Possible problems with String reversing using charAt method

Question

I saw a comment here that all solutions with charAt are wrong. I could not exactly understand and find something about charAt on internet. As I look the source code it just returns an element from the char array. So my question is that if there any problem or issue about using charAt?

Comment is like that

Strictly speaking, all the solutions based on charAt are wrong, as charAt doesn't give you the "character at", but the "code unit at", and there are code units that are not characters and characters that need multiple code units.

Answer 1

Different characters are encoded with a different numbers of bytes (using UTF-16 scheme). For example, the "A" character is represented as follows:

01000001

So far so good.

But if you have a character like 𝔴, you'll have a problem. Its UTF-16 representation (BE) is:

11011000 00110101 11011101 00110100

And then charAt can indeed return the second code unit for that character.

See the JDK 7 implementation of String#charAt:

public char charAt(int index) {
    if ((index < 0) || (index >= count)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index + offset];
}