Truncating Strings in Java by Bytes

Question

Hey all.

I create the following for truncating a string in java to a new string with a given number of bytes.

        String truncatedValue = "";
        String currentValue = string;
        int pivotIndex = (int) Math.round(((double) string.length())/2);
        while(!truncatedValue.equals(currentValue)){
            currentValue = string.substring(0,pivotIndex);
            byte[] bytes = null;
            bytes = currentValue.getBytes(encoding);
            if(bytes==null){
                return string;
            }
            int byteLength = bytes.length;
            int newIndex =  (int) Math.round(((double) pivotIndex)/2);
            if(byteLength > maxBytesLength){
                pivotIndex = newIndex;
            } else if(byteLength < maxBytesLength){
                pivotIndex = pivotIndex + 1;
            } else {
                truncatedValue = currentValue;
            }
        }
        return truncatedValue;

This is the first thing that came to my mind, and I know I could improve on it. I saw another post that was asking a similar question there, but they were truncating Strings using the bytes instead of String.substring. I think I would rather use String.substring in my case.

EDIT: I just removed the UTF8 reference because I would rather be able to do this for different storage types aswell.

Answer 1

Why not convert to bytes and walk forward--obeying UTF8 character boundaries as you do it--until you've got the max number, then convert those bytes back into a string?

Or you could just cut the original string if you keep track of where the cut should occur:

// Assuming that Java will always produce valid UTF8 from a string, so no error checking!
// (Is this always true, I wonder?)
public class UTF8Cutter {
  public static String cut(String s, int n) {
    byte[] utf8 = s.getBytes();
    if (utf8.length < n) n = utf8.length;
    int n16 = 0;
    boolean extraLong = false;
    int i = 0;
    while (i < n) {
      n16 += (extraLong) ? 2 : 1;
      extraLong = false;
      if ((utf8[i] & 0x80) == 0) i += 1;
      else if ((utf8[i] & 0xC0) == 0x80) i += 2;
      else if ((utf8[i] & 0xE0) == 0xC0) i += 3;
      else { i += 4; extraLong = true; }
    }
    return s.substring(0,n16);
  }
}

Answer 2

you could convert the string to bytes and convert just those bytes back to a string.

public static String substring(String text, int maxBytes) {
   StringBuilder ret = new StringBuilder();
   for(int i = 0;i < text.length(); i++) {
       // works out how many bytes a character takes, 
       // and removes these from the total allowed.
       if((maxBytes -= text.substring(i, i+1).getBytes().length) < 0) break;
       ret.append(text.charAt(i));
   }
   return ret.toString();
}