The **`kotlin.collections` package** is part of Kotlin's standard library. It contains all collection types, including `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 `addAll()` method
The `ArrayList` class contains the **`addAll()` method**, which adds all elements of the specified input collection to the end of the array list.

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

## Arguments 
This method takes a collection of elements as input.

## Return value
This method returns the boolean value `true` if the list is modified during the `addAll()` operation. Otherwise, it returns `false`.

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

- It is allowed to contain duplicate elements.

- The elements are appended in the order they appear in the specified collection.

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

## Code
The ```ArrayList``` class is present in the ```kotlin.collection``` package and is imported by default in every Kotlin program.

fun main(args : Array<String>) {
    val countryList = ArrayList<String>()
    
    countryList.add("India")
    countryList.add("US")   
    
    println("Printing ArrayList elements--")  
    println(countryList)  

    val islandList = ArrayList<String>() 

    islandList.add("Maldives")
    islandList.add("Mauritius")

    countryList.addAll(islandList)

    println("Printing ArrayList elements after calling addAll()--")  
    println(countryList)  
}

##   
### Explanation
- First, we create an empty ```ArrayList``` to store the strings.

- Next, we add a few country names to the ```ArrayList``` object using the ```add()``` method, such as: ```"India"```and  ```"US"```.
- Next, we create another array list of strings and add few island names to the new ```ArrayList``` object using the ```add()``` method, such as: ```"Maldives"```and  ```"Mauritius"```.
- Next, we call the  ```addAll()``` method by passing the island names array list as the input parameter. It then adds all the elements of the island names array list to the country name array list.
- The `Arraylist` elements are displayed before and after calling the ```addAll()``` method by using the ```print()``` function of the ```kotlin.io``` package. The list contains four names after calling the ```addAll()``` method.



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

The addAll() method in Kotlin's ArrayList adds all elements from a specified collection to the end of the list, modifying it.