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    <p>This is the fastest version I have found so far, about 6 times faster than readLines. On a 150MB log file this takes 0.35 seconds, versus 2.40 seconds when using readLines(). Just for fun, linux' wc -l command takes 0.15 seconds.</p> <pre><code>public static int countLines(String filename) throws IOException { InputStream is = new BufferedInputStream(new FileInputStream(filename)); try { byte[] c = new byte[1024]; int count = 0; int readChars = 0; boolean empty = true; while ((readChars = is.read(c)) != -1) { empty = false; for (int i = 0; i &lt; readChars; ++i) { if (c[i] == '\n') { ++count; } } } return (count == 0 &amp;&amp; !empty) ? 1 : count; } finally { is.close(); } } </code></pre> <p>EDIT, 9 1/2 years later: I have practically no java experience, but anyways I have tried to benchmark this code against the <code>LineNumberReader</code> solution below since it bothered me that nobody did it. It seems that especially for large files my solution is faster. Although it seems to take a few runs until the optimizer does a decent job. I've played a bit with the code, and have produced a new version that is consistently fastest:</p> <pre><code>public static int countLinesNew(String filename) throws IOException { InputStream is = new BufferedInputStream(new FileInputStream(filename)); try { byte[] c = new byte[1024]; int readChars = is.read(c); if (readChars == -1) { // bail out if nothing to read return 0; } // make it easy for the optimizer to tune this loop int count = 0; while (readChars == 1024) { for (int i=0; i&lt;1024;) { if (c[i++] == '\n') { ++count; } } readChars = is.read(c); } // count remaining characters while (readChars != -1) { System.out.println(readChars); for (int i=0; i&lt;readChars; ++i) { if (c[i] == '\n') { ++count; } } readChars = is.read(c); } return count == 0 ? 1 : count; } finally { is.close(); } } </code></pre> <p>Benchmark resuls for a 1.3GB text file, y axis in seconds. I've performed 100 runs with the same file, and measured each run with <code>System.nanoTime()</code>. You can see that <code>countLines</code> has a few outliers, and <code>countLinesNew</code> has none and seems to be also slightly faster. <code>LineNumberReader</code> is clearly slower.</p> <p><a href="https://i.stack.imgur.com/fjQQB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fjQQB.png" alt="Benchmark Plot"></a></p>
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    1. COyou were right david, I thought the JVM would be good enough for this... I have updated the code, this one is faster.
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    2. COBufferedInputStream should be doing the buffering for you, so I don't see how using an intermediate byte[] array will make it any faster. You're unlikely to do much better than using readLine() repeatedly anyway (since that will be optimized towards by the API).
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    3. COIve benchmarked it with and without the buffered inputstream, and it is afaster when using it.
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