How to convert an alphanumeric phone number to digits

Question

UPDATE:

The final version of my utility looks like this:

StringBuilder b = new StringBuilder();

for(char c : inLetters.toLowerCase(         


        

Answer 1

Use a Map, where the keys are the letters and digits, and the value is the number on the keypad. (So each keypad number will be indexed by three or four letters and one digit).

Map keypad = new HashMap();
...
StringBuilder buf = new StringBuilder(inLetters.length());
for (int idx = 0; idx < inLetters.length(); ++idx) {
  Character ch = keypad.get(inLetters.charAt(idx));
  if (ch != null)
    buf.append(ch);
}

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Update: I was curious whether a hand-coded lookup table would perform better than a dense set switch cases. In my casual testing, I found the following code to be the fastest I could come up with:

  private static final char[] lut = 
    "0123456789:;<=>?@22233344455566677778889999[\\]^_`22233344455566677778889999".toCharArray();

  private static final char min = lut[0];

  String fastest(String letters)
  {
    int n = letters.length();
    char[] buf = new char[n];
    while (n-- > 0) {
      int ch = letters.charAt(n) - min;
      buf[n] = ((ch < 0) || (ch >= lut.length)) ? letters.charAt(n) : lut[ch];
    }
    return new String(buf);
  }

Surprisingly, it was more than twice as fast as similar code using a switch statement (which compiled to a tableswitch instruction). This was just for fun, mind you, but on my laptop, running in a single thread, I could convert 10 million 10-letter-"numbers" in about 1.3 seconds. I was really surprised, because as I understand it, a tableswitch operates in essentially the same way, but I expected it to be faster since it is a JVM instruction.

Of course, unless I were getting paid only for each of a limitless supply of phone numbers I could convert, I would never write code like this. A switch is much more readable, performs well as-is, and is likely to get a free performance boost in some future JVM.

Far and away, the greatest improvement to the original code comes from using a StringBuilder instead of concatenating strings, and that does nothing to impair readability of the code. Using charAt instead of converting the input to a char[] also makes the code simpler and easier to understand and improves performance too. Finally, appending char literals instead of String literals ('1' rather than "1") is a performance improvement that aids readability a little too.