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?

ArrayList's retainAll() method in Kotlin removes elements not in a specified collection, returns true if elements are removed, or false otherwise.

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

The retainAll() method

Syntax

Arguments

Return value

Things to note

Explanation

The `retainAll()` method