Lesson 6: Queue Data Structure

A queue is a linear structure that follows First-In, First-Out (FIFO) order. The first item added is the first item removed.

A queue uses two ends:

End	Purpose
Rear	New items are inserted here.
Front	Items are removed here.

A real-world example is a line of people waiting for service. The person who arrives first is served first.

Operation	Description
`enqueue`	Adds an item to the rear of the queue.
`dequeue`	Removes an item from the front of the queue.
`peek`	Views the front item without removing it.
`isEmpty`	Checks if the queue has no items.

Method	Description
`add(e)`	Inserts an element and may throw an exception if insertion fails.
`offer(e)`	Inserts an element and returns whether insertion succeeded.
`element()`	Returns the front item but may throw an exception if empty.
`peek()`	Returns the front item or `null` if empty.
`remove()`	Removes and returns the front item; may throw an exception if empty.
`poll()`	Removes and returns the front item or `null` if empty.

Queues may be implemented using arrays, linked lists, pointers, or built-in collection classes.

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End

Purpose

Rear

New items are inserted here.

Front

Items are removed here.

Operation

Description

enqueue

Adds an item to the rear of the queue.

dequeue

Removes an item from the front of the queue.

peek

Views the front item without removing it.

isEmpty

Checks if the queue has no items.

Method

Description

add(e)

Inserts an element and may throw an exception if insertion fails.

offer(e)

Inserts an element and returns whether insertion succeeded.

element()

Returns the front item but may throw an exception if empty.

peek()

Returns the front item or null if empty.

remove()

Removes and returns the front item; may throw an exception if empty.

poll()

Removes and returns the front item or null if empty.