3 Defining and calling functions

By now, you should be fairly comfortable with using Kotlin the same way you use Java. You’ve seen how the concepts familiar to you from Java translate to Kotlin, and how Kotlin often makes them more concise and readable.

In this chapter, you’ll see how Kotlin improves on one of the key elements of every program: declaring and calling functions. We’ll also look into the possibilities for adapting Java libraries to the Kotlin style through the use of extension functions, allowing you to gain the full benefits of Kotlin in mixed-language projects.

To make our discussion more useful and less abstract, we’ll focus on Kotlin collections, strings, and regular expressions as our problem domain. As an introduction, let’s look at how to create collections in Kotlin.

3.1 Creating collections in Kotlin

Before you can do interesting things with collections, you need to learn how to create them. In section 2.3.4, you bumped into the way to create a new set: the setOf function. You created a set of colors then, but for now, let’s keep it simple and work with numbers:

You create a list or a map in a similar way:

3.2 Making functions easier to call

3.2.1 Named arguments

3.2.2 Default parameter values

3.2.3 Getting rid of static utility classes: top-level functions and properties

3.3 Adding methods to other people’s classes: extension functions and properties

3.3.1 Imports and extension functions

3.3.2 Calling extension functions from Java

3.3.3 Utility functions as extensions

3.3.4 No overriding for extension functions

3.3.5 Extension properties

3.4 Working with collections: varargs, infix calls, and library support

3.4.1 Extending the Java Collections API