COSC 1P03 Data Structures and Abstraction 18.1

COSC 1P03
Data Structures and Abstraction
COSC 1P03
The Queue
Data Structures and Abstraction 18.1
COSC 1P03
Queue
a list (initially empty) of items (of some type) to which items may
be added at one end (called the rear) and from which items may
be removed at the other end (called the front)
examples
waiting lines
print queues
behaviour
FIFO ordering
error conditions:
underflow
overflow
Data Structures and Abstraction 18.2
Figure 18.1 Bank waiting line
18.1

COSC 1P03
Data Structures and Abstraction
COSC 1P03
Queue Interface
generic
E – items to be stored
operations:
enter (enqueue, add, insert)
leave e (dequeue, remove, delete)
front (head, first)
length (count, size)
empty
exceptions
NoItemException
NoSpaceException
Data Structures and Abstraction 18.4
COSC 1P03
Queue ADT
Contiguous Implementation
based on variable-sized array
two indices: front & rear
add at rear, remove at front
queue moves towards rear
repositioning on delete: O(n)
circular array
at end of array reuse front
index modulo array size
Data Structures and Abstraction 18.5
front
rear
front
rear
0 1 2 3 4
0 1 2 3
45 6 7
(a)
(b)
front
rear
rear
front
0 1 2 3
0 1 2 3
(c)
Figure 18.3 Contiguous queue representation
(d)
18.2

COSC 1P03
Data Structures and Abstraction
COSC 1P03
implementation
instance variables
count
constructors
empty state
methods
enter
overflow
increment
leave, front
underflow
length, empty
compute?
empty vs full
Data Structures and Abstraction 18.7
COSC 1P03
Queue ADT
Linked Implementation
sequentially-linked structure of items
deletion from front
insertion at end
keep pointer to rear O(1)
length?
keep count else O(n)
comparison with contiguous
all operations O(1)
space tradeoffs
Data Structures and Abstraction 18.8
front
rear
Figure 18.5 Linked queue representation
18.3

COSC 1P03
Data Structures and Abstraction
COSC 1P03
Java Collections Framework
Queue interface
generic in element type (E)
some duplication of methods (Collection)
standard queue methods
add = enter
remove = leave
element = front
size = length
isEmpty = empty
LinkedList class
generic in element type (E)
implements Queue as symmetrically-linked structure
also implements List
Data Structures and Abstraction 18.10
package java.util;
public interface Queue < E > extends Collection {
public boolean add ( E o ); // from Collection
public void clear ( ); // from Collection
public boolean contains ( Object o ); // from Collection
public E element ( );
public boolean equals ( Object o ); // from Collection
public boolean isEmpty ( ); // from Collection
public Iterator iterator ( ); // from Collection
public boolean offer ( E o );
public E peek ( );
public E poll ( );
public E remove ( );
public boolean remove ( Object o ); // from Collection
public int size ( ); // from Collection
public Object[] toArray ( ); // from Collection
} // Queue
18.4