How to read plain text file in kotlin?

问题

There may be various way to read plain text file in kotlin.

I want know what are the possible ways and how I can use them.

回答1:

Using BufferedReader

import java.io.File
import java.io.BufferedReader

fun main(args: Array<String>) {
    val bufferedReader: BufferedReader = File("example.txt").bufferedReader()    
    val inputString = bufferedReader.use { it.readText() }
    println(inputString)
}

Using InputStream

Read By Line

import java.io.File
import java.io.InputStream

fun main(args: Array<String>) {
    val inputStream: InputStream = File("example.txt").inputStream()
    val lineList = mutableListOf<String>()

    inputStream.bufferedReader().useLines { lines -> lines.forEach { lineList.add(it)} }
    lineList.forEach{println(">  " + it)}
}

Read All Lines

import java.io.File
import java.io.InputStream

fun main(args: Array<String>) {
    val inputStream: InputStream = File("example.txt").inputStream()
    val inputString = inputStream.bufferedReader().use { it.readText() }
    println(inputString)
}

Use File directly

import java.io.File
import java.io.BufferedReader

fun main(args: Array<String>) {
    val lineList = mutableListOf<String>()

    File("example.txt").useLines { lines -> lines.forEach { lineList.add(it) }}
    lineList.forEach { println(">  " + it) }
}

回答2:

The answers above here are all based on Kotlin Java. Here is a Kotlin Native way to read text files:

val bufferLength = 64 * 1024
val buffer = allocArray<ByteVar>(bufferLength)

 for (i in 1..count) {
    val nextLine = fgets(buffer, bufferLength, file)?.toKString()
    if (nextLine == null || nextLine.isEmpty()) break

    val records = parseLine(nextLine, ',')
    val key = records[column]
    val current = keyValue[key] ?: 0
    keyValue[key] = current + 1
}


fun parseLine(line: String, separator: Char) : List<String> {
    val result = mutableListOf<String>()
    val builder = StringBuilder()
    var quotes = 0
    for (ch in line) {
        when {
            ch == '\"' -> {
                quotes++
                builder.append(ch)
            }
            (ch == '\n') || (ch ==  '\r') -> {}
            (ch == separator) && (quotes % 2 == 0) -> {
                result.add(builder.toString())
                builder.setLength(0)
            }
            else -> builder.append(ch)
        }
    }
    return result
}

See: https://github.com/JetBrains/kotlin-native/blob/master/samples/csvparser/src/csvParserMain/kotlin/CsvParser.kt

回答3:

Anisuzzaman's answer lists several possibilities.

The main differences between them are in whether the file is read into memory as a single String, read into memory and split into lines, or read line-by-line.

Obviously, reading the entire file into memory in one go can take a lot more memory, so that's something to avoid unless it's really necessary. (Text files can get arbitrarily big!) So processing line-by-line with BufferedReader.useLines() is often a good approach.

The remaining differences are mostly historical. Very early versions of Java used InputStream &c which didn't properly distinguish between characters and bytes; Reader &c were added to correct that. Java 8 added ways to read line-by-line more efficiently using streams (e.g. Files.lines()). And more recently, Kotlin has added its own extension functions (e.g. BufferedReader.useLines()) which make it even simpler.

来源：https://stackoverflow.com/questions/55182578/how-to-read-plain-text-file-in-kotlin

标签

file

kotlin

text-files

bufferedreader