Mastering High Performance with Kotlin

Closures

A closure is a function that has access to variables that are defined in the outer scope. In our example, the { p1: P1 -> this(f(p1)) } closure captures the f: (P1) -> R1 argument. Unlike in Java, variables captured in a closure can be modified in Kotlin. The following example shows how to modify a captured variable in Kotlin: 

var totalAges = 0students.forEach {    totalAges += it.age}

But in Java, this similar example will lead to a compilation error:

int totalAges = 0;students.forEach(student -> totalAges += student.getAge());

The compilation error will read:

Error:(19, 49) java: local variables referenced from a lambda expression must be final or effectively final