Java 文件操作读取文本(txt)文件内容的方法代码总结

1、常用读取文本文件内容代码

private String readFile(String file) throws IOException {
    BufferedReader reader = new BufferedReader(new FileReader (file));
    String         line = null;
    StringBuilder  stringBuilder = new StringBuilder();
    String         ls = System.getProperty("line.separator");
    try {
        while((line = reader.readLine()) != null) {
            stringBuilder.append(line);
            stringBuilder.append(ls);
        }
        return stringBuilder.toString();
    } finally {
        reader.close();
    }
}

2、使用Files.readString读取文本

Java 11添加了readString()方法来读取小文件String，保留行终止符。

String content = Files.readString(path, StandardCharsets.US_ASCII);

3、使用Files.readAllBytes读取文本内容(指定字符编码)

static String readFile(String path, Charset encoding) 
  throws IOException 
{
  byte[] encoded = Files.readAllBytes(Paths.get(path));
  return new String(encoded, encoding);
}

在StandardCharsets类中定义的所有Java运行时所需要的编码的一些常量：

String content = readFile("test.txt", StandardCharsets.UTF_8);

该平台默认可从该Charset类本身：

String content = readFile("test.txt", Charset.defaultCharset());

4、使用外部库org.apache.commons.io.FileUtils.readFileToString()

使用外部库，请查看Apache Commons IO（200KB JAR）。它包含一个org.apache.commons.io.FileUtils.readFileToString()，读取File为String用一行代码。例如：

import java.io.*;
import java.nio.charset.*;
import org.apache.commons.io.*;
public String readFile() throws IOException {
    File file = new File("data.txt");
    return FileUtils.readFileToString(file, StandardCharsets.UTF_8);
}

5、使用BufferedReader读取

BufferedReader会将文件所有行都存入内存中，适用于读取文本较小的情况。

例如，

读取文本数据保存为二维数组，

private static double[][] getFile(String pathName) throws Exception {
        File file = new File(pathName);
        if (!file.exists())
            throw new RuntimeException("Not File!");
        BufferedReader br = new BufferedReader(new FileReader(file));
        String str;
        List<double[]> list = new ArrayList<double[]>();
        while ((str = br.readLine()) != null) {
            int s = 0;
            String[] arr = str.split(",");
            double[] dArr = new double[arr.length];
            for (String ss : arr) {
                if (ss != null) {
                    dArr[s++] = Double.parseDouble(ss);
                }
            }
            list.add(dArr);
        }
        int max = 0;
        for (int i = 0; i < list.size(); i++) {
            if (max < list.get(i).length)
                max = list.get(i).length;
        }
        double[][] array = new double[list.size()][max];
        for (int i = 0; i < array.length; i++) {
            for (int j = 0; j < list.get(i).length; j++) {
                array[i][j] = list.get(i)[j];
            }
        }
        return array;
 }

6、使用FileInputStream读取

如果读取文本文件比较大，就不能使用BufferedReader，则需要使用FileInputStream。

例如，

读取文本数据保存为二维数组，

private static double[][] getFileInput(String pathName) throws Exception {
    FileInputStream inputStream = null;
    Scanner sc = null;
    List<double[]> list = new ArrayList<>();
    inputStream = new FileInputStream(pathName);
    sc = new Scanner(inputStream, "utf-8");
    while (sc.hasNextLine()) {
      String line = sc.nextLine();
      int s = 0;
      String[] arr = line.split(",");
      double[] dArr = new double[arr.length];
      for (String ss : arr) {
        if (ss != null) {
          dArr[s++] = Double.parseDouble(ss);
        }
      }
      list.add(dArr);
    }
    int max = 0;
    for (int i = 0; i < list.size(); i++) {
      if (max < list.get(i).length)
        max = list.get(i).length;
    }
    double[][] array = new double[list.size()][max];
    for (int i = 0; i < array.length; i++) {
      for (int j = 0; j < list.get(i).length; j++) {
        array[i][j] = list.get(i)[j];
      }
    }
    inputStream.close();
    sc.close();
    return array;
}