Queue

A Queue is an interface provided in the java.util package that extends collections to provide additional methods to access or manipulate items at the head of the queue. Where the “head” of the queue is defined by the particular implementation.

Queue elements are commonly accessed in a FIFO (first-in-first-out) manner. In a priority queue implementation, the first item out will be defined by its specified priority.

Syntax

import java.util.Queue;

Queue<DataType> q = new QueueClass<DataType>();

Where DataType is the data type to be stored in the queue, and QueueClass is some class implementing the Queue interface.

Methods

The Queue interface utilizes the following methods:

• .add(item): adds item to the Queue if possible, otherwise it throws an exception.
• .offer(item): adds item to the Queue if possible, otherwise it returns false.
• .remove(): removes and returns the head item of the Queue, throwing an exception when the Queue is empty.
• .poll(): removes and returns the head item of the Queue, returning null if the Queue is empty.
• .element(): returns the head of the Queue without removing it, throwing an exception when the Queue is empty.
• .peek(): returns the head of the Queue without removing it, returning null if the Queue is empty.

Example

This is an example of the Queue interface implemented by a LinkedList:

import java.util.LinkedList;
import java.util.Queue;

// Main.java
public class Main {
  public static void main(String[] args) {
    Queue<String> food = new LinkedList<String>();
    food.offer("Cabbage");
    food.offer("Pizza");
    food.offer("Sausage");
    food.offer("Potatoes");
    food.offer("Salad");
    System.out.println(food.peek());
    System.out.println(food.poll());
    System.out.println(food);
  }
}

This will output the following:

Cabbage
Cabbage
[Pizza, Sausage, Potatoes, Salad]