这个有趣的解决方案真的很好!

public static int countLines(File input) throws IOException {
    try (InputStream is = new FileInputStream(input)) {
        int count = 1;
        for (int aChar = 0; aChar != -1;aChar = is.read())
            count += aChar == '\n' ? 1 : 0;
        return count;
    }
}

在基于unix的系统上，在命令行上使用wc命令。

我知道这是一个老问题，但公认的解决方案并不完全符合我所需要的。因此，我将其改进为接受各种行结束符(而不仅仅是换行)并使用指定的字符编码(而不是ISO-8859-n)。所有在一个方法(适当重构):

public static long getLinesCount(String fileName, String encodingName) throws IOException {
    long linesCount = 0;
    File file = new File(fileName);
    FileInputStream fileIn = new FileInputStream(file);
    try {
        Charset encoding = Charset.forName(encodingName);
        Reader fileReader = new InputStreamReader(fileIn, encoding);
        int bufferSize = 4096;
        Reader reader = new BufferedReader(fileReader, bufferSize);
        char[] buffer = new char[bufferSize];
        int prevChar = -1;
        int readCount = reader.read(buffer);
        while (readCount != -1) {
            for (int i = 0; i < readCount; i++) {
                int nextChar = buffer[i];
                switch (nextChar) {
                    case '\r': {
                        // The current line is terminated by a carriage return or by a carriage return immediately followed by a line feed.
                        linesCount++;
                        break;
                    }
                    case '\n': {
                        if (prevChar == '\r') {
                            // The current line is terminated by a carriage return immediately followed by a line feed.
                            // The line has already been counted.
                        } else {
                            // The current line is terminated by a line feed.
                            linesCount++;
                        }
                        break;
                    }
                }
                prevChar = nextChar;
            }
            readCount = reader.read(buffer);
        }
        if (prevCh != -1) {
            switch (prevCh) {
                case '\r':
                case '\n': {
                    // The last line is terminated by a line terminator.
                    // The last line has already been counted.
                    break;
                }
                default: {
                    // The last line is terminated by end-of-file.
                    linesCount++;
                }
            }
        }
    } finally {
        fileIn.close();
    }
    return linesCount;
}

这个解决方案在速度上与公认的解决方案相当，在我的测试中大约慢了4%(尽管Java中的计时测试是出了名的不可靠)。

接受的答案有一个错误关闭多行文件，不以换行符结束。一个没有换行符的单行文件将返回1，但是一个没有换行符的两行文件也将返回1。下面是解决这个问题的公认解决方案的实现。endsWithoutNewLine检查对于除最终读取外的所有内容都是浪费的，但与整个函数相比，应该是微不足道的时间。

public int count(String filename) throws IOException {
    InputStream is = new BufferedInputStream(new FileInputStream(filename));
    try {
        byte[] c = new byte[1024];
        int count = 0;
        int readChars = 0;
        boolean endsWithoutNewLine = false;
        while ((readChars = is.read(c)) != -1) {
            for (int i = 0; i < readChars; ++i) {
                if (c[i] == '\n')
                    ++count;
            }
            endsWithoutNewLine = (c[readChars - 1] != '\n');
        }
        if(endsWithoutNewLine) {
            ++count;
        } 
        return count;
    } finally {
        is.close();
    }
}

一个直接的方式使用扫描器

static void lineCounter (String path) throws IOException {

        int lineCount = 0, commentsCount = 0;

        Scanner input = new Scanner(new File(path));
        while (input.hasNextLine()) {
            String data = input.nextLine();

            if (data.startsWith("//")) commentsCount++;

            lineCount++;
        }

        System.out.println("Line Count: " + lineCount + "\t Comments Count: " + commentsCount);
    }

要知道文件里有多少行，唯一的方法就是数数。当然，你可以从你的数据中创建一个指标，给你一个平均长度的一行，然后得到文件大小，并除以平均长度，但这不会是准确的。