用Java读取文本文件 [英] Read text file in Java

问题描述

我有一个文本文件。我想从一行到另一行检索内容。
例如，该文件可能是200K行。我想读第78行到第2735行的内容。由于文件可能非常大，我不想将整个内容读入内存。

解决方案

这是一个可能解决方案的开始：

  public static List< String> linesFromTo（int from，int to，String fileName）
抛出FileNotFoundException，IllegalArgumentException {
返回linesFromTo（from，to，fileName，UTF-8）; 
} 
 
 public static List< String> linesFromTo（int from，int to，String fileName，String charsetName）
抛出FileNotFoundException，IllegalArgumentException {
 $ b $ if（from> to）{
抛出new IllegalArgumentException（'from '>'到'）; 
 
 if（from< 1 || to< 1）{
 throw new IllegalArgumentException（'from'or'to'is negative）; 
} 
 
列表< String> lines = new ArrayList< String>（）; 
扫描仪扫描=新的扫描仪（新文件（文件名），charsetName）; 
 int lineNumber = 0; （scan.hasNextLine（）&& lineNumber< to）{
 lineNumber ++; 
 
。 
 String line = scan.nextLine（）; 
 if（lineNumber< from）continue; 
 lines.add（line）; 
 
 $ b $ if（lineNumber！= to）{
 throw new IllegalArgumentException（fileName +没有++行）; 
} 
 
 return lines; 
} 
  

I have a text file. I would like to retrieve the content from one line to another line. For example, the file may be 200K lines. I want to read the content from line 78 to line 2735. Since the file may be very large, I do not want to read the whole content into the memory.

解决方案

Here's a start of a possible solution:

public static List<String> linesFromTo(int from, int to, String fileName)
        throws FileNotFoundException, IllegalArgumentException {
    return linesFromTo(from, to, fileName, "UTF-8");
}

public static List<String> linesFromTo(int from, int to, String fileName, String charsetName)
        throws FileNotFoundException, IllegalArgumentException {

    if(from > to) {
        throw new IllegalArgumentException("'from' > 'to'");
    }
    if(from < 1 || to < 1) {
        throw new IllegalArgumentException("'from' or 'to' is negative");
    }

    List<String> lines = new ArrayList<String>();
    Scanner scan = new Scanner(new File(fileName), charsetName);
    int lineNumber = 0;

    while(scan.hasNextLine() && lineNumber < to) {
        lineNumber++;
        String line = scan.nextLine();
        if(lineNumber < from) continue;
        lines.add(line);
    }

    if(lineNumber != to) {
        throw new IllegalArgumentException(fileName+" does not have "+to+" lines");
    }

    return lines;
}

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