Fast double to string conversion with given precision
Disclaimer: I only recommend that you use this if speed is an absolute requirement.
On my machine, the following can do 1 million conversions in about 130ms:
private static final int POW10[] = {1, 10, 100, 1000, 10000, 100000, 1000000};
public static String format(double val, int precision) {
StringBuilder sb = new StringBuilder();
if (val < 0) {
sb.append('-');
val = -val;
}
int exp = POW10[precision];
long lval = (long)(val * exp + 0.5);
sb.append(lval / exp).append('.');
long fval = lval % exp;
for (int p = precision - 1; p > 0 && fval < POW10[p]; p--) {
sb.append('0');
}
sb.append(fval);
return sb.toString();
}
The code as presented has several shortcomings: it can only handle a limited range of doubles
, and it doesn't handle NaNs. The former can be addressed (but only partially) by extending the POW10
array. The latter can be explicitly handled in the code.
If you need both speed and precision, I've developed a fast DoubleFormatUtil class at xmlgraphics-commons: http://xmlgraphics.apache.org/commons/changes.html#version_1.5rc1
You can see the code there: http://svn.apache.org/viewvc/xmlgraphics/commons/trunk/src/java/org/apache/xmlgraphics/util/DoubleFormatUtil.java?view=markup
It's faster than both DecimalFormat/BigDecimal, as fast as Double.toString, it's precise, it's well tested. It's licensed under Apache License 2.0, so you can use it as you want.