Kotlin Cookbook

Chapter 10. Input/Output

Kotlin makes doing input/output (I/O) operations easy, but the style is different from what a Java developer may expect. Resources in Kotlin are frequently closed by employing a use function that does so on the user’s behalf. This chapter includes a couple of recipes that focus on that approach, specifically for files, but that can be generalized to other resources.

10.1 Managing Resources with use

Problem

You want to process a resource such as a file and be sure that it is closed when you are finished, but Kotlin does not support Java’s try-with-resources construct.

Solution

Use the extension functions use or useLines on readers or input/output streams.

Discussion

Java 1.7 introduced the try-with-resources construct, which allows a developer to open a resource inside parentheses between the try keyword and its corresponding block; the JVM will automatically close the resource when the try block ends. The only requirement is that the resource come from a class that implements the Closeable interface. File, Stream, and many other classes implement Closeable, as the example in Example 10-1 shows.

Example 10-1. Using try-with-resources from Java

package io;

import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;

public class TryWithResourcesDemo {
    public static void main(String[] args) throws IOException {
        String path = "src/main/resources/book_data.csv";

        File file = new File(path);
        String ...