How does Java store UTF-16 characters in its 16-bit char type?

Question

According to the Java SE 7 Specification, Java uses the Unicode UTF-16 standard to represent characters. When imagining a String as a simple array of 1

Answer 1

The answer is in the javadoc :

The char data type (and therefore the value that a Character object encapsulates) are 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 legal code points is now U+0000 to U+10FFFF, known as Unicode scalar value. (Refer to the definition of the U+n notation in the Unicode standard.) The set of characters from U+0000 to U+FFFF is sometimes referred to as the Basic Multilingual Plane (BMP). Characters whose code points are greater than U+FFFF are called supplementary characters. The Java 2 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 Basic Multilingual Plane (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.

Unless otherwise specified, the behavior with respect to supplementary characters and surrogate char values is as follows: The methods that only accept a char value cannot support supplementary characters. 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. The methods that accept an int value support all Unicode characters, including supplementary characters. For example, Character.isLetter(0x2F81A) returns true because the code point value represents a letter (a CJK ideograph). In the Java SE API documentation, Unicode code point is used for character values in the range between U+0000 and U+10FFFF, and Unicode code unit is used for 16-bit char values that are code units of the UTF-16 encoding. For more information on Unicode terminology, refer to the Unicode Glossary.

Simply said :

• the 16 bits for a char rule was designed for an old version of the Unicode standard
• you sometimes need two chars to represent a unicode rune (code point) which isn't in the Basic Multilingual Plane. This kindof "works" because you don't frequently use chars, especially to handle unicode runes outside the BMP.

Even simpler said :

• a java char doesn't represent a Unicode codepoint (well, not always).

As an aside, it can be noted that the evolution of Unicode to extend past the BMP made UTF-16 globally irrelevant, now that UTF-16 doesn't even enable a fixed byte-chars ratio. That's why more modern languages are based on UTF-8. This manifesto helps understand it.

Answer 2

Basically, strings store a sequence of UTF-16 code units... which isn't the same as storing a sequence of Unicode code points.

When a character outside the Basic Multilingual Plane is required, that takes up two UTF-16 code units within the String.

Most String operations - length(), charAt, substring() etc deal in numbers of UTF-16 code units. However, there are operations like codePointAt() which will deal with full Unicode code points... although the indexes are still expressed in terms of UTF-16 code units.

EDIT: If you want to store a non-BMP code point in a single char, you're basically out of luck. It's like wanting to store more than 256 distinct values in a byte variable... it just doesn't work. Following the conventions for representing a code point elsewhere (e.g. in String) it's best to just use an int variable.