In Java, how to read from a file a specific line, given the line number?

Question

In Java, is there any method to read a particular line from a file, for example, read line 32?

Answer 1

Unless you have previous knowledge about the lines in the file, there's no way to directly access the 32nd line without reading the 31 previous lines.

That's true for all languages and all modern file systems.

So effectively you'll simply read lines until you've found the 32nd one.

Answer 2

If you are talking about a text file, then there is really no way to do this without reading all the lines that precede it - After all, lines are determined by the presence of a newline, so it has to be read.

Use a stream that supports readline, and just read the first X-1 lines and dump the results, then process the next one.

Answer 3

Not that I know of, but what you could do is loop through the first 31 lines doing nothing using the readline() function of BufferedReader

FileInputStream fs= new FileInputStream("someFile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(fs));
for(int i = 0; i < 30; ++i)
  br.readLine();
String lineIWant = br.readLine();

Answer 4

No, unless in that file format the line lengths are pre-determined (e.g. all lines with a fixed length), you'll have to iterate line by line to count them.

Answer 5

Joachim is right on, of course, and an alternate implementation to Chris' (for small files only because it loads the entire file) might be to use commons-io from Apache (though arguably you might not want to introduce a new dependency just for this, if you find it useful for other stuff too though, it could make sense).

For example:

String line32 = (String) FileUtils.readLines(file).get(31);

http://commons.apache.org/io/api-release/org/apache/commons/io/FileUtils.html#readLines(java.io.File, java.lang.String)