Deque

A Deque extends the Queue interface and is a double-ended queue. It provides methods to add, access, and remove items at the top and end of the deque. Thereby, it can be used as a queue or stack. The Deque interface is included in the java.util package.

Syntax

import java.util.deque;

Deque<DataType> myDeque = new DequeClass<DataType>();

Where DataType is the data type to be stored in the deque, and DequeClass is a class implementing the Deque interface, for example, ArrayDeque or LinkedList.

Example

The following is an example of the Deque interface implemented by an ArrayDeque:

import java.util.Deque;
import java.util.ArrayDeque;

public class Main {
  public static void main(String[] args) {
    Deque<String> food = new ArrayDeque<String>();
    food.addFirst("Cabbage");
    food.addLast("Sausage");
    food.addFirst("Potatoes");
    food.addLast("Salad");
    System.out.println(food);
    System.out.println(food.pollFirst());
    System.out.println(food.peekFirst());
    System.out.println(food);
  }
}

The above example results in the following output:

[Potatoes, Cabbage, Sausage, Salad]
Potatoes
Cabbage
[Cabbage, Sausage, Salad]

Methods

The following list contains a selection of methods provided by the Deque interface:

• .addFirst(item): Adds item at the top of the Deque if possible, otherwise it throws an exception.
• .addLast(item): Adds item at the end of the Deque if possible, otherwise it throws an exception.
• .getFirst(): Returns, without removal, the first item of the Deque. It throws an exception if the Deque is empty.
• .offerLast(item): Adds item at the end of the Deque if possible, otherwise it returns false.
• .peekFirst(): Returns, without removal, the first element of the Deque. Returns null if the Deque is empty.
• .pollFirst(): Returns and removes the first item of the Deque. Returns null if the Deque is empty.
• .removeFirst(): Returns and removes the first item of the Deque. Throws an exception if the Deque is empty.

Equivalent methods Stack and Queue

The following table lists the equivalent methods for Queue and Stack: