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 and uses a dynamic resizable array as the backing storage.

#    
## The `indexOf()` method
The `ArrayList` class contains the **`indexOf()` method**, which returns the index of the first occurrence of a specified element in the array list, or `-1`, if the element is not contained in the list.

##    
##  Syntax
```
fun indexOf(element: E): Int
```

##   
### Arguments 
- This method takes the element to search as the input.

###   
### Return value
- It returns the integer index of the element in the array list, or `-1` if the input element is not present in the list.

## 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 `indexOf()` 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 fruitList = ArrayList<String>()
    
    fruitList.add("Orange")
    fruitList.add("Apple")  
    fruitList.add("Grapes")  
    fruitList.add("Banana")  
    
    println("Printing ArrayList elements--")  
    println(fruitList)  

    println("Index of 'Apple' in array list using indexOf() : ${fruitList.indexOf("Apple")}")
    println("Index of 'Apple' in array list using indexOf() : ${fruitList.indexOf("Banana")}")
    println("Index of 'Strawberry' in array list using indexOf() : ${fruitList.indexOf("Strawberry")}")
}

## 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 call the  ```indexOf()``` method by passing `Apple` as input, and it returns `1`.
- We call the  ```indexOf()``` method by passing `Banana` as input, and it returns `3`.
- Next, we call the  ```indexOf()``` method by passing `Strawberry` as input, and it returns `-1`, as `Strawberry` is not present in the array list.
- The `Arraylist` elements and result of the `indexOf()` method are displayed using the ```print()``` function of the ```kotlin.io``` package.



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

The indexOf() method in Kotlin's ArrayList returns the index of the first occurrence of an element or -1 if not found.