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10.3.9 Differences between C/C++ and Java Grammars

The different structure of the Java language forces several differences between C/C++ grammars, and grammars designed for Java parsers. This section summarizes these differences.

• Java has no a preprocessor, so obviously the YYERROR, YYACCEPT, YYABORT symbols (see Bison Symbols) cannot be macros. Instead, they should be preceded by return when they appear in an action. The actual definition of these symbols is opaque to the Bison grammar, and it might change in the future. The only meaningful operation that you can do, is to return them. See Special Features for Use in Java Actions.

  Note that of these three symbols, only YYACCEPT and YYABORT will cause a return from the yyparse method⁸.

• Java lacks unions, so %union has no effect. Instead, semantic values have a common base type: Object or as specified by ‘%define api.value.type’. Angle brackets on %token, type, $n and $$ specify subtypes rather than fields of an union. The type of $$, even with angle brackets, is the base type since Java casts are not allow on the left-hand side of assignments. Also, $n and @n are not allowed on the left-hand side of assignments. See Java Semantic Values, and Special Features for Use in Java Actions.
• The prologue declarations have a different meaning than in C/C++ code.

  %code imports

  blocks are placed at the beginning of the Java source code. They may include copyright notices. For a package declarations, use ‘%define api.package’ instead.

  unqualified %code

  blocks are placed inside the parser class.

  %code lexer

  blocks, if specified, should include the implementation of the scanner. If there is no such block, the scanner can be any class that implements the appropriate interface (see Java Scanner Interface).

  Other %code blocks are not supported in Java parsers. In particular, %{ … %} blocks should not be used and may give an error in future versions of Bison.

  The epilogue has the same meaning as in C/C++ code and it can be used to define other classes used by the parser outside the parser class.