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 where() method
The `HashSet<E>` class provides the  **`where()` method**, which returns all the elements of the HashSet that satisfy the specified input condition.
>`HashSet<E>` is a collection of unique elements that provide 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 `where()` method works.

## Syntax
The syntax of the `where()` method is as follows:
```
Iterable<E> where(bool test(E element))
```
## Parameters
The `where()` method takes the predicate `test` as input. It then checks every element of the HashSet against this predicate.

## Return value
`where()` returns a lazy iterable of all HashSet elements that satisfy the input predicate in the same order as in the HashSet.

As long as the returned iterable is not iterated over, the input predicate ```test``` will not be invoked. 

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


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

  var result = monthSet.where((e)=>e.length==5);
  print('Does set contains month names of length = 5 : $result');
   
}  

## Explanation
* **Line 3**: We create an instance of a `HashSet` class of type `String`.

* **Lines 5-9**: We use the `add()` method to add a few month names to the HashSet: `"January"`, `"February"`, `"March"`,  `"April"`, and `"May"`.

* **Line 13**: We call the `where()` method with a predicate that `returns` true if the string's length is equal to five. The method returns an iterable with two elements: `"March"` and `"April"`. 

* **Line 14**: We use the `print()` function of the core library to display the set elements and the result of the `where()` method.

What is HashSet.where() in Dart?

The Dart HashSet's where() method returns a lazy iterable of elements that satisfy the given predicate.