Type inference

Kotlin is a statically-typed language. It executes the type inference for us, so we don't have to specify types. Consider the code for 3a_TypeInference.kts:

val message = "Hi there"
println(message)

When we run this code, we get the following output:

We can also explicitly specify the type. Consider the following code:

val message : String = "Hi there"
println(message)

This is an example of creating a variable of the String type and printing it in the console. This looks like Scala syntax and it is quite different from what we do in Java.

In Java, we would write this as follows:

String message = "Hi there";

Languages such as Kotlin emphasize that the name of a variable is more important than the nature of the variable. Kotlin puts the name of a variable or constant first and the type after in the var declaration syntax. The type is, in fact, optional; we don't have to specify it. We might think that this is the same as the dynamic type, where the type is resolved during runtime. Kotlin, however, actually infers the type at compile time.

Kotlin uses the String class from the JDK library. We can query the class that it uses as follows:

val message = "Hi there"
println(message.javaClass)

The output is as follows:

As a general rule, it's a good idea to specify type information when we write public-facing interfaces and, when using local variables, we can let the language infer the type.