Local variables

Local variables can be declared in two ways: first, with the val  keyword, then they are immutable (the variable cannot be reassigned). If you are coming from Java, val would be equal to variables declared with a final keyword.

Secondly, you can declare a local variable with a var keyword; then it is considered mutable and the value can be reassigned after a declaration.

The following command demonstrates that it will not compile, because the bar local variable cannot be reassigned:

fun immutable() {
   val bar: String = "Kotlin"
   bar = "Kotlin is awesome" // compiler error
}

And, when declared with var, the compiler allows a local variable to be reassigned:

fun mutable() {
   var bar: String = "Kotlin"
   bar = "Kotlin is awesome" 
}

You should favor immutable variables. The compiler will...