How can I get the count of line in a file in an efficient way?

BufferedReader reader = new BufferedReader(new FileReader("file.txt"));
int lines = 0;
while (reader.readLine() != null) lines++;
reader.close();

Update: To answer the performance-question raised here, I made a measurement. First thing: 20.000 lines are too few, to get the program running for a noticeable time. I created a text-file with 5 million lines. This solution (started with java without parameters like -server or -XX-options) needed around 11 seconds on my box. The same with wc -l (UNIX command-line-tool to count lines), 11 seconds. The solution reading every single character and looking for '\n' needed 104 seconds, 9-10 times as much.

use LineNumberReader

something like

public static int countLines(File aFile) throws IOException {
    LineNumberReader reader = null;
    try {
        reader = new LineNumberReader(new FileReader(aFile));
        while ((reader.readLine()) != null);
        return reader.getLineNumber();
    } catch (Exception ex) {
        return -1;
    } finally { 
        if(reader != null) 
            reader.close();
    }
}

I found some solution for this, it might useful for you

Below is the code snippet for, count the no.of lines from the file.

  File file = new File("/mnt/sdcard/abc.txt");
  LineNumberReader lineNumberReader = new LineNumberReader(new FileReader(file));
  lineNumberReader.skip(Long.MAX_VALUE);
  int lines = lineNumberReader.getLineNumber();
  lineNumberReader.close();

Files.lines

Java 8+ has a nice and short way using NIO using Files.lines. Note that you have to close the stream using try-with-resources:

long lineCount;
try (Stream<String> stream = Files.lines(path, StandardCharsets.UTF_8)) {
  lineCount = stream.count();
}

If you don't specify the character encoding, the default one used is UTF-8. You may specify an alternate encoding to match your particular data file as shown in the example above.