How to add an element to a DoubleLinkedQueue in Dart

The **`dart:collection` library** provides advanced collection support for the Dart language. The library contains the **`DoubleLinkedQueue<E>`** class, which uses a doubly-linked list to implement the abstract `Queue<E>` class.

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## The `addFirst()` method
The ```DoubleLinkedQueue<E>``` class contains the **`addFirst()` method** which adds an element to the start of a queue.

>A **queue** is a *FIFO (First In First Out)* data structure.  In a queue, the element that is added first will be deleted first. 

The figure below illustrates how the `addFirst()` method works:

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## Syntax

The syntax of the `addFirst()` method is as follows:

```
void addFirst(E value)
```

## Parameters
- The `addFirst` method takes a value of type ```E``` as input and adds it to the beginning of the queue.

## Return value
`addFirst` does not return anything and it is a constant-time operation.

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## Code

The code below shows how the `addFirst()` method works in Dart:

import 'dart:collection'; 
void main() { 
   var monthQueue = DoubleLinkedQueue<String>();
   
   monthQueue.add("February"); 
   monthQueue.add("March"); 
   monthQueue.add("April");   
   
   print('Printing Queue Elements');
   print(monthQueue); 

   monthQueue.addFirst("January");
   print('Printing Queue Elements');
   print(monthQueue); 
   
}  

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## Explanation
- We create an instance of a ```DoubleLinkedQueue``` class of type ```String```.
- Next we use the `add()` method to add a few strings to the queue: ```"February"```, ```"March"```, and ```"April"```.
- Notice that all of the elements are added to the end of the queue, as the queue is a FIFO data structure.
- We now add ```"January"``` to the beginning of the queue with the ```addFirst()``` method.
- We use the `print()` function of the `code` library to display the queue elements, and it can be observed that the queue contains ```"January"``` at the start.

What is DoubleLinkedQueue.addFirst() in Dart?

DoubleLinkedQueue<E>'s addFirst() in Dart adds an element to the start of a FIFO queue, taking constant time.

What is DoubleLinkedQueue.addFirst() in Dart?

The addFirst() method

Syntax

Parameters

Return value

Code

Explanation

The `addFirst()` method