Computer programs are for executing operations using input data to produce output data, sometimes by manipulating data taken from a database during that operation. The word database here is used in the most general sense: It could be a file, some memory storage, or a full-fledged database product.

Many different programming languages exist nowadays, each with its own merits and drawbacks. Some of them aim at execution stability, some at high performance, some are tailored to solve specific tasks, and some exist only because a company wants to establish a strong market position. Looking at the way programming languages have developed over time is an interesting subject in and of itself, and it has implications for various aspects of information technology. One could write a separate book about that, but for this book I simply want to stress one important fact about computer language development, which I think has a direct effect on the way modern computer programs are written. If you are looking at the historical development of computer languages, you will notice a substantial change in the abstraction level the languages exhibit. Whereas in the infancy of the industry a programmer needed to have a fairly good knowledge of computer hardware, now different levels of abstraction have been introduced into the languages, meaning an increased conceptional and linguistic distance from hardware features. This has increasingly alleviated the requirement that software developers know what is occurring in a computer’s central processing unit (CPU).

This already looks more expressive compared to the first version, and a lot of current programming languages allow for this kind of programming style. We can do even better, though: You can see the definition of the list being written in one line, separated from the list processing in the loop. There’s nothing preventing us from writing a lot of overly complex code between the list definition and the loop, and this is what you see quite often, making the program hard to read and understand. Wouldn’t it be better to have it all in one statement? Using a more expressive snippet allows us to write such a combined statement. In pseudo-code, it could look like this:

This is about the maximum of expressiveness you can get, if you see the dot “ . ” as some kind of “do something with it” command and “{ … }” as a block of code doing something, with the identifier in front of the -> in this case designating a loop variable.

The programming language Kotlin is capable of getting us to such an extent of expressiveness, and in this book I want to introduce Kotlin as a programming language for Android that allows you to accomplish things in an expressive and concise way. As a matter of fact, in Kotlin the little looping example, with a filter added, reads:

If you run this, it will print the text Green Gray on two lines of the console. With the notion of parameters being placed inside round brackets, you should be able to understand this snippet without knowing a single Kotlin idiom.

Once you reach the end of the book, you should be an advanced developer able to address problems in the Kotlin language, with particular attention on Android matters. Of course, you will not know all possible libraries that are out there in the wild for solving specific problems, as only experience will help you there. Knowing most of the language constructs and having good ideas concerning programming techniques, however, will set you on the way to become an expert programmer for Android.

The Kotlin version referred to in this book is 1.3. Most of the examples and most of what gets explained here is likely valid for later versions as well.