 0

14. Add Check isFull first or Throw Exception items[rear ++] = obj;
front
rear
Add
Check isFull first or
Throw Exception
front
rear
rear
front
rear
front
items[rear ++] = obj;
if (rear == items.length)
rear = 0;
count ++;
items[rear] = obj;
rear = (rear + 1) % items.length;
count ++;

15. Remove Check isEmpty first or Throw Exception obj = items[front ++];
rear
front
rear
front
front
rear
front
rear
obj = items[front ++];
if (front == items.length)
front = 0;
count --;
obj = items[front];
front = (front + 1) % items.length;
count --;

16. public class Queue {
private Object[] items;
private int front, rear, count;
public Queue( int size )
items = new Object[size];
front = rear = count = 0; }
public boolean isEmpty()
return count == 0;
public boolean isFull ()
return count == items.length;
...
} // class Queue

17. public class Queue {
private Object[] items;
private int front, rear, count;
...
// what if queue is full?
public void add ( Object x )
items[rear] = x;
rear = (rear + 1) % items.length;
count ++; }
// what if queue is empty?
public Object remove()
Object x = items[front];
front = (front + 1) % items.length;
count --;
return x;
} // class Queue

18. // Generic Queue class
public class Queue <E> {
private E[] items;
private int front, rear, count;
// Java warning possible and it’s OK
public Queue( int size )
items = (E[]) new Object[size];
front = rear = count = 0; }
. . .

19. Due October 31, at 11 pm No Grace Date!
Lab 8
Due October 31, at 11 pm
No Grace Date!

20. Schedule Lab 7 (5points) Prog4 (20 points) Quiz 4 Test 2
Due 11 pm, November 2
Prog4 (20 points)
Due 11 pm, November 9
Quiz 4
Monday, Nov 5
Test 2
Wednesday, Nov 14