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The Java platform uses the UTF-16 representation in char arrays and in the String and StringBuffer classes. In this representation, supplementary characters are represented as a pair of char values, the first from the high-surrogates range, (\uD800-\uDBFF), the second from the low-surrogates range (\uDC00-\uDFFF)..

A char value, therefore, represents BMP code points, including the surrogate code points, or code units of the UTF-16 encoding. An int value represents all Unicode code points, including supplementary code points. The lower (least significant) 21 bits of int are used to represent Unicode code points, and the upper (most significant) 11 bits must be zero. Similar to UTF-8 (see STR00-J. Don't form strings containing partial characters from variable-width encodings), UTF-16 is a variable-width encoding. Because the UTF-16 representation is also used in char arrays and in the String and StringBuffer classes, care must be taken when manipulating string data in Java. In particular, do not write code that assumes that a value of the primitive type char (or a Character object) fully represents a Unicode code point. Conformance with this requirement typically requires using methods that accept a Unicode code point as an int value and avoiding methods that accept a Unicode code unit as a char value because these latter methods cannot support supplementary characters.

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They treat char values from the surrogate ranges as undefined characters. For example, Character.isLetter('\uD840') returns false, even though this specific value if followed by any low-surrogate value in a string would represent a letter.

Compliant Solution

This noncompliant code example compliant solution corrects the problem with supplementary characters by using the integer form of the Character.isLetter() method that accepts a Unicode code point as an int argument. Java library methods that accept an int value support all Unicode characters, including supplementary characters.  

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Forming strings consisting of partial characters can result in unexpected behavior.

Automated Detection

Bibliography

[API 2014]	Classes Character and BreakIterator
[Java Tutorials]	Character Boundaries
[Seacord 2015]	Image Modified STR01-J. Do not assume that a Java char fully represents a Unicode code point LiveLesson

[Tutorials 2008]

Character Boundaries

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