CHAPTER 2: Grammars

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Grammars

This chapter describes the context-free grammars used in this specification to define the lexical and syntactic structure of a Java program.

2.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of one or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the infinite set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

2.2 The Lexical Grammar

A lexical grammar for Java is given in Chapter 3. This grammar has as its terminal symbols the characters of the Unicode character set. It defines a set of productions, starting from the goal symbol Input (S3.5), that describe how sequences of Unicode characters (S3.1) are translated into a sequence of input elements (S3.5).

These input elements, with white space (S3.6) and comments (S3.7) discarded, form the terminal symbols for the syntactic grammar for Java and are called Java tokens (S3.5). These tokens are the identifiers (S3.8), keywords (S3.9), literals (S3.10), separators (S3.11), and operators (S3.11) of the Java language.

2.3 The Syntactic Grammar

The syntactic grammar for Java is given in Chapters 4, 6-10, 14, and 15. This grammar has Java tokens defined by the lexical grammar as its terminal symbols. It defines a set of productions, starting from the goal symbol CompilationUnit (S7.3), that describe how sequences of tokens can form syntactically correct Java programs.

A LALR(1) version of the syntactic grammar is presented in Chapter 19. The grammar in the body of this specification is very similar to the LALR(1) grammar but more readable.

2.4 Grammar Notation

Terminal symbols are shown in fixed width font in the productions of the lexical and syntactic grammars, and throughout this specification whenever the text is directly referring to such a terminal symbol. These are to appear in a program exactly as written.

Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined followed by a colon. One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

IfThenStatement:

	if ( Expression ) Statement

states that the nonterminal IfThenStatement represents the token if , followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. As another example, the syntactic definition:

ArgumentList:

	Argument

	ArgumentList , Argument

states that an ArgumentList may represent either a single Argument or an ArgumentList, followed by a comma, followed by an Argument. This definition of ArgumentList is recursive, that is to say, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments. Such recursive definitions of nonterminals are common.

The subscripted suffix "opt", which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

BreakStatement:

	break Identifieropt ;

is a convenient abbreviation for:

BreakStatement:

	break ;

	break Identifier ;

and that:

ForStatement:

	for ( ForInitopt ; Expressionopt ; ForUpdateopt ) Statement

is a convenient abbreviation for:

ForStatement:

	for ( ; Expressionopt ; ForUpdateopt ) Statement

	for ( ForInit ; Expressionopt ; ForUpdateopt ) Statement

which in turn is an abbreviation for:

ForStatement:

	for ( ; ; ForUpdateopt ) Statement

	for ( ; Expression ; ForUpdateopt ) Statement

	for ( ForInit ; ; ForUpdateopt ) Statement

	for ( ForInit ; Expression ; ForUpdateopt ) Statement

which in turn is an abbreviation for:

ForStatement:

	for ( ; ; ) Statement

	for ( ; ; ForUpdate ) Statement

	for ( ; Expression ; ) Statement

	for ( ; Expression ; ForUpdate ) Statement

	for ( ForInit ; ; ) Statement

	for ( ForInit ; ; ForUpdate ) Statement

	for ( ForInit ; Expression ; ) Statement

	for ( ForInit ; Expression ; ForUpdate ) Statement

so the nonterminal ForStatement actually has eight alternative right-hand sides.

A very long right-hand side may be continued on a second line by substantially indenting this second line, as in:

ConstructorDeclaration:

	ConstructorModifiersopt ConstructorDeclarator

		Throwsopt ConstructorBody

which defines one right-hand side for the nonterminal ConstructorDeclaration. (This right-hand side is an abbreviation for four alternative right-hand sides, because of the two occurrences of "opt".)

When the words "one of" follow the colon in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for Java contains the production:

ZeroToThree: one of

	0 1 2 3

which is merely a convenient abbreviation for:

ZeroToThree:

	0

	1

	2

	3

When an alternative in a lexical production appears to be a token, it represents the sequence of characters that would make up such a token. Thus, the definition:

BooleanLiteral: one of

	true false

in a lexical grammar production is shorthand for:

BooleanLiteral:

	t r u e

	f a l s e

The right-hand side of a lexical production may specify that certain expansions are not permitted by using the phrase "but not" and then indicating the expansions to be excluded, as in the productions for InputCharacter (S3.4) and Identifier (S3.8):

InputCharacter:

	UnicodeInputCharacter but not CR or LF

Identifier:

	IdentifierName but not a Keyword or BooleanLiteral or NullLiteral

Finally, a few nonterminal symbols are described by a descriptive phrase in roman type in cases where it would be impractical to list all the alternatives:

RawInputCharacter:

	any Unicode character