Java SE >Java SE Specifications >Java Language Specification
Table of Contents
This chapter describes the context-free grammars used in this specification to define the lexical and syntactic structure of a program.
Acontext-free grammar consists of a number ofproductions. Each production has an abstract symbol called anonterminal as itsleft-hand side, and a sequence of one or more nonterminal andterminal symbols as itsright-hand side. For each grammar, the terminal symbols are drawn from a specifiedalphabet.
Starting from a sentence consisting of a single distinguished nonterminal, called thegoal symbol, a given context-free grammar specifies a language, namely, the 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.
Alexical grammar for the Java programming language is given in§3 (Lexical Structure). This grammar has as its terminal symbols the characters of the Unicode character set. It defines a set of productions, starting from the goal symbolInput (§3.5), that describe how sequences of Unicode characters (§3.1) are translated into a sequence of input elements (§3.5).
These input elements, with white space (§3.6) and comments (§3.7) discarded, form the terminal symbols for the syntactic grammar for the Java programming language and are calledtokens (§3.5). These tokens are the identifiers (§3.8), keywords (§3.9), literals (§3.10), separators (§3.11), and operators (§3.12) of the Java programming language.
Thesyntactic grammar for the Java programming language is given in Chapters 4, 6-10, 14, and 15. This grammar has tokens defined by the lexical grammar as its terminal symbols. It defines a set of productions, starting from the goal symbolCompilationUnit (§7.3), that describe how sequences of tokens can form syntactically correct programs.
For convenience, the syntactic grammar is presented all together in Chapter 19.
Terminal symbols are shown infixed 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 initalic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined, followed by a colon. One or more alternative definitions for the nonterminal then follow on succeeding lines.
For example, the syntactic production:
states that the nonterminalIfThenStatement represents the tokenif, followed by a left parenthesis token, followed by anExpression, followed by a right parenthesis token, followed by aStatement.
The syntax{x} on the right-hand side of a production denotes zero or more occurrences ofx.
For example, the syntactic production:
states that anArgumentList consists of anArgument, followed by zero or more occurrences of a comma and anArgument. The result is that anArgumentList may contain any positive number of arguments.
The syntax[x] on the right-hand side of a production denotes zero or one occurrences ofx. That is,x is anoptional symbol. The alternative which contains the optional symbol actually defines two alternatives: one that omits the optional symbol and one that includes it.
This means that:
is a convenient abbreviation for:
As another example, it means that:
is a convenient abbreviation for:
which in turn is an abbreviation for:
which in turn is an abbreviation for:
so the nonterminalBasicForStatement actually has eight alternative right-hand sides.
A very long right-hand side may be continued on a second line by clearly indenting the second line.
For example, the syntactic grammar contains this production:
which defines one right-hand side for the nonterminalNormalClassDeclaration.
The phrase(one of) on the right-hand side of a production signifies that each of the terminal symbols on the following line or lines is an alternative definition.
For example, the lexical grammar contains the production:
which is merely a convenient abbreviation for:
When an alternative in a production appears to be a token, it represents the sequence of characters that would make up such a token.
Thus, the production:
is shorthand for:
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase "but not" and then indicating the expansions to be excluded.
Finally, a few nonterminals are defined by a narrative phrase in roman type where it would be impractical to list all the alternatives.