Lambda Expression in Java/Kotlin

 

Higher-order function

In computer science, a higher-order function is a function that does at least one of the following:

  • Takes one or more functions as arguments
  • Returns a function as its result.

All other functions are first-order functions.

Anonymous class

In Java, the anonymous class enables you to declare and instantiate a class at the same time. If you only need to use a local class once, then you should use the anonymous class. For example, if you wish to define a runnable class to execute a task:

Executors.NewSingleThreadExecutor().execute(new Runnable() {
    @Override
    public void run() {
        // Your task execution.
    }
})

As you can see on the above example, the Runnable is a interface with one function run defined. The anonymous class implemented the interface.

Lambda Expression

Besides the anonymous class, Java also supports anonymous functions, named Lambda Expression.

While anonymous class enables you to declare the new class in a statement, it is sometimes not concise enough when there is only one function in the class.

For the example on the above section, we could simplify it’s implementation with a Lambda expression.

Executors.NewSingleThreadExecutor().execute(()-> {// Your task execution })

The lambda expression provides a few functionalities:

  • Enable to treat functionality as a method argument, or code as data
  • A function that can be created without belonging to any class.
  • A lambda expression can be passed around as if was an object and executed on demand.

In the mean time, the functions are first-class in Kotlin. They can be stored in variables and data structures, passed as arguments and returned from other higher-order functions.

The kotlin Lambda expression follows the following syntax:

  • It is always surrounded by curly braces,
  • Parameter declarations in full syntactic from go inside curly braces and have optional type annotations.
  • The body goes after an -> sign.
  • If the expression return type is not Unit, the last expression inside the body is treated as the return type.

As you can tell, the Lambda expression can’t specify the return types. If you wish the define the return type, you could use an alternative solution: anonymous function.

fun(x: Int, y: Int): Int = x + y

The major difference between Kotlin and Java is that Kotlin is a functional programming language. Kotlin has a dedicated type for functions, for example:

val initFunction: (Int) -> Int

The above expression means that the initFunctions is a function type, and the function takes in a integer and return a integer.

The above function be rewrite as:

val a = {i: Int -> i +1}

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