How can I iterate through the unicode codepoints of a Java String?

Question

So I know about String#codePointAt(int), but it's indexed by the char offset, not by the codepoint offset.

I'm thinking about trying something like:

• using String#charAt(int) to get the char at an index
• testing whether the char is in the high-surrogates range
  • if so, use String#codePointAt(int) to get the codepoint, and increment the index by 2
  • if not, use the given char value as the codepoint, and increment the index by 1

But my concerns are

• I'm not sure whether codepoints which are naturally in the high-surrogates range will be stored as two char values or one
• this seems like an awful expensive way to iterate through characters
• someone must have come up with something better.

Answer 1

Yes, Java uses a UTF-16-esque encoding for internal representations of Strings, and, yes, it encodes characters outside the Basic Multilingual Plane (BMP) using the surrogacy scheme.

If you know you'll be dealing with characters outside the BMP, then here is the canonical way to iterate over the characters of a Java String:

final int length = s.length();
for (int offset = 0; offset < length; ) {
   final int codepoint = s.codePointAt(offset);

   // do something with the codepoint

   offset += Character.charCount(codepoint);
}