Number of lines in a file in Java

Question

I use huge data files, sometimes I only need to know the number of lines in these files, usually I open them up and read them line by line until I reach the end of the file<

Answer 1

I concluded that wc -l:s method of counting newlines is fine but returns non-intuitive results on files where the last line doesn't end with a newline.

And @er.vikas solution based on LineNumberReader but adding one to the line count returned non-intuitive results on files where the last line does end with newline.

I therefore made an algo which handles as follows:

@Test
public void empty() throws IOException {
    assertEquals(0, count(""));
}

@Test
public void singleNewline() throws IOException {
    assertEquals(1, count("\n"));
}

@Test
public void dataWithoutNewline() throws IOException {
    assertEquals(1, count("one"));
}

@Test
public void oneCompleteLine() throws IOException {
    assertEquals(1, count("one\n"));
}

@Test
public void twoCompleteLines() throws IOException {
    assertEquals(2, count("one\ntwo\n"));
}

@Test
public void twoLinesWithoutNewlineAtEnd() throws IOException {
    assertEquals(2, count("one\ntwo"));
}

@Test
public void aFewLines() throws IOException {
    assertEquals(5, count("one\ntwo\nthree\nfour\nfive\n"));
}

And it looks like this:

static long countLines(InputStream is) throws IOException {
    try(LineNumberReader lnr = new LineNumberReader(new InputStreamReader(is))) {
        char[] buf = new char[8192];
        int n, previousN = -1;
        //Read will return at least one byte, no need to buffer more
        while((n = lnr.read(buf)) != -1) {
            previousN = n;
        }
        int ln = lnr.getLineNumber();
        if (previousN == -1) {
            //No data read at all, i.e file was empty
            return 0;
        } else {
            char lastChar = buf[previousN - 1];
            if (lastChar == '\n' || lastChar == '\r') {
                //Ending with newline, deduct one
                return ln;
            }
        }
        //normal case, return line number + 1
        return ln + 1;
    }
}

If you want intuitive results, you may use this. If you just want wc -l compatibility, simple use @er.vikas solution, but don't add one to the result and retry the skip:

try(LineNumberReader lnr = new LineNumberReader(new FileReader(new File("File1")))) {
    while(lnr.skip(Long.MAX_VALUE) > 0){};
    return lnr.getLineNumber();
}