File IO in Kotlin.
// Kotlin simply supplements the existing Java IO libraries with kotlin.io package
// Note text file in base directory of project <-
fun main() {
File("testfile.txt").reader()
// Abbreviates the Java declaration: new InputStreamReader(new FileInputStreamReader(new File("testfile.txt")), UTF-8")
val lines = File("testfile.txt").reader().readLines() // DO NOT USE THIS FUNCTION FOR HUGE FILES. Reads entire file into memory.
// Closes the Stream for us. Returns listOf<String>
lines.forEach { println(it)}
val reader2 = File("testfile.txt").reader()
val lines2 = reader2.readText() // Returns one String which we would have to parse and close the Stream ourselves
println("Lines2: $lines2") // Prints fine as text contains newlines
reader2.close()
println("======")
val lines3 = File("testfile.txt").reader().use { it.readText() } // Closes the Reader itself - .use functions close the resource
println("Lines3: $lines3")
println("======")
val lines4 = File("testfile.txt").bufferedReader().use { it.readText() } // BufferedReader
println("Lines4: $lines4")
println("======")
val lines5 = File("testfile.txt").readText() // Version of readText which runs from the File class - limit of 2GB
println("Lines5: $lines5")
println("======")
// If we don't want to read the whole file at once:
File("testfile.txt").reader().forEachLine { println(it) } // Closes the resource for us
println("======")
// If we don't want to read the whole file at once:
File("testfile.txt").reader().useLines { it.forEach { println(it) } } // useLines returns a reference so we need forEach. More verbose
println("======")
// Binary files - best done by using Java methods. No great enhancements in Kotlin.
val di = DataInputStream(FileInputStream("testfile.bin"))
var si: String
try {
while (true) {
si = di.readUTF()
println("Binary file: $si")
}
} catch (e: EOFException) {
}
// Writing files is no different to Java
// Try with resources. To make sure a resource is released after using it use try...finally block, or better: try with resources. Kotlin method is to use one of the .use functions (see above)
}