Re: Optimizing Java method

"Ingo R. Homann" <>
Wed, 11 Jul 2007 15:01:55 +0200

some suggestions (note that they may depend upon your hardware, os and

Benjamin White wrote:

     public static byte[] stringToPack(String str) throws DataException {
        int i; // string index
        int j; // byte array index
        boolean nibble_ordinal = false;
        char ch1;
        byte nibble;
        int strlen = str.length();
        byte[] pknum = new byte[16];
        for (i = 0; i < 16; i++) { // initialize to zero
            pknum[i] = 0;

This schould not be necessary and can be done within the loop below!

         i = strlen - 1;
        j = 15; /* byte index */
        pknum[j] = 12; // start with positive sign
        while (i > -1) {
            ch1 = str.charAt(i);

It might (!) be faster to first get the complete char[] from the String
and then to iterate over it.

Of course it would of course be great (if possible) if the
method-signature would accept a char[] instead of a String.

Note that a loop with an increasing array-index is *much* faster in most
cases (due to processor-caches) - if your logic allows that!

             switch (ch1) {
            case '0':
            case '1':
            case '2':
            case '3':
            case '4':
            case '5':
            case '6':
            case '7':
            case '8':
            case '9':

Here, an if might be faster (but might be slower as well - measure it):

if(c1>=0 && ch1<=9) {
} else if (...

An if-cascade, of course, is much faster if you know the statistical
probability of the cases. If e.g. you know, that 99% of all characters
are an '-', then put the corresponding if-statement at the beginning!

                 nibble = (byte) Character.getNumericValue(ch1);
                if (nibble_ordinal) {
                    pknum[j] = (byte) (pknum[j] | nibble);
                    nibble_ordinal ^= true;
                } else {
                    pknum[j] = (byte) (pknum[j] | nibble << 4);
                    nibble_ordinal ^= true;
                --i; // get next char
            case ',':
            case ' ':
            case '.':
            case '$':
            case '+':
                --i; // get next char
            case '-':
                pknum[15] = (byte) (pknum[15] & 0xf0);
                pknum[15] = (byte) (pknum[15] | 0x0d);

This is slow: Why not write the result in a temporary variable first:

byte tmp = (byte) (pknum[15] & 0xf0);
pknum[15] = (byte) (tmp | 0x0d);

....or even do it in one single line:

pknum[15] = (byte) ((pknum[15] & 0xf0)| 0x0d);

                 --i; // get next char
                throw new DataException("Invalid decimal digit: " + ch1);
        return (pknum);

And, of course, if you have to convert millions of Strings but there are
only a hand full of *different* Strings (perhaps 1000 or 100.000), then
it is often a good idea to initialize a (Hash)Map with the Strings as
key and the result as value and only lookup the results.


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