For Loops: Java vs Kotlin

A very quick for-loop variations in Java and Kotlin!

Loops are one of the most basic building block of any programming language. Irrespective of your language of choice, for loops or a variant of it is sure to exist. In that spirit, today I would like to do a side by side comparison of what for loops look like in two very popular JVM languages: Java and Kotlin.

Accessing elements directly

As an example let us consider a list of numbers that we need to iterate over and print.

In Java:

List<Integer> numbers = new ArrayList(){{
  add(3);
  add(6); 
  add(9); 
  add(12);
}};for(Integer number: numbers) {
  System.out.println(number);
}

The Kotlin version of the code would be (dropping the type decleration as a part of the for loop):

val numbers = listOf(3, 6, 9, 12)for(number in numbers) {
  println(number)
}

Output:

3
6
9
12

Accessing elements using the index

Having to access elements in a collection like an array or a list is probably programming 101.

In good old Java that would be:

List<Integer> numbers = new ArrayList(){{
    add(3);
    add(6);
    add(9);
    add(12);
}};for(int index=0; index<numbers.size(); index++) {
    System.out.println(numbers.get(index));
}

Kotlin has support for ranges that we could for the index (note that both the bounds of the for loop are inclusive):

val numbers = listOf(3, 6, 9, 12)for(index in 0..numbers.size-1) {
  println(numbers[index])
}

Output:

3
6
9
12

Accessing elements using the index in reverse

If we wanted to access element of the list in reverse order, we would do the following in Java:

List<Integer> numbers = new ArrayList(){{
    add(3);
    add(6);
    add(9);
    add(12);
}};for(int index=numbers.size()-1; index>=0; index--) {
    System.out.println(numbers.get(index));
}

In Kotlin:

val numbers = listOf(3, 6, 9, 12)for(index in numbers.size-1 downTo 0) {
  println(numbers[index])
}

Output:

12
9
6
3

Accessing elements using the index with custom increment

And finally if we are looking for a non-1 increment or decrement (using increments of 2 as an example here):

Java version:

List<Integer> numbers = new ArrayList(){{
  add(3);
  add(6); 
  add(9); 
  add(12);
}};for(int index=0; index<numbers.size(); index=index+2) {
  System.out.println(numbers.get(index));
}

Kotlin version:

val numbers = listOf(3, 6, 9, 12)for(index in 0..numbers.size-1 step 2) {
  println(numbers[index])
}

Output:

3
9

And that is a wrap! I hope this helps with being able to fluently switch between writing for loops in Java and Kotlin!