java 用Java读取文本文件
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Read text file in Java
提问by frank wang
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.
我有一个文本文件。我想将内容从一行检索到另一行。例如，文件可能是 200K 行。我想读取第78行到第2735行的内容。由于文件可能很大，我不想将整个内容读入内存。
采纳答案by Bart Kiers
Here's a start of a possible solution:
这是一个可能的解决方案的开始：
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;
}
回答by Michael Borgwardt
Use BufferedReader.readLine()and count the lines. You'll keep only the buffer size and the current line in memory.
使用BufferedReader.readLine()并计算行数。您将只保留缓冲区大小和内存中的当前行。
And no, it's not possible to get to line 3412 without reading the whole file up to that point (unless your lines all have a fixed size).
不，不可能在没有读取整个文件的情况下到达第 3412 行（除非您的行都具有固定大小）。
回答by flopex
I would suggest using a RandomAccessFile, this class enables you to jump to a specific location in a file. So if you want to read the last line of the file you don't have to read all of the previous lines you can just jump to that line.
我建议使用 RandomAccessFile，这个类使您能够跳转到文件中的特定位置。因此，如果您想阅读文件的最后一行，则不必阅读前面的所有行，只需跳到该行即可。
回答by khmarbaise
Just simply read line by line first and count the line numbers and start getting the contents you need at the line position you mentioned.
只需简单地首先逐行阅读并计算行号，然后在您提到的行位置开始获取您需要的内容。
