Java to Kotlin

Chapter 13. Streams to Iterables to Sequences

Java and Kotlin both allow us to transform and reduce collections. They have different design goals and implementations, though. What does Kotlin use instead of Java streams, when should we convert, and how?

Java Streams

Java 8 introduced streams in 2014, making good use of the new lambdas. Say we want to work out the average length of some strings, except that blank strings (those with only whitespace characters) should be treated as if they are empty. Previously we might have written:

public static double averageNonBlankLength(List<String> strings) {
    var sum = 0;
    for (var s : strings) {
        if (!s.isBlank())
            sum += s.length();
    }
    return sum / (double) strings.size();
}

With Java streams, we can express this algorithm as filter, map, and reduce by first converting the List to a Stream and applying transformations:

public static double averageNonBlankLength(List<String> strings) {
    return strings
        .stream()
        .filter(s -> !s.isBlank())
        .mapToInt(String::length)
        .sum()
        / (double) strings.size();
}

Rather than having to run the for-loop in our heads to see what this code is doing, we can see the steps of the algorithm declared line by line and rely on the runtime to implement those steps for us.

If we are really in a hurry for those results, we can even write:

public static double averageNonBlankLength(List<String> strings) {
    return strings
        .parallelStream() 
        .filter(s -> !s.isBlank())
        .mapToInt(String::length)
        .sum()
        / (double) strings ...