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

## The **`followedBy()`** method
The `HashSet<E>` class provides  **`followedBy()` method**, which takes an Iterable as input and lazily concatenates the elements of the input to the hashset.

>**Note**: **`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 `followedBy()` method works.

## Syntax
The syntax of the `followedBy()` method is as follows:
```
Iterable<E> followedBy(Iterable<E> other)
```
## Parameters
* **`other`**: The `followedBy()` method takes an Iterable, `other`, as input and adds the `other` elements after the HashSet elements in the same order.

## Return value
* **`Iterable`**: `followedBy()` returns a lazy Iterable with the concatenation of HashSet elements and the input iterable.

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


import 'dart:collection'; 
void main() { 
   var firstMonthSet = HashSet<String>();   
   firstMonthSet.add("January"); 
   firstMonthSet.add("February"); 
   firstMonthSet.add("March");
   print('Printing HashSet Elements');
   print(firstMonthSet); 
   
  var otherMonthSet = HashSet<String>(); 
   otherMonthSet.add("April");
   otherMonthSet.add("May");
   print('Printing Other Set Elements');
   print(otherMonthSet); 

  var result = firstMonthSet.followedBy(otherMonthSet);
  print('Concatenated Set using followedBy():  $result');
   
}  

## Code explanation
In the above code, we see the following:
- Line 3: We create an instance of a `HashSet` class of type `String`.
- Lines 4–6: Next, we use the `add()` method to add a few month names to the HashSet: `"January"`, `"February"`, `"March"`.
- Lines 10–12: We create another HashSet and add the names of two months to it.
- Line 16: We call the `followedBy()` on the HashSet passing the second HashSet as input. It returns an Iterable with five elements.
- Line 17: We use the `print()` function of the core library to display the set elements and the result of the `followedBy()` method.

What is the HashSet.followedBy() method in Dart?

The followedBy() method in Dart's HashSet class lazily concatenates elements of an input Iterable to the hashset.