Chapter 3. Lexical Structure

An integer literal may be expressed in decimal (base 10), hexadecimal (base 16), octal (base 8), or binary (base 2).

An integer literal is of type long if it is suffixed with an ASCII letter L or l (ell); otherwise it is of type int (§4.2.1).

The suffix L is preferred, because the letter l (ell) is often hard to distinguish from the digit 1 (one).

Underscores are allowed as separators between digits that denote the integer.

In a hexadecimal or binary literal, the integer is only denoted by the digits after the 0x or 0b characters and before any type suffix. Therefore, underscores may not appear immediately after 0x or 0b, or after the last digit in the numeral.

In a decimal or octal literal, the integer is denoted by all the digits in the literal before any type suffix. Therefore, underscores may not appear before the first digit or after the last digit in the numeral. Underscores may appear after the initial 0 in an octal numeral (since 0 is a digit that denotes part of the integer) and after the initial non-zero digit in a non-zero decimal literal.

A decimal numeral is either the single ASCII digit 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 interspersed with underscores, representing a positive integer.

A hexadecimal numeral consists of the leading ASCII characters 0x or 0X followed by one or more ASCII hexadecimal digits interspersed with underscores, and can represent a positive, zero, or negative integer.

Hexadecimal digits with values 10 through 15 are represented by the ASCII letters a through f or A through F, respectively; each letter used as a hexadecimal digit may be uppercase or lowercase.

The HexDigit production above comes from §3.3.

An octal numeral consists of an ASCII digit 0 followed by one or more of the ASCII digits 0 through 7 interspersed with underscores, and can represent a positive, zero, or negative integer.

Note that octal numerals always consist of two or more digits, as 0 alone is always considered to be a decimal numeral - not that it matters much in practice, for the numerals 0, 00, and 0x0 all represent exactly the same integer value.

A binary numeral consists of the leading ASCII characters 0b or 0B followed by one or more of the ASCII digits 0 or 1 interspersed with underscores, and can represent a positive, zero, or negative integer.

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. The decimal literal 2147483648 may appear only as the operand of the unary minus operator - (§15.15.4).

It is a compile-time error if the decimal literal 2147483648 appears anywhere other than as the operand of the unary minus operator; or if a decimal literal of type int is larger than 2147483648 (2³¹).

The largest positive hexadecimal, octal, and binary literals of type int - each of which represents the decimal value 2147483647 (2³¹-1) - are respectively:

The most negative hexadecimal, octal, and binary literals of type int - each of which represents the decimal value -2147483648 (-2³¹) - are respectively:

The following hexadecimal, octal, and binary literals represent the decimal value -1:

It is a compile-time error if a hexadecimal, octal, or binary int literal does not fit in 32 bits.

The largest decimal literal of type long is 9223372036854775808L (2⁶³).

All decimal literals from 0L to 9223372036854775807L may appear anywhere a long literal may appear. The decimal literal 9223372036854775808L may appear only as the operand of the unary minus operator - (§15.15.4).

It is a compile-time error if the decimal literal 9223372036854775808L appears anywhere other than as the operand of the unary minus operator; or if a decimal literal of type long is larger than 9223372036854775808L (2⁶³).

The largest positive hexadecimal, octal, and binary literals of type long - each of which represents the decimal value 9223372036854775807L (2⁶³-1) - are respectively:

The most negative hexadecimal, octal, and binary literals of type long - each of which represents the decimal value -9223372036854775808L (-2⁶³) - are respectively:

The following hexadecimal, octal, and binary literals represent the decimal value -1L:

It is a compile-time error if a hexadecimal, octal, or binary long literal does not fit in 64 bits.

0    2    0372    0xDada_Cafe    1996    0x00_FF__00_FF

0l    0777L    0x100000000L    2_147_483_648L    0xC0B0L

A floating-point literal has the following parts: a whole-number part, a decimal or hexadecimal point (represented by an ASCII period character), a fraction part, an exponent, and a type suffix.

A floating-point literal may be expressed in decimal (base 10) or hexadecimal (base 16).

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. The exponent, if present, is indicated by the ASCII letter e or E followed by an optionally signed integer.

For hexadecimal floating-point literals, at least one digit is required (in either the whole number or the fraction part), and the exponent is mandatory, and the float type suffix is optional. The exponent is indicated by the ASCII letter p or P followed by an optionally signed integer.

Underscores are allowed as separators between digits that denote the whole-number part, and between digits that denote the fraction part, and between digits that denote the exponent.

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.

The elements of the types float and double are those values that can be represented using the IEEE 754 binary32 and IEEE 754 binary64 floating-point formats, respectively (§4.2.3).

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 and smallest positive literals of type float are as follows:

The largest and smallest positive literals of type double are as follows:

It is a compile-time error if a non-zero 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.

It is a compile-time error if a non-zero 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 non-zero floating-point literal has a small value that, on rounded conversion to its internal representation, becomes a non-zero subnormal number.

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

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

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

The boolean type has two values, represented by the boolean literals true and false, formed from ASCII letters.

A boolean literal is always of type boolean (§4.2.5).

A character literal is expressed as a character or an escape sequence (§3.10.7), enclosed in ASCII single quotes. (The single-quote, or apostrophe, character is \u0027.)

A character literal is always of type char (§4.2.1).

The content of a character literal is the SingleCharacter or the EscapeSequence which follows the opening '.

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

It is a compile-time error for a line terminator (§3.4) to appear after the opening ' and before the closing '.

The characters CR and LF are never an InputCharacter; each is recognized as constituting a LineTerminator, so may not appear in a character literal, even in the escape sequence \ LineTerminator.

The character represented a character literal is the content of the character literal with any escape sequence interpreted, as if by execution of String.translateEscapes on the content.

Character literals can only represent UTF-16 code units (§3.1), i.e., they are limited to values from \u0000 to \uffff. Supplementary characters must be represented either as a surrogate pair within a char sequence, or as an integer, depending on the API they are used with.

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), so the character literal is not valid in step 3. Instead, one should use the escape sequence '\n'. Similarly, it is not correct to write '\u000d' for a character literal whose value is carriage return (CR). Instead, use '\r'. Finally, it is not possible to write '\u0027' for a character literal containing an apostrophe (').

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.

A string literal consists of zero or more characters enclosed in double quotes. Characters such as newlines may be represented by escape sequences (§3.10.7).

A string literal is always of type String (§4.3.3).

The content of a string literal is the sequence of characters that begins immediately after the opening " and ends immediately before the matching closing ".

It is a compile-time error for a line terminator (§3.4) to appear after the opening " and before the matching closing ".

The characters CR and LF are never an InputCharacter; each is recognized as constituting a LineTerminator, so may not appear in a string literal, even in the escape sequence \ LineTerminator.

The string represented by a string literal is the content of the string literal with every escape sequence interpreted, as if by execution of String.translateEscapes on the content.

""                    // 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), so the string literal is not valid in step 3. Instead, one should use the escape sequence "\n". Similarly, it is not correct to write "\u000d" for a string literal containing a single carriage return (CR). Instead, use "\r". Finally, it is not possible to write "\u0022" for a string literal containing a double quotation mark (").

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).

At run time, a string literal is a reference to an instance of class String (§4.3.3) that denotes the string represented by the string literal.

Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.29) - are "interned" so as to share unique instances, as if by execution of the method String.intern (§12.5).

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

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

true
true
true
true
false
true

A text block consists of zero or more characters enclosed by opening and closing delimiters. Characters may be represented by escape sequences (§3.10.7), but the newline and double quote characters that must be represented with escape sequences in a string literal (§3.10.5) may be represented directly in a text block.

The following productions from §3.3, §3.4, and §3.6 are shown here for convenience:

A text block is always of type String (§4.3.3).

The opening delimiter is a sequence that starts with three double quote characters ("""), continues with zero or more space, tab, and form feed characters, and concludes with a line terminator.

The closing delimiter is a sequence of three double quote characters.

The content of a text block is the sequence of characters that begins immediately after the line terminator of the opening delimiter, and ends immediately before the first double quote of the closing delimiter.

Unlike in a string literal (§3.10.5), it is not a compile-time error for a line terminator to appear in the content of a text block.

String html = "<html>\n" +
              "    <body>\n" +
              "        <p>Hello, world</p>\n" +
              "    </body>\n" +
              "</html>\n";

String html = """
              <html>
                  <body>
                      <p>Hello, world</p>
                  </body>
              </html>
              """;

class Test {
    public static void main(String[] args) {
        // The six characters w i n t e r
        String season = """
                        winter""";

        // The seven characters w i n t e r LF
        String period = """
                        winter
                        """;

        // The ten characters H i , SP " B o b " LF
        String greeting = """
                          Hi, "Bob"
                          """;

        // The eleven characters H i , LF SP " B o b " LF
        String salutation = """
                            Hi,
                             "Bob"
                            """;        

        // The empty string (zero length)
        String empty = """
                       """;      

        // The two characters " LF
        String quote = """
                       "
                       """; 

        // The two characters \ LF
        String backslash = """
                           \\
                           """;  
    }
}

Using the escape sequences \n and \" to represent a newline character and a double quote character, respectively, is permitted in a text block, though not usually necessary. The exception is where three consecutive double quote characters appear that are not intended to be the closing delimiter """ - in this case, it is necessary to escape at least one of the double quote characters in order to avoid mimicking the closing delimiter.

class Story1 {
    public static void main(String[] args) {
        String story = """
            "When I use a word," Humpty Dumpty said,
            in rather a scornful tone, "it means just what I
            choose it to mean - neither more nor less."
            "The question is," said Alice, "whether you
            can make words mean so many different things."
            "The question is," said Humpty Dumpty,
            "which is to be master - that's all."
        """;
    }
}

class Story2 {
    public static void main(String[] args) {
        String story = """
            "When I use a word," Humpty Dumpty said,
            in rather a scornful tone, "it means just what I
            choose it to mean - neither more nor less."
            "The question is," said Alice, "whether you
            can make words mean so many different things."
            "The question is," said Humpty Dumpty,
            "which is to be master - that's all."""";  // error
    }
}

class Story3 {
    public static void main(String[] args) {
        String story = """
            "When I use a word," Humpty Dumpty said,
            in rather a scornful tone, "it means just what I
            choose it to mean - neither more nor less."
            "The question is," said Alice, "whether you
            can make words mean so many different things."
            "The question is," said Humpty Dumpty,
            "which is to be master - that's all.\"""";  // OK
    }
}

class Code {
    public static void main(String[] args) {
        String text = """
            The quick brown fox jumps over the lazy dog
        """;

        String code =
            """
            String text = \"""
                The quick brown fox jumps over the lazy dog
            \""";
            """;
    }
}

The string represented by a text block is not the literal sequence of characters in the content. Instead, the string represented by a text block is the result of applying the following transformations to the content, in order:

When this specification says that a text block contains a particular character or sequence of characters, or that a particular character or sequence of characters is in a text block, it means that the string represented by the text block (as opposed to the literal sequence of characters in the content) contains the character or sequence of characters.

String html = """
              <html>\r
                  <body>\r
                      <p>Hello, world</p>\r
                  </body>\r
              </html>\r
              """;

|<html>\u000D\u000A
|    <body>\u000D\u000A
|        <p>Hello, world</p>\u000D\u000A
|    </body>\u000D\u000A
|</html>\u000D\u000A

At run time, a text block is a reference to an instance of class String that denotes the string represented by the text block.

Moreover, a text block always refers to the same instance of class String. This is because the strings represented by text blocks - or, more generally, strings that are the values of constant expressions (§15.29) - are "interned" so as to share unique instances, as if by execution of the method String.intern (§12.5).

System.out.println("ab" + """
                          cde
                          """);

String cde = """
             abcde""".substring(2);

String math = """
              1+1 equals \
              """ + String.valueOf(2);

@Preconditions("""
    rate > 0 &&
    rate <= MAX_REFRESH_RATE
""")
public void setRefreshRate(int rate) { ... }

In character literals, string literals, and text blocks (§3.10.4, §3.10.5, §3.10.6), the escape sequences allow for the representation of some nongraphic characters without using Unicode escapes (§3.3), as well as the single quote, double quote, and backslash characters.

The OctalDigit production above comes from §3.10.1. Octal escapes are provided for compatibility with C, but can express only Unicode values \u0000 through \u00FF, so Unicode escapes are usually preferred.

It is a compile-time error if the character following a backslash in an escape sequence is not a LineTerminator or an ASCII b, s, t, n, f, r, ", ', \, 0, 1, 2, 3, 4, 5, 6, or 7.

An escape sequence in the content of a character literal, string literal, or text block is interpreted by replacing its \ and trailing character(s) with the single character denoted by the Unicode escape in the EscapeSequence grammar. The line continuation escape sequence has no corresponding Unicode escape, so is interpreted by replacing it with nothing.

The line continuation escape sequence can appear in a text block, but cannot appear in a character literal or a string literal because each disallows a LineTerminator.

The null type has one value, the null reference, represented by the null literal null, which is formed from ASCII characters.

A null literal is always of the null type (§4.1).