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 called the basic multilingual plane (BMP), and 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):
The Java platform uses the UTF-16 representation in
chararrays and in the
StringBufferclasses. In this representation, supplementary characters are represented as a pair of
charvalues, the first from the high-surrogates range, (\uD800-\uDBFF), the second from the low-surrogates range (\uDC00-\uDFFF).
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
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.
Noncompliant Code Example
This noncompliant code example attempts to trim leading letters from
trim() method may fail because it is using the character form of the
Character.isLetter() method. Methods that accept only a
char value cannot support supplementary characters. According to the Java API [API 2014] class
charvalues from the surrogate ranges as undefined characters. For example,
false, even though this specific value if followed by any low-surrogate value in a string would represent a letter.
This 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.
Forming strings consisting of partial characters can result in unexpected behavior.
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