kotlin

“Kotlin Quick Reference” book

Somewhere around a year ago I started working on a Kotlin programming book, but then I had to get away from it to work on other things. When I got back to it recently I looked around and felt like the world didn’t need another “Introduction to Kotlin” book — there are a couple of good ones out there, including Kotlin in Action, and the kotlinlang.org documentation is excellent — so I decided to ditch the project completely.

Kotlin: A for loop that counts up to some maximum integer value

As a brief note to self, I was just converting some Java code to Kotlin, and the correct way to convert this Java for loop that uses i as a counter:

for (int i=0; i<tabLayout.tabCount; i++) { ...

is with this Kotlin for loop:

for (i in 0 until tabLayout.tabCount) { ...

The key there for me is the 0 until part of the syntax.

Android/Kotlin: A FloatingActionButton setOnClickListener Snackbar example

As a quick note to self, this is an example of how to implement a setOnClickListener on a FloatingActionButton with Android and Kotlin:

fab.setOnClickListener { view ->
    Snackbar.make(
        view,
        "An exciting message!",
        Snackbar.LENGTH_LONG
    ).setAction("Action", null).show()
}

Hopefully that’s enough to get “future me” pointed in the right direction when I need this again. :)

(There are more lambda details at this link.)

How to generate a small random number in Kotlin

If you need to generate a small random number in Kotlin, I find that this approach works:

val r = (1..10).shuffled().first()

That code generates a single random number in the range from 1 to 10 (including 1 on the low end and 10 on the high end).

You can use the technique to simulate the rolling of a dice (die):

val r = (1..6).shuffled().first()

Or simulate flipping a coin (0=heads, 1=tails, or vice versa):

val r = (1..2).shuffled().first()

This also works for the coin flip simulation:

Pulled a Steve Jobs and canceled my own project

I’ve been working on a Kotlin book on and off for the past few months, and this morning I pulled a Steve Jobs on myself and canceled the project, even though it’s about 75% complete (by chapter count).

The problem with the book is that at this point it doesn’t contain anything unique, although arguably my way of explaining things might be better than other approaches. Unlike the Scala Cookbook, which provides solutions to common Scala problems, and Functional Programming, Simplified, which provides a unique approach to explaining functional programming in Scala, I don’t feel like there’s anything new here.

So, in short, without getting into the details of what’s next, the “vision guy” part of me decided that there are better things to do with my time. (And if you’ve ever been on a project that was canceled and you thought it was hard to take, imagine canceling your own project.)

Kotlin collections methods: examples and syntax

As a quick note today, if you ever need some examples of how the Kotlin collections methods work, I hope these examples are helpful.

Sample data

First, here’s some sample data:

val a = listOf(10, 20, 30, 40, 10)
val names = listOf("joel", "ed", "chris", "maurice")

The Kotlin forEach println syntax

It’s a little hard to move back and forth between Scala and Kotlin because of some of the differences between the languages. Skipping the long story, here’s an example of how to print every line in a list of strings in Kotlin using forEach and println. First the setup:

import java.io.File
fun readFile(filename: String): List<String> = File(filename).readLines()
val lines = readFile("/etc/passwd")

Then here are two different ways to use forEach with println: