The **```kotlin.collections``` package** is part of Kotlin's standard library, and it contains all collection types, such as ```Map```, ```List```, ```Set```, etc.

The package provides the **```ArrayList``` class**, which is a mutable list that uses a dynamic resizable array as the backing storage.

#    
## The `retainAll()` method
The `ArrayList` class contains the **`retainAll()` method**, which retains only the elements which are present in the specified input collection and removes all other elements from the array list.

##    
##  Syntax
```
fun retainAll(elements: Collection<E>): Boolean
```

### Arguments 
- This method takes a collection of elements as input.
### Return value
- This method returns `true` if any of the element is removed in the operation or `false` if the array list is not modified.

###   
### Things to note
- `ArrayList` follows the sequence of insertion of elements.

- It is allowed to contain duplicate elements.

The illustration below shows the function of the `retainAll()` method.

fun main(args : Array<String>) {
    val fruitList = ArrayList<String>()
    
    fruitList.add("Orange")
    fruitList.add("Apple")  
    fruitList.add("Grapes")  
    fruitList.add("Banana")      
    println("Printing ArrayList elements --")  
    println(fruitList)  

    var retainList = ArrayList<String>()
    retainList.add("Apple")
    retainList.add("Grapes")
    fruitList.retainAll(retainList)
    println("Printing ArrayList elements after calling retainAll() --")  
    println(fruitList)  

   
}

## Explanation
- First, we create an empty array list named `fruitList` to store the strings.

- Next, we add a few fruit names to the ```ArrayList``` object using the ```add()``` method, such as: ```"Orange"```, ```"Apple"```, ```"Grapes"```, and  ```"Banana"```.
- Next, we create a new list `retainList` and add two fruit names: ```"Apple"``` and ```"Grapes"```, using the  ```add()``` method.
- Next, we call the  ```retainAll()``` method by passing `retainList` as input. It removes all elements of the array list except the elements present in the `retainList`.
- The `Arraylist` elements and result of the `retainAll()` method are displayed using the ```print()``` function of the ```kotlin.io``` package.

What is the ArrayList.retainAll() method in Kotlin?

The kotlin.collections package is part of Kotlin s standard library, and it contains all collection types, such as Map, List, Set, etc. The package provides the ArrayList class, which is a mutable list that uses a dynamic resizable array as the backing storage.  The retainAll() method The ArrayList class contains the retainAll() method, which retains only the elements which are present in the specified input collection and removes all other elements from the array list.  Syntax fun retainAll(elements: Collection): Boolean  Arguments  This method takes a collection of elements as input.  Return value  This method returns true if any of the element is removed in the operation or false if the array list is not modified.   Things to note   ArrayList follows the sequence of insertion of elements.   It is allowed to contain duplicate elements.   The illustration below shows the function of the retainAll() method. 