The char data type is based on the original Unicode specification, which defined characters as fixed-width 16-bit entities. The Unicode Standard has since been changed to allow for characters whose representation requires more than 16 bits. The range of Unicode code points is now U+0000 to U+10FFFF. The set of characters from U+0000 to U+FFFF is referred to as the basic multilingual plane (BMP) while characters whose code points are greater than U+FFFF are called supplementary characters. Such characters are generally rare, but some are used, for example, as part of Chinese and Japanese personal names. To support supplementary characters without changing the char primitive data type and causing incompatibility with previous Java programs, supplementary characters are defined by a pair of Unicode code units called surrogates. According to the Java API [API 2014] class Character documentation (Unicode Character Representations):
...
Unfortunately, the trim() method may fail because it is using the character form of the Character.isLetter() method. Methods that only accept a char value cannot support supplementary characters. According to the Java API [API 2014] class Character documentation:
...
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
STR01-J | low | unlikely | medium | P2 | L3 |
Bibliography
[API 2014] | Classes |
Character Boundaries |