Kotlin的特性
1.Kotlin提供了非常多，File、Stream、Reader、Writer的拓展方法
2.使用use拓展自动关闭资源
3.小文件一次性读写操作
一、首先来看看繁琐的JavaIO操作，来读取一个文件

package cn.kotliner.java.io;

import java.io.*;

/**
 * Created by wangdong
 * Java中的IO操作
 */
public class IO {
    public static void main(String... args) {
        BufferedReader bufferedReader = null;
        try {
             bufferedReader = new BufferedReader(new FileReader(new File("build.gradle")));
            String line;
            while((line = bufferedReader.readLine()) != null){
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                bufferedReader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

二、如果用Kotlin直接译一中的JavaIO代码

package cn.kotliner.kotlin.io

import java.io.*

/**
 * Created by wangdong
 * Kotlin直译Java中的IO操作
 */
fun main(args: Array<String>) {
    val file = File("build.gradle")
    val bufferedReader = BufferedReader(FileReader(file))
    var line: String

    while(true){
        line = bufferedReader.readLine()?:break
        println(line)
    }
    bufferedReader.close()
}

三、使用Kotlin中use方法去改写

package cn.kotliner.kotlin.io

import java.io.*

/**
 * Created by wangdong
 * 使用kotlin中的use的方法改写
 */
fun main(args: Array<String>) {
    val file = File("build.gradle")
    BufferedReader(FileReader(file)).use {
        var line: String
        while(true){
            line = it.readLine()?:break
            println(line)
        }
    }
}

/**
 * 第十行use点进去，就可以看到use是怎么实现的
 */
/**
@InlineOnly
public inline fun <T : Closeable?, R> T.use(block: (T) -> R): R {
    var closed = false
    try {
        return block(this)
    } catch (e: Exception) {
        closed = true
        try {
            this?.close()
        } catch (closeException: Exception) {
        }
        throw e
    } finally {
        if (!closed) {
            this?.close()
        }
    }
}*/                                                                                                 

四、使用kotlin中的readlin()方法改写一下

package cn.kotliner.kotlin.io

import java.io.*

/**
 * Created by wangdong
 * 使用readlines（）方法改写
 */
fun main(args: Array<String>) {
    File("build.gradle").readLines().forEach(::println)
}

/**
 * Reads the file content as a list of lines.
 *
 * Do not use this function for huge files.
 *
 * @param charset character set to use. By default uses UTF-8 charset.
 * @return list of file lines.

public fun File.readLines(charset: Charset = Charsets.UTF_8): List<String> {
    val result = ArrayList<String>()
    forEachLine(charset) { result.add(it); }
    return result
}*/