# Server side Kotlin-04

Introduction:-  In the last post of this series, I discussed the Kotlin Collection API. In this post, I wil share how the Map APIs work.   
Map   
The mapping transformation creates a new collection from  another collection.   lambda function is applied to each subsequent element and returns the list of the lambda results. The order of results is the same.   
Listing 1 
```
val numbers = setOf(4, 5, 6)
println(numbers.map { it * 4 })
```
output
[16, 20, 24]   
    
. mapIndexed() should be used for element index   

listing 2    
``` 
val numbers = setOf(4, 5, 6)
println(numbers.mapIndexed { idx, value -> value * idx })     
```  
output   
[0, 5, 12]    
Listing 3  filter out the nulls from the result collection by calling the mapNotNull().   
Listing 3   
```   
val numbers = setOf(4, 5, 6 )
println(numbers.mapNotNull { if ( it == 5) null else it * 4 })
 ```   
output   
[16, 24]    
Listing 4 applies on IndexedNotNull    
```    
val numbers = setOf(4, 5, 6 )
println(numbers.mapIndexedNotNull { idx, value -> if (idx == 0) null else value * idx })    
```   
output   
[5, 12]     
 Listing 5 shows that transformation can also be applied on keys, use mapKeys(); and , mapValues() transforms values.   
Listing 5  
```  
val numbersMap = mapOf("key11" to 11, "key22" to 22, "key33" to 33)
println(numbersMap.mapKeys { it.key.uppercase() })
println(numbersMap.mapValues { it.value + it.key.length })   
```   
output  
{KEY11=11, KEY22=22, KEY33=33}   

{key11=16, key22=27, key33=38}    
   
Conclusion
You can see how this Map works. In the next post, I will explore the zip APIs.