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1. Introduction

When we learn a new programing language, it’s common to start with console I/O. In this tutorial, we’ll explore some alternatives for handling console I/O with Kotlin.

2. Using the Kotlin Standard Library

The Kotlin standard library provides us extensions for handling I/O based on the JDK’s built-in support.

To print to the console, we can use the print function. If we run the following snippet:

print("Hello from Kotlin")

We’ll see the following message displayed on our terminal:

Hello from Kotlin

Behind the scenes, this function uses Java’s System.out.print method. Also, the library offers us the println alternative function, which adds the line separator at the end of the message.

In order to read from the console, we can use the readLine function:

val inputText = readLine()

Interestingly, this isn’t a synonym for Scanner.readLine like print is for System.out.print. Let’s see now where Scanner does come in, though.

3. Using the Java Standard Library

Kotlin has great interoperability with Java. Thus, we can use the standard I/O classes from the JDK in our programs in case we need them.

Let’s explore some of them here.

3.1. Using the Scanner Class

Using the Scanner class is very straightforward; we only need to create an instance and use the nextLine method:

val scanner = Scanner(System.`in`)
val readText = scanner.nextLine()

Note that we’re escaping the in property with backticks because it’s a keyword in Kotlin.

3.2. Using the BufferedReader Class

To use the BufferedReader class to read from the standard input stream, we first need to instantiate it with

val reader = BufferedReader(InputStreamReader(System.`in`))

And then we can use its methods — for example, readLine():

val readText = reader.readLine()

3.3. Using the Console Class

Unlike the two previous classes, the Console class has additional methods for handling console I/O, like readPassword and printf.

In order to use the Console class, we need to get an instance from the System class:

val console = System.console()

Now, we can use its readLine() method, among others:

val readText = console.readLine()

4. Conclusion

In this tutorial, we saw an introduction to handling I/O with Kotlin and how to use the equivalent classes from the JDK. For more details about these JDK classes, be sure to check out our tutorial on Scanner, BufferedReader, and Console.

Also, thanks to Kotlin’s interoperability with Java, we can use additional Java libraries for handling I/O.

As usual, all the code samples shown in this article are available over on GitHub.

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I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:


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