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

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## The ```contains()``` method
The ```DoubleLinkedQueue<E>``` class contains the **`contains()` method** which checks the presence of an element in the queue.

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

The `contains()` method does not change the state of the queue.

The illustration below shows how the `contains()` method works.

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## Syntax

The prototype of the `contains()` method is shown below.

```
bool contains(
Object? element
)
```

## Arguments

The `contains()` method takes the object to search in the queue as the argument.

## Return values
- The `contains()` method returns a Boolean value. It returns `true` if the input element is present in the queue and `false` otherwise.
- The `contains()` method is a *O(n)* operation where *n* is the number of the elements in the queue.

## Code

The code below shows how the `contains()` method can be used in Dart.


import 'dart:collection'; 
void main() { 
   var faangCompanies = DoubleLinkedQueue<String>();
   
   faangCompanies.add("Facebook"); 
   faangCompanies.add("Apple"); 
   faangCompanies.add("Amazon");   
   faangCompanies.add("Netflix");   
   faangCompanies.add("Google");   
   print('Printing Queue Elements');
   print(faangCompanies); 

   bool isGooglePresent = faangCompanies.contains("Google");
   print('Checking presence of "Google" using contains(): ${isGooglePresent}');

   bool isMicrosoftPresent = faangCompanies.contains("Microsoft");
   print('Checking presence of "Microsoft" using contains(): ${isMicrosoftPresent}');


}  

## Explanation

The code above performs the following actions:

- An instance of the ```DoubleLinkedQueue``` class of type ```String``` is initialized in line 3.

- Next, a few strings, ```"Facebook"```, ```"Apple"```, ```"Amazon"```, ```"Netflix"```, and ```"Google"```,  are added to the queue using the ```add()``` method. 
- The queue elements are displayed as `{Facebook, Apple, Amazon, Netflix, Google}`. All the elements are added to the end of the queue, as the queue is a FIFO data structure.
- The ```contains()``` method in line 13 checks for the presence of ```"Google"``` in the queue and stores the boolean result in a variable. It is ```true``` because ```"Google"``` is present in the queue and is printed in the next line.
- The ```contains()``` method in line 16 checks for the presence of ```"Microsoft"``` in the queue and stores the boolean result in a variable. It is ```false``` because ```"Microsoft"``` is not present in the queue and is printed in the next line.

What is the DoubleLinkedQueue.contains() method in Dart?

The `contains()` method in `DoubleLinkedQueue<E>` checks for an element's presence without altering the queue, returning true if found.

What is the DoubleLinkedQueue.contains() method in Dart?

The contains() method

Syntax

Arguments

Return values

Code

Explanation

The `contains()` method