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CHAPTER 3

Lexical Structure

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

http://www.unicode.org

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.

3.2 Lexical Translations

1. A translation of Unicode escapes (§3.3) in the raw stream of Unicode characters to the corresponding Unicode character. A Unicode escape of the form \uxxxx, where xxxx is a hexadecimal value, represents the UTF-16 code unit whose encoding is xxxx. This translation step allows any program to be expressed using only ASCII characters.
2. A translation of the Unicode stream resulting from step 1 into a stream of input characters and line terminators (§3.4).
3. A translation of the stream of input characters and line terminators resulting from step 2 into a sequence of input elements (§3.5) which, after white space (§3.6) and comments (§3.7) are discarded, comprise the tokens (§3.5) that are the terminal symbols of the syntactic grammar (§2.3).

3.3 Unicode Escapes

UnicodeInputCharacter:
        UnicodeEscape
        RawInputCharacter

UnicodeEscape:
        \ UnicodeMarker HexDigit HexDigit HexDigit HexDigit

UnicodeMarker:
        u
        UnicodeMarker u

RawInputCharacter:
        any Unicode character

HexDigit: one of
        0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

In addition to the processing implied by the grammar, for each raw input character that is a backslash \, input processing must consider how many other \ characters contiguously precede it, separating it from a non-\ character or the start of the input stream. If this number is even, then the \ is eligible to begin a Unicode escape; if the number is odd, then the \ is not eligible to begin a Unicode escape. For example, the raw input "\\u2297=\u2297" results in the eleven characters " \ \ u 2 2 9 7 = " (\u2297 is the Unicode encoding of the character "").

If an eligible \ is not followed by u, then it is treated as a RawInputCharacter and remains part of the escaped Unicode stream. If an eligible \ is followed by u, or more than one u, and the last u is not followed by four hexadecimal digits, then a compile-time error occurs.

The character produced by a Unicode escape does not participate in further Unicode escapes. For example, the raw input \u005cu005a results in the six characters \ u 0 0 5 a, because 005c is the Unicode value for \. It does not result in the character Z, which is Unicode character 005a, because the \ that resulted from the \u005c is not interpreted as the start of a further Unicode escape.

The Java programming language specifies a standard way of transforming a program written in Unicode into ASCII that changes a program into a form that can be processed by ASCII-based tools. The transformation involves converting any Unicode escapes in the source text of the program to ASCII by adding an extra u-for example, \uxxxx becomes \uuxxxx-while simultaneously converting non-ASCII characters in the source text to Unicode escapes containing a single u each.

This transformed version is equally acceptable to a compiler for the Java programming language ("Java compiler") and represents the exact same program. The exact Unicode source can later be restored from this ASCII form by converting each escape sequence where multiple u's are present to a sequence of Unicode characters with one fewer u, while simultaneously converting each escape sequence with a single u to the corresponding single Unicode character.

Implementations should use the \uxxxx notation as an output format to display Unicode characters when a suitable font is not available.

3.4 Line Terminators

LineTerminator:
        the ASCII LF character, also known as "newline"
        the ASCII CR character, also known as "return"
  the ASCII CR character followed by the ASCII LF character

InputCharacter:
        UnicodeInputCharacter but not CR or LF

The result is a sequence of line terminators and input characters, which are the terminal symbols for the third step in the tokenization process.

3.5 Input Elements and Tokens

This process is specified by the following productions:

Input:
        InputElementsopt Subopt

InputElements:
        InputElement
        InputElements InputElement

InputElement:
        WhiteSpace
        Comment
        Token

Token:
        Identifier
        Keyword
        Literal
        Separator
        Operator

Sub:
        the ASCII SUB character, also known as "control-Z"

As a special concession for compatibility with certain operating systems, the ASCII SUB character (\u001a, or control-Z) is ignored if it is the last character in the escaped input stream.

Consider two tokens x and y in the resulting input stream. If x precedes y, then we say that x is to the left of y and that y is to the right of x.

For example, in this simple piece of code:

class Empty {
}

3.6 White Space

WhiteSpace:
        the ASCII SP character, also known as "space"
        the ASCII HT character, also known as "horizontal tab"
        the ASCII FF character, also known as "form feed"
        LineTerminator

3.7 Comments

/* text */               A traditional comment: all the 
                        text from the ASCII characters /* 
                        to the ASCII characters */ is ignored
                        (as in C and C++).

// text                  A end-of-line comment: all the text 
                        from the ASCII characters // to 
                        the end of the line is ignored (as in C++).

Comment:
        TraditionalComment
        EndOfLineComment

TraditionalComment:
        / * CommentTail

EndOfLineComment:
        / / CharactersInLineopt 

CommentTail:
        * CommentTailStar
        NotStar CommentTail

CommentTailStar:
        /
        * CommentTailStar
        NotStarNotSlash CommentTail

NotStar:
        InputCharacter but not *
        LineTerminator

NotStarNotSlash:
        InputCharacter but not * or /
        LineTerminator

CharactersInLine:
        InputCharacter
        CharactersInLine InputCharacter
        

• Comments do not nest.
• /* and */ have no special meaning in comments that begin with //.
• // has no special meaning in comments that begin with /* or /**.

/* this comment /* // /** ends here: */

The lexical grammar implies that comments do not occur within character literals (§3.10.4) or string literals (§3.10.5).

3.8 Identifiers

Identifier:
        IdentifierChars but not a Keyword or BooleanLiteral or NullLiteral

IdentifierChars:
        JavaLetter
        IdentifierChars JavaLetterOrDigit

JavaLetter:
        any Unicode character that is a Java letter (see below)

JavaLetterOrDigit:
        any Unicode character that is a Java letter-or-digit (see below)

A "Java letter" is a character for which the method Character.isJavaIdentifierStart(int) returns true. A "Java letter-or-digit" is a character for which the method Character.isJavaIdentifierPart(int) returns true.

The Java letters include uppercase and lowercase ASCII Latin letters A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical reasons, the ASCII underscore (_, or \u005f) and dollar sign ($, or \u0024). The $ character should be used only in mechanically generated source code or, rarely, to access preexisting names on legacy systems.

The "Java digits" include the ASCII digits 0-9 (\u0030-\u0039).

Two identifiers are the same only if they are identical, that is, have the same Unicode character for each letter or digit.

Identifiers that have the same external appearance may yet be different. For example, the identifiers consisting of the single letters LATIN CAPITAL LETTER A (A, \u0041), LATIN SMALL LETTER A (a, \u0061), GREEK CAPITAL LETTER ALPHA (A, \u0391), CYRILLIC SMALL LETTER A (a, \u0430) and MATHEMATICAL BOLD ITALIC SMALL A (a, \ud835\udc82) are all different.

Unicode composite characters are different from the decomposed characters. For example, a LATIN CAPITAL LETTER A ACUTE (Á, \u00c1) could be considered to be the same as a LATIN CAPITAL LETTER A (A, \u0041) immediately followed by a NON-SPACING ACUTE (´, \u0301) when sorting, but these are different in identifiers. See The Unicode Standard, Volume 1, pages 412ff for details about decomposition, and see pages 626-627 of that work for details about sorting. Examples of identifiers are:

       String                  i3                                   MAX_VALUE                               isLetterOrDigit

3.9 Keywords

Keyword: one of
        abstract    continue    for           new          switch
        assert      default     if            package      synchronized
        boolean     do          goto          private      this
        break       double      implements    protected    throw
        byte        else        import        public       throws
        case        enum        instanceof    return       transient
        catch       extends     int           short        try
        char        final       interface     static       void 
        class       finally     long          strictfp     volatile
        const       float       native        super        while                        
        

The keywords const and goto are reserved, even though they are not currently used. This may allow a Java compiler to produce better error messages if these C++ keywords incorrectly appear in programs.

While true and false might appear to be keywords, they are technically Boolean literals (§3.10.3). Similarly, while null might appear to be a keyword, it is technically the null literal (§3.10.7).

3.10 Literals

Literal:
        IntegerLiteral
        FloatingPointLiteral
        BooleanLiteral
        CharacterLiteral
        StringLiteral
        NullLiteral
        

3.10.1 Integer Literals

An integer literal may be expressed in decimal (base 10), hexadecimal (base 16), or octal (base 8):

IntegerLiteral:
        DecimalIntegerLiteral
        HexIntegerLiteral       
        OctalIntegerLiteral

DecimalIntegerLiteral:
        DecimalNumeral IntegerTypeSuffixopt

HexIntegerLiteral:
        HexNumeral IntegerTypeSuffixopt

OctalIntegerLiteral:    
        OctalNumeral IntegerTypeSuffixopt

IntegerTypeSuffix: one of
        l L

A decimal numeral is either the single ASCII character 0, representing the integer zero, or consists of an ASCII digit from 1 to 9, optionally followed by one or more ASCII digits from 0 to 9, representing a positive integer:

DecimalNumeral:
        0
        NonZeroDigit Digitsopt

Digits:
        Digit
        Digits Digit

Digit:
        0
        NonZeroDigit

NonZeroDigit: one of
        1 2 3 4 5 6 7 8 9

HexNumeral:
        0 x HexDigits
        0 X HexDigits

HexDigits:
        HexDigit
        HexDigit HexDigits

HexDigit: one of
        0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

OctalNumeral:
        0 OctalDigits

OctalDigits:
        OctalDigit
        OctalDigit OctalDigits

OctalDigit: one of
        0 1 2 3 4 5 6 7

The largest decimal literal of type int is 2147483648 (2³¹). All decimal literals from 0 to 2147483647 may appear anywhere an int literal may appear, but the literal 2147483648 may appear only as the operand of the unary negation operator -.

The largest positive hexadecimal and octal literals of type int are 0x7fffffff and 017777777777, respectively, which equal 2147483647 (2³¹-1). The most negative hexadecimal and octal literals of type int are 0x80000000 and 020000000000, respectively, each of which represents the decimal value -2147483648 (-2³¹). The hexadecimal and octal literals 0xffffffff and 037777777777, respectively, represent the decimal value -1.

A compile-time error occurs if a decimal literal of type int is larger than 2147483648 (2³¹), or if the literal 2147483648 appears anywhere other than as the operand of the unary - operator, or if a hexadecimal or octal int literal does not fit in 32 bits.

Examples of int literals:

       0       2       0372    0xDadaCafe      1996     0x00FF00FF

The largest positive hexadecimal and octal literals of type long are 0x7fffffffffffffffL and 0777777777777777777777L, respectively, which equal 9223372036854775807L (2⁶³-1). The literals 0x8000000000000000L and 01000000000000000000000L are the most negative long hexadecimal and octal literals, respectively. Each has the decimal value -9223372036854775808L (-2⁶³). The hexadecimal and octal literals 0xffffffffffffffffL and 01777777777777777777777L, respectively, represent the decimal value -1L.

A compile-time error occurs if a decimal literal of type long is larger than 9223372036854775808L (2⁶³), or if the literal 9223372036854775808L appears anywhere other than as the operand of the unary - operator, or if a hexadecimal or octal long literal does not fit in 64 bits.

Examples of long literals:

       0l      0777L   0x100000000L    2147483648L     0xC0B0L

3.10.2 Floating-Point Literals

A floating-point literal has the following parts: a whole-number part, a decimal or hexadecimal point (represented by an ASCII period character), a fractional part, an exponent, and a type suffix. A floating point number may be written either as a decimal value or as a hexadecimal value. For decimal literals, the exponent, if present, is indicated by the ASCII letter e or E followed by an optionally signed integer. For hexadecimal literals, the exponent is always required and is indicated by the ASCII letter p or P followed by an optionally signed integer.

For decimal floating-point literals, at least one digit, in either the whole number or the fraction part, and either a decimal point, an exponent, or a float type suffix are required. All other parts are optional. For hexadecimal floating-point literals, at least one digit is required in either the whole number or fraction part, the exponent is mandatory, and the float type suffix is optional.

A floating-point literal is of type float if it is suffixed with an ASCII letter F or f; otherwise its type is double and it can optionally be suffixed with an ASCII letter D or d.

FloatingPointLiteral:
        DecimalFloatingPointLiteral
        HexadecimalFloatingPointLiteral

DecimalFloatingPointLiteral:
        Digits . Digitsopt ExponentPartopt FloatTypeSuffixopt
      . Digits ExponentPartopt FloatTypeSuffixopt
      Digits ExponentPart FloatTypeSuffixopt
        Digits ExponentPartopt FloatTypeSuffix

ExponentPart:
        ExponentIndicator SignedInteger

ExponentIndicator: one of
        e E

SignedInteger:
        Signopt Digits

Sign: one of
        + -

FloatTypeSuffix: one of
        f F d D

HexadecimalFloatingPointLiteral:
        HexSignificand BinaryExponent FloatTypeSuffixopt

HexSignificand:
        HexNumeral
        HexNumeral .
        0x HexDigitsopt . HexDigits
        0X HexDigitsopt . HexDigits

BinaryExponent:
        BinaryExponentIndicator SignedInteger

BinaryExponentIndicator:one of
        p P

The details of proper input conversion from a Unicode string representation of a floating-point number to the internal IEEE 754 binary floating-point representation are described for the methods valueOf of class Float and class Double of the package java.lang.

The largest positive finite float literal is 3.4028235e38f. The smallest positive finite nonzero literal of type float is 1.40e-45f. The largest positive finite double literal is 1.7976931348623157e308. The smallest positive finite nonzero literal of type double is 4.9e-324.

A compile-time error occurs if a nonzero floating-point literal is too large, so that on rounded conversion to its internal representation it becomes an IEEE 754 infinity. A program can represent infinities without producing a compile-time error by using constant expressions such as 1f/0f or -1d/0d or by using the predefined constants POSITIVE_INFINITY and NEGATIVE_INFINITY of the classes Float and Double.

A compile-time error occurs if a nonzero floating-point literal is too small, so that, on rounded conversion to its internal representation, it becomes a zero. A compile-time error does not occur if a nonzero floating-point literal has a small value that, on rounded conversion to its internal representation, becomes a nonzero denormalized number.

Predefined constants representing Not-a-Number values are defined in the classes Float and Double as Float.NaN and Double.NaN.

Examples of float literals:

       1e1f    2.f     .3f     0f      3.14f   6.022137e+23f

Examples of double literals:

        1e1     2.      .3      0.0     3.14    1e-9d   1e137

Besides expressing floating-point values in decimal and hexadecimal, the method intBitsToFloat of class Float and method longBitsToDouble of class Double provide a way to express floating-point values in terms of hexadecimal or octal integer literals.For example, the value of:

Double.longBitsToDouble(0x400921FB54442D18L)

3.10.3 Boolean Literals

A boolean literal is always of type boolean.

BooleanLiteral: one of
        true false

3.10.4 Character Literals

A character literal is always of type char.

CharacterLiteral:
        ' SingleCharacter '
        ' EscapeSequence '

SingleCharacter:
        InputCharacter but not ' or \

As specified in §3.4, the characters CR and LF are never an InputCharacter; they are recognized as constituting a LineTerminator.

It is a compile-time error for the character following the SingleCharacter or EscapeSequence to be other than a '.

It is a compile-time error for a line terminator to appear after the opening ' and before the closing '.

The following are examples of char literals:

'a'
'%'
'\t'
'\\'
'\''
'\u03a9'
'\uFFFF'
'\177'
''
''

Because Unicode escapes are processed very early, it is not correct to write '\u000a' for a character literal whose value is linefeed (LF); the Unicode escape \u000a is transformed into an actual linefeed in translation step 1 (§3.3) and the linefeed becomes a LineTerminator in step 2 (§3.4), and so the character literal is not valid in step 3. Instead, one should use the escape sequence '\n' (§3.10.6). Similarly, it is not correct to write '\u000d' for a character literal whose value is carriage return (CR). Instead, use '\r'.

In C and C++, a character literal may contain representations of more than one character, but the value of such a character literal is implementation-defined. In the Java programming language, a character literal always represents exactly one character.

3.10.5 String Literals

A string literal is always of type String (§4.3.3). A string literal always refers to the same instance (§4.3.1) of class String.

StringLiteral:
        " StringCharactersopt "

StringCharacters:
        StringCharacter
        StringCharacters StringCharacter

StringCharacter:
        InputCharacter but not " or \
        EscapeSequence

The escape sequences are described in §3.10.6.

As specified in §3.4, neither of the characters CR and LF is ever considered to be an InputCharacter; each is recognized as constituting a LineTerminator.

It is a compile-time error for a line terminator to appear after the opening " and before the closing matching ". A long string literal can always be broken up into shorter pieces and written as a (possibly parenthesized) expression using the string concatenation operator + (§15.18.1).

The following are examples of string literals:

""                                      // the empty string
"\""                                    // a string containing " alone
"This is a string"                      // a string containing 16 characters
"This is a " +                          // actually a string-valued constant expression,
        "two-line string"               //              formed from two string literals

Because Unicode escapes are processed very early, it is not correct to write "\u000a" for a string literal containing a single linefeed (LF); the Unicode escape \u000a is transformed into an actual linefeed in translation step 1 (§3.3) and the linefeed becomes a LineTerminator in step 2 (§3.4), and so the string literal is not valid in step 3. Instead, one should write "\n" (§3.10.6). Similarly, it is not correct to write "\u000d" for a string literal containing a single carriage return (CR). Instead use "\r".

Each string literal is a reference (§4.3) to an instance (§4.3.1, §12.5) of class String (§4.3.3). String objects have a constant value. String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are "interned" so as to share unique instances, using the method String.intern.

Thus, the test program consisting of the compilation unit (§7.3):

package testPackage;
class Test {
        public static void main(String[] args) {
                String hello = "Hello", lo = "lo";
                System.out.print((hello == "Hello") + " ");
                System.out.print((Other.hello == hello) + " ");
                System.out.print((other.Other.hello == hello) + " ");
                System.out.print((hello == ("Hel"+"lo")) + " ");
                System.out.print((hello == ("Hel"+lo)) + " ");
                System.out.println(hello == ("Hel"+lo).intern());
        }
}
class Other { static String hello = "Hello"; }

package other;
public class Other { static String hello = "Hello"; }

true true true true false true

• Literal strings within the same class (§8) in the same package (§7) represent references to the same String object (§4.3.1).
• Literal strings within different classes in the same package represent references to the same String object.
• Literal strings within different classes in different packages likewise represent references to the same String object.
• Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.
• Strings computed by concatenation at run time are newly created and therefore distinct.

The result of explicitly interning a computed string is the same string as any pre-existing literal string with the same contents.

3.10.6 Escape Sequences for Character and String Literals

EscapeSequence:
        \ b                     /* \u0008: backspace BS                   */
        \ t                     /* \u0009: horizontal tab HT              */
        \ n                     /* \u000a: linefeed LF                    */
        \ f                     /* \u000c: form feed FF           */
        \ r                     /* \u000d: carriage return CR             */
        \ "                     /* \u0022: double quote "             */
        \ '                     /* \u0027: single quote '          */
        \ \                     /* \u005c: backslash \                     */
        OctalEscape               /* \u0000 to \u00ff: from octal value   */

OctalEscape:
        \ OctalDigit
        \ OctalDigit OctalDigit
        \ ZeroToThree OctalDigit OctalDigit

OctalDigit: one of
        0 1 2 3 4 5 6 7

ZeroToThree: one of
        0 1 2 3

3.10.7 The Null Literal

NullLiteral:
        null

3.11 Separators

Separator: one of
        (       )       {       }       [       ]       ;       ,       .

3.12 Operators

Operator: one of
        =       >    <    !       ~       ?       :
        ==      <=   >=   !=      &&      ||      ++      --
        +       -       *       /       &   |       ^       %       <<        >>        >>>
        +=      -=      *=      /=      &=  |=      ^=      %=      <<=       >>=       >>>=

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Java Language Specification Third Edition

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