Java SE > Java SE Specifications > Java Language Specification
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This chapter describes the context-free grammars used in this specification to define the lexical and syntactic structure of a program.本章介绍本规范中用于定义程序词汇和语法结构的上下文无关语法。
A context-free grammar consists of a number of productions. 上下文无关语法由许多结果组成。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. 每个产品都有一个称为非终结符的抽象符号作为其左侧，一个或多个非终结符和终结符的序列作为其右侧。For each grammar, the terminal symbols are drawn from a specified alphabet.对于每种语法，终端符号都是从指定的字母表中提取的。
Starting from a sentence consisting of a single distinguished nonterminal, called the goal 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.给定的上下文无关语法从一个由单个可分辨的非终结符（称为目标符号）组成的句子开始指定一种语言，即，一组可能的终端符号序列，可以通过将序列中的任何非终端反复替换为产品的右侧（非终端为左侧）而得到。
A lexical grammar for the Java programming language is given in 3 (Lexical Structure). Java编程语言的词汇语法在3“词汇结构”中给出。This grammar has as its terminal symbols the characters of the Unicode character set. 此语法将Unicode字符集的字符作为其终端符号。It defines a set of productions, starting from the goal symbol Input (3.5), that describe how sequences of Unicode characters (3.1) are translated into a sequence of input elements (3.5).它定义了一组产品，从目标符号输入（3.5）开始，描述如何将Unicode字符序列（3.1）转换为输入元素序列（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 called tokens (3.5). 这些输入元素，连同空格（3.6）和注释（3.7）一起被丢弃，形成了Java编程语言语法的终端符号，被称为标记（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.这些标记是Java编程语言的标识符（3.8）、关键字（3.9）、文本（3.10）、分隔符（3.11）和运算符（3.12）。
The syntactic grammar for the Java programming language is given in Chapters 4, 6-10, 14, and 15. Java编程语言的语法在第4、6-10、14和15章中给出。This grammar has as its terminal symbols the tokens defined by the lexical grammar. 该语法将词汇语法定义的标记作为其终端符号。It defines a set of productions, starting from the goal symbol CompilationUnit (7.3), that describe how sequences of tokens can form syntactically correct programs.它定义了一组产品，从目标符号编译单元（3.12）开始，描述标记序列如何形成语法正确的程序。
For convenience, the syntactic grammar is presented all together in Chapter 19.为方便起见，第19章将一并介绍句法语法。
Terminal symbols are shown in 在词汇语法和句法语法的生成中，以及在本规范中，只要文本直接引用这样的终端符号，终端符号都以等宽字体显示。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.它们将完全按照编写的那样出现在程序中。
Nonterminal symbols are shown in italic type. 非终端符号以斜体显示。The definition of a nonterminal is introduced by the name of the nonterminal being defined, followed by a colon. 非终结符的定义由被定义的非终结符的名称引入，后跟冒号。One or more alternative definitions for the nonterminal then follow on succeeding lines.非终结符的一个或多个替代定义随后出现在后续行中。
For example, the syntactic production:例如，语法生成：
states that the nonterminal IfThenStatement represents the token 声明非终结符IfThenStatement表示标记if, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement.if，后跟左括号标记，后跟Expression，后跟右括号标记，后跟Statement。
The syntax {x} on the right-hand side of a production denotes zero or more occurrences of x.产品右侧的语法{x}表示零次或多次出现x。
For example, the syntactic production:例如，语法生成：
states that an ArgumentList consists of an Argument, followed by zero or more occurrences of a comma and an Argument. 语句ArgumentList由一个Argument，后跟零次或多次出现的逗号和Argument组成。The result is that an ArgumentList may contain any positive number of arguments.结果是ArgumentList可以包含任意数量的正数参数。
The syntax [x] on the right-hand side of a production denotes zero or one occurrences of x. 产品右侧的语法[x]表示零次或一次出现x。That is, x is an optional symbol. 也就是说，x是可选符号。The alternative which contains the optional symbol actually defines two alternatives: one that omits the optional symbol and one that includes it.包含可选符号的备选方案实际上定义了两个备选方案：一个省略可选符号，另一个包含可选符号。
This means that:这意味着：
is a convenient abbreviation for:是一个以下内容方便的缩写：
As another example, it means that:另一个例子是，这意味着：
is a convenient abbreviation for:是一个以下内容方便的缩写：
which in turn is an abbreviation for:这反过来又是以下内容的缩写：
which in turn is an abbreviation for:这反过来又是以下内容的缩写：
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 ) Statementso the nonterminal BasicForStatement actually has eight alternative right-hand sides.所以非终结符的BasicForStatement实际上有八个可选的右侧。
A very long right-hand side may be continued on a second line by clearly indenting the second line.通过清楚地缩进第二行，可以在第二行上继续非常长的右侧。
For example, the syntactic grammar contains this production:例如，语法语法包含以下结果：
which defines one right-hand side for the nonterminal NormalClassDeclaration.它定义了非终端NormalClassDeclaration的一个右侧。
The phrase (one of) on the right-hand side of a production signifies that each of the symbols on the following line or lines is an alternative definition.作品右侧的短语(one of)表示以下一行或几行上的每个符号都是一个N选一定义。
For example, the lexical grammar contains the production:例如，词汇语法包含以下结果：
which is merely a convenient abbreviation for:这只是一个方便的缩写：
When an alternative in a production appears to be a token, it represents the sequence of characters that would make up such a token.当产品中的替代项看起来是一个标记时，它表示将构成该标记的字符序列。
Thus, the production:因此，生产：
is shorthand for:是以下的简写：
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.产品的右侧可以通过使用短语“但不是”并指示要排除的扩展来指定不允许某些扩展。
Finally, a few nonterminals are defined by a narrative phrase in roman type where it would be impractical to list all the alternatives.最后，一些非终结符是由罗马式的叙述性短语定义的，在这种情况下，列出所有备选项是不切实际的。