Kotlin – Interfaces, Inline and Higher-Order Functions

Kotlin is a modern, statically typed programming language that runs on the Java Virtual Machine and can also be compiled to JavaScript. It is expressive, concise, and powerful, and has quickly become a popular choice for Android development. In this blog post, we will take a look at some of the advanced features of Kotlin that can make your code more concise, readable, and efficient.

Type Inference

Type inference is a feature that allows the Kotlin compiler to infer the type of a variable or expression based on the context in which it is used. This means that you don’t always have to specify the type explicitly, and the compiler will fill it in for you.

For example, consider the following code:

val x = 1
val y = 2
val z = x + y

In this code, the variables x, y, and z are all inferred to be of type Int, because they are all assigned integer values. This is convenient because it saves you from having to specify the type explicitly, and it also makes the code more readable because it is less cluttered with type annotations.

Type inference can also be used with generic types. For example, consider the following code:

val list = listOf(1, 2, 3)

Here, the type of the list variable is inferred to be List, because it is assigned a list of integers.

Inline Functions

In Kotlin, you can use the inline keyword to annotate a function as inline. This means that the function will be inlined at the call site, which can improve the performance of your code by avoiding the overhead of function calls.

For example, consider the following code:

inline fun foo(block: () -> Unit) {
    println("Before")
    block()
    println("After")
}

foo {
    println("Inside")
}

In this code, the foo function is annotated as inline, which means that the code inside the function will be inlined at the call site. The output of this code will be:

Before
Inside
After

Inlining functions can be especially useful when working with higher-order functions, which we will discuss next.

Higher-Order Functions

A higher-order function is a function that takes one or more functions as arguments or returns a function as a result. Kotlin has excellent support for higher-order functions, which can be used to write concise and expressive code.

For example, consider the following code:

fun foo(list: List<Int>, predicate: (Int) -> Boolean): List<Int> {
    return list.filter(predicate)
}

val result = foo(listOf(1, 2, 3, 4, 5), { it % 2 == 0 })

In this code, the foo function is a higher-order function because it takes a function (predicate) as an argument. The predicate function is a lambda expression that takes an Int and returns a Boolean. The foo function uses the filter higher-order function to filter the list based on the predicate.

Higher-order functions can be used in combination with inline functions to write even more concise and expressive code. For example, consider the following code:

inline fun <T> Iterable<T