java utf8 encoding - char, string types

Nothing in your code example is directly using UTF-8. Java strings are encoded in memory using UTF-16 instead. Unicode codepoints that do not fit in a single 16-bit char will be encoded using a 2-char pair known as a surrogate pair.

If you do not pass a parameter value to String.getBytes(), it returns a byte array that has the String contents encoded using the underlying OS's default charset. If you want to ensure a UTF-8 encoded array then you need to use getBytes("UTF-8") instead.

Calling String.charAt() returns an original UTF-16 encoded char from the String's in-memory storage only.

So in your example, the Unicode character ｮ is stored in the String in-memory storage using two bytes that are UTF-16 encoded (0x6E 0xFF or 0xFF 0x6E depending on endian), but is stored in the byte array from getBytes() using three bytes that are encoded using whatever the OS default charset is.

In UTF-8, that particular Unicode character happens to use 3 bytes as well (0xEF 0xBD 0xAE).

String.getBytes() returns the bytes using the platform's default character encoding which does not necessary match internal representation.

You're best of never using this method in most cases, because in most cases it does not make sense to rely on platform's default encoding. Use String.getBytes(String charsetName) instead and explicit specify the character set that should be used for encoding your String into bytes.