The `dart:collection` library provides advanced collection support for the Dart language. The library contains the **`HashSet<E>`** class, an unordered hash-table-based implementation of the abstract `Set<E>` class.

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## The `addAll()` method
The `HashSet<E>` class contains the **`addAll()`** method, which adds all elements of the specified input collection to the HashSet.


>**`HashSet<E>`** is a collection of unique elements that provides constant-time `add`, `remove`, and `contains` operations. `HashSet<E>` does not have a specified iteration order. However, multiple iterations on the set produce the same element order.


The figure below illustrates how the `addAll()` method works.


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## Syntax
The syntax of the `addAll()` method is as follows:
```
void addAll(Iterable<E> elements)
```
## Parameters
The `addAll()` method takes elements as input and adds them to the HashSet.


## Return value
This method does not return a value.

## Code
The code below shows how the `addAll()` method works in Dart.

import 'dart:collection'; 
void main() { 
   var monthSet = HashSet<String>();   
   monthSet.add("January"); 
   print('Hash Set Elements : $monthSet');
   List<String> nextMonths = ["February", "March","April", "May"];
   print('Input List Elements : $nextMonths');
   var result = monthSet.addAll(nextMonths);
   print('Hash Set Elements : $monthSet');  
}  

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## Explanation
- We create an instance of a `HashSet` class of type `String`.
- Next, we use the `add()` method and add `"January"` to the HashSet.
- We create a list of strings and add `"February"`, `"March"`, `"April"`, and `"May"` to it. 
- We call the `addAll()` method and pass the previous list as input. The `addAll()` method adds all elements of the list to the HashSet.

- We use the `print()` function of the core library to display the set elements and the result of the `addAll()` method.

What is HashSet.addAll() in Dart?

The addAll() method in Dart's HashSet adds all elements from a specified collection to the HashSet.