Java charAt used with characters that have two code units

It sounds like tho book is saying that ‘ℤ’ is not a UTF-16 character in the basic multilingual plane, but in fact it is.

Java uses UTF-16 with surrogate pairs for characters that are not in the basic multilingual plane. Since ‘ℤ’ (0x2124) is in the basic multilingual plane it is represented by a single code unit. In your example sentence.charAt(0) will return ‘ℤ’, and sentence.charAt(1) will return ‘ ‘.

A character represented by surrogate pairs has two code units making up the character. sentence.charAt(0) would return the first code unit, and sentence.charAt(1) would return the second code unit.


A String represents a string in the UTF-16 format in which
supplementary characters are represented by surrogate pairs (see the
section Unicode Character Representations in the Character class for
more information). Index values refer to char code units, so a
supplementary character uses two positions in a String.

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