Question

This is a function I have to create timestamp string in %.7f format. This function takes only 2-3 ms to execute. But its called from many places in my code, by even optimizing it by 1 ms I will save 1 second in one particular user action. Any ideas?

public static String makeTimestamp()
{
    long millis = System.currentTimeMillis();
    String result;
    Double ts = new Double((millis) / 1000.0);

    ByteArrayOutputStream b = new ByteArrayOutputStream();
    PrintStream p = new PrintStream(b);
    p.printf("%.7f", ts );
    result = b.toString();
    try
    {
        p.close();
        b.close();
    } catch (IOException ioe) {};
    return result;
}

Answer 1

If you absolutely need it, this is roughly 10 times faster than string formatting:

public static String makeTimestamp() {
    return formatTime(System.currentTimeMillis(), 7);
}

public static String formatTime(long millis, int fractionDigits) {
    int integerDigits = (int) Math.log10(millis / 1000.0) + 1;

    char[] chars = new char[integerDigits + fractionDigits + 1];
    for (int i = 0; i < chars.length; i++) {
        chars[i] = '0';
    }

    millis *= Math.pow(10, fractionDigits - 3);
    for (int i = chars.length - 1; i >= 0; i--) {
        if (i == integerDigits) {
            chars[i] = '.';
            i--;
        }

        chars[i] = (char) (millis % 10);
        chars[i] += '0';

        millis /= 10;
    }

    return new String(chars);
}