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<h4 class="subsection">10.2.7 Differences between C/C++ and Java Grammars</h4>

<p>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.

     <ul>
<li>Java lacks a preprocessor, so the <code>YYERROR</code>, <code>YYACCEPT</code>,
<code>YYABORT</code> symbols (see <a href="Table-of-Symbols.html#Table-of-Symbols">Table of Symbols</a>) cannot obviously be
macros.  Instead, they should be preceded by <code>return</code> 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 see <a href="Java-Action-Features.html#Java-Action-Features">Java Action Features</a>.

     <p>Note that of these three symbols, only <code>YYACCEPT</code> and
<code>YYABORT</code> will cause a return from the <code>yyparse</code>
method<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.

     <li>Java lacks unions, so <code>%union</code> has no effect.  Instead, semantic
values have a common base type: <code>Object</code> or as specified by
&lsquo;<samp><span class="samp">%define stype</span></samp>&rsquo;.  Angle brackets on <code>%token</code>, <code>type</code>,
<code>$</code><var>n</var> and <code>$$</code> specify subtypes rather than fields of
an union.  The type of <code>$$</code>, even with angle brackets, is the base
type since Java casts are not allow on the left-hand side of assignments. 
Also, <code>$</code><var>n</var> and <code>@</code><var>n</var> are not allowed on the
left-hand side of assignments. See see <a href="Java-Semantic-Values.html#Java-Semantic-Values">Java Semantic Values</a> and
see <a href="Java-Action-Features.html#Java-Action-Features">Java Action Features</a>.

     <li>The prologue declarations have a different meaning than in C/C++ code.
          <dl>
<dt><code>%code imports</code><dd>blocks are placed at the beginning of the Java source code.  They may
include copyright notices.  For a <code>package</code> declarations, it is
suggested to use <code>%define package</code> instead.

          <br><dt>unqualified <code>%code</code><dd>blocks are placed inside the parser class.

          <br><dt><code>%code lexer</code><dd>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 see <a href="Java-Scanner-Interface.html#Java-Scanner-Interface">Java Scanner Interface</a>). 
</dl>

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

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

   <div class="footnote">
<hr>
<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Java parsers include the actions in a separate
method than <code>yyparse</code> in order to have an intuitive syntax that
corresponds to these C macros.</p>

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