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The Lexical Structure of Java  from the Horse's Mouth

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This chapter from The Java Language Specification specifies the lexical structure of the Java programming language. It's written by James Gosling, Bill Joy, Guy Steele, and Gilad Bracha, so it doesn't get more authoritative than this. The chapter covers Unicode, lexical translations, Unicode escapes, line terminators, input elements and tokens, white space, comments, identifiers, keywords, literals, separators, and operators. If this chapter doesn't help you settle a Java programmer's bar bet, nothing will.


        Lexicographer: A writer of dictionaries, a harmless drudge.
                                             —Samuel Johnson, Dictionary (1755)

This chapter specifies the lexical structure of the Java programming language. Programs are written in Unicode (§3.1), but lexical translations are provided (§3.2) so that Unicode escapes (§3.3) can be used to include any Unicode character using only ASCII characters. Line terminators are defined (§3.4) to support the different conventions of existing host systems while maintaining consistent line numbers.

The Unicode characters resulting from the lexical translations are reduced to a sequence of input elements (§3.5), which are white space (§3.6), comments (§3.7), and tokens. The tokens are the identifiers (§3.8), keywords (§3.9), literals (§3.10), separators (§3.11), and operators (§3.12) of the syntactic grammar.

3.1 Unicode

Programs are written using the Unicode character set. Information about this character set and its associated character encodings may be found at:

http://www.unicode.org

The Java platform tracks the Unicode specification as it evolves. The precise version of Unicode used by a given release is specified in the documentation of the class Character.

Versions of the Java programming language prior to 1.1 used Unicode version 1.1.5. Upgrades to newer versions of the Unicode Standard occurred in JDK 1.1 (to Unicode 2.0), JDK 1.1.7 (to Unicode 2.1), J2SE 1.4 (to Unicode 3.0), and J2SE 5.0 (to Unicode 4.0).

The Unicode standard was originally designed as a fixed-width 16-bit character encoding. It 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, using the hexadecimal U+n notation. Characters whose code points are greater than U+FFFF are called supplementary characters. To represent the complete range of characters using only 16-bit units, the Unicode standard defines an encoding called UTF-16. In this encoding, supplementary characters are represented as pairs of 16-bit code units, the first from the high-surrogates range, (U+D800 to U+DBFF), the second from the low-surrogates range (U+DC00 to U+DFFF). For characters in the range U+0000 to U+FFFF, the values of code points and UTF-16 code units are the same.

The Java programming language represents text in sequences of 16-bit code units, using the UTF-16 encoding. A few APIs, primarily in the Character class, use 32-bit integers to represent code points as individual entities. The Java platform provides methods to convert between the two representations.

This book uses the terms code point and UTF-16 code unit where the representation is relevant, and the generic term character where the representation is irrelevant to the discussion.

Except for comments (§3.7), identifiers, and the contents of character and string literals (§3.10.4, §3.10.5), all input elements (§3.5) in a program are formed only from ASCII characters (or Unicode escapes (§3.3) which result in ASCII characters). ASCII (ANSI X3.4) is the American Standard Code for Information Interchange. The first 128 characters of the Unicode character encoding are the ASCII characters.

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