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<a name="Java-Scanner-Interface-1"></a>
<h4 class="subsection">10.2.5 Java Scanner Interface</h4>

<p>There are two possible ways to interface a Bison-generated Java parser
with a scanner: the scanner may be defined by <code>%code lexer</code>, or
defined elsewhere.  In either case, the scanner has to implement the
<code>Lexer</code> inner interface of the parser class.  This interface also
contain constants for all user-defined token names and the predefined
<code>EOF</code> token.
</p>
<p>In the first case, the body of the scanner class is placed in
<code>%code lexer</code> blocks.  If you want to pass parameters from the
parser constructor to the scanner constructor, specify them with
<code>%lex-param</code>; they are passed before <code>%parse-param</code>s to the
constructor.
</p>
<p>In the second case, the scanner has to implement the <code>Lexer</code> interface,
which is defined within the parser class (e.g., <code>YYParser.Lexer</code>).
The constructor of the parser object will then accept an object
implementing the interface; <code>%lex-param</code> is not used in this
case.
</p>
<p>In both cases, the scanner has to implement the following methods.
</p>
<dl>
<dt><a name="index-yyerror-on-Lexer"></a>Method on Lexer: <em>void</em> <strong>yyerror</strong> <em>(Location <var>loc</var>, String <var>msg</var>)</em></dt>
<dd><p>This method is defined by the user to emit an error message.  The first
parameter is omitted if location tracking is not active.  Its type can be
changed using <code>%define api.location.type {<var>class-name</var>}</code>.
</p></dd></dl>

<dl>
<dt><a name="index-yylex-on-Lexer"></a>Method on Lexer: <em>int</em> <strong>yylex</strong> <em>()</em></dt>
<dd><p>Return the next token.  Its type is the return value, its semantic
value and location are saved and returned by the their methods in the
interface.
</p>
<p>Use &lsquo;<samp>%define lex_throws</samp>&rsquo; to specify any uncaught exceptions.
Default is <code>java.io.IOException</code>.
</p></dd></dl>

<dl>
<dt><a name="index-getStartPos-on-Lexer"></a>Method on Lexer: <em>Position</em> <strong>getStartPos</strong> <em>()</em></dt>
<dt><a name="index-getEndPos-on-Lexer"></a>Method on Lexer: <em>Position</em> <strong>getEndPos</strong> <em>()</em></dt>
<dd><p>Return respectively the first position of the last token that
<code>yylex</code> returned, and the first position beyond it.  These
methods are not needed unless location tracking is active.
</p>
<p>The return type can be changed using <code>%define api.position.type
{<var>class-name</var>}</code>.
</p></dd></dl>

<dl>
<dt><a name="index-getLVal-on-Lexer"></a>Method on Lexer: <em>Object</em> <strong>getLVal</strong> <em>()</em></dt>
<dd><p>Return the semantic value of the last token that yylex returned.
</p>
<p>The return type can be changed using &lsquo;<samp>%define api.value.type
{<var>class-name</var>}</samp>&rsquo;.
</p></dd></dl>

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