Find out number of characters in a UTF-8 string in Java/Android

The shortest "length" is in Unicode code points, as notion of numbered character, UTF-32.

Correction: As @liudongmiao mentioned probably one should use:

int length = string.codePointCount(0, s.length);

In java 8:

int length = (int) string.codePoints().count();

Prior javas:

int length(String s) {
   int n = 0;
   for (int i = 0; i < s.length(); ++n) {
       int cp = s.codePointAt(i);
       i += Character.charCount(cp);
   }
   return n;
}

A Unicode code point can be encoded in UTF-16 as one or two chars.

The same Unicode character might have diacritical marks. They can be written as separate code points: basic letter + zero or more diacritical marks. To normalize the string to one (C=) compressed code point:

string = java.text.Normalizer.normalize(string, Normalizer.Form.NFC);

BTW for database purposes, the UTF-16 length seems more useful:

string.length() // Number of UTF-16 chars, every char two bytes.

(In the example mentioned UTF-32 length == UTF-16 length.)

---

A dump function

A commenter had some unexpected result:

void dump(String s) {
   int n = 0;
   for (int i = 0; i < s.length(); ++n) {
       int cp = s.codePointAt(i);
       int bytes = Character.charCount(cp);
       i += bytes;
       System.out.printf("[%d] #%dB: U+%X = %s%n",
           n, bytes, cp, Character.getName(cp));
   }
   System.out.printf("Length:%d%n", n);
}

Take a look at the http://rosettacode.org/wiki/String_length#Grapheme_Length_4:

import java.text.BreakIterator;

public class Grapheme {
  public static void main(String[] args) {
    printLength("møøse");
    printLength("");
    printLength("J̲o̲s̲é̲");
  }

  public static void printLength(String s) {
    BreakIterator it = BreakIterator.getCharacterInstance();
    it.setText(s);
    int count = 0;
    while (it.next() != BreakIterator.DONE) {
      count++;
    }
    System.out.println("Grapheme length: " + count+ " " + s);
  }
}

Output:

Grapheme length: 5 møøse
Grapheme length: 7 
Grapheme length: 4 J̲o̲s̲é̲

What you are looking for is not string length bu grapeme length. It gives you the number of "visible" characters.

String.length() actually returns the number of characters in a string encoded in UTF-16 (where two bytes are used to encode a character). However this should work for most UTF-8 chars too unless you have a character with an ASCII value greater than 127. If you want to do things by hand without encoding it to UTF-8, you can do something like this

public static int utf8Length(CharSequence sequence) {
        int count = 0;
        for (int i = 0; i < sequence.length(); i++) {
            char ch = sequence.charAt(i);
            if (ch <= 0x7F) {
                count++;
            } else if (ch <= 0x7FF) {
                count += 2;
            } else if (Character.isHighSurrogate(ch)) {
                count += 4;
                ++i;
            } else {
                count += 3;
            }
        }
        return count;
    }

Here's the UTF-8 spec.