1. Introduction

In Kotlin, converting a file to a byte array is a common operation, especially in scenarios where file data needs to be processed or transmitted over a network.

This tutorial will demonstrate how to efficiently convert a File into a ByteArray in Kotlin, considering various aspects like file size and error handling.

2. Using readBytes() for Small Files

The simplest way to convert a file to a byte array in Kotlin is by using the File.readBytes() method. This method reads the entire file into memory as a ByteArray:

fun fileToByteArray(filePath: String): ByteArray {
    val file = File(filePath)
    return file.readBytes()

This approach is convenient but may not be suitable for very large files due to memory constraints.

3. Reading Files With reader()

For a more controlled approach to reading files, especially when we need to process the file data (like parsing or modifying the content), Kotlin’s reader() extension function is quite handy. This function returns a FileReader to read the file’s text:

fun readFileUsingReader(filePath: String): ByteArray {
    val file = File(filePath)
    val contentBuilder = StringBuilder()

    file.reader().use { reader ->
        reader.forEachLine { line ->

    return contentBuilder.toString().toByteArray()

In this example, the reader() function opens a FileReader. The use() function ensures that the file is properly closed after reading its content. The forEachLine() lambda iterates over each line in the file, allowing us to process or modify the content as needed. In this case, we’ll append each line to a StringBuilder. Finally, we convert our String to a ByteArray.

This approach allows us to process text files line-by-line.

4. Efficient Approach for Large Files With bufferedReader()

Finally, to handle large files efficiently, we’ll look at Kotlin’s bufferedReader(). Furthermore, we’ll combine the buffered reader with a BufferedOutputStream.

This method reads the file line by line and processes it before moving on, which is memory-efficient. In our case, it passes each line to a BufferedOutputStream. This BufferedOutputStream writes to another file:

fun largeFileToByteArray(filePath: String, outputFilePath: String) {
    File(filePath).bufferedReader().use { reader ->
        BufferedOutputStream(FileOutputStream(outputFilePath)).use { bufferOut ->
              .forEach { line -> bufferOut.write(line.toByteArray()) }

Again, the use() function automatically closes the resource when the execution of the block is complete, thus preventing resource leaks. Using BufferedOutputStream writes the data to a file instead of holding it all in memory, making it ideal for very large files.

5. Handling Exceptions

When working with file I/O, it’s important to handle exceptions that might occur, such as FileNotFoundException or IOException. Kotlin’s trycatch block can be used for this purpose:

fun safeFileToByteArray(filePath: String): ByteArray? {
    return try {
    } catch (e: IOException) {
        println("Error reading file: ${e.message}")

With this strategy, we’re able to catch any errors that might arise while processing the file.

6. Conclusion

In this article, we explored different methods to convert a file to a byte array in Kotlin, ranging from a basic approach to more efficient methods for large files. We also discussed the importance of handling exceptions that may occur during file operations.

By choosing the appropriate method, we can effectively handle file data in our Kotlin applications.

As always, the code used in this article is available over on GitHub.

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