java: document new features

* data/skeletons/lalr1.java: More comments.
(Context.EMPTY): Remove.
* doc/bison.texi (Java Parser Context Interface): New.

data/skeletons/lalr1.java

@@ -876,17 +876,20 @@ b4_dollar_popdef[]dnl
 
     private YYStack yystack;
 
+
+    /**
+     * The symbol kind of the lookahead token.
+     */
     public SymbolKind getToken ()
     {
       return yytoken;
     }
 
-    /**
-     * Value returned by getToken when there is no token.
-     */
-    public static final int EMPTY = ]b4_parser_class[.YYEMPTY_;
-
     private SymbolKind yytoken;]b4_locations_if([[
+
+    /**
+     * The location of the lookahead.
+     */
     public ]b4_location_type[ getLocation ()
     {
       return yylocation;
@@ -895,10 +898,12 @@ b4_dollar_popdef[]dnl
     private ]b4_location_type[ yylocation;]])[
     static final int NTOKENS = ]b4_parser_class[.YYNTOKENS_;
 
-    /* Put in YYARG at most YYARGN of the expected tokens given the
-       current YYCTX, and return the number of tokens stored in YYARG.  If
-       YYARG is null, return the number of expected tokens (guaranteed to
-       be less than YYNTOKENS).  */
+    /**
+     * Put in YYARG at most YYARGN of the expected tokens given the
+     * current YYCTX, and return the number of tokens stored in YYARG.  If
+     * YYARG is null, return the number of expected tokens (guaranteed to
+     * be less than YYNTOKENS).
+     */
     int getExpectedTokens (SymbolKind yyarg[], int yyargn)
     {
       return getExpectedTokens (yyarg, 0, yyargn);
@@ -933,8 +938,10 @@ b4_dollar_popdef[]dnl
       return yycount - yyoffset;
     }
 
-    /* The user-facing name of the symbol whose (internal) number is
-       YYSYMBOL.  No bounds checking.  */
+    /**
+     * The user-facing name of the symbol whose (internal) number is
+     * YYSYMBOL.  No bounds checking.
+     */
     static String yysymbolName (SymbolKind yysymbol)
     {
       return ]b4_parser_class[.yysymbolName (yysymbol);

doc/bison.texi

@@ -433,6 +433,7 @@ Java Parsers
 * Java Semantic Values::        %token and %nterm vs. Java
 * Java Location Values::        The position and location classes
 * Java Parser Interface::       Instantiating and running the parser
+* Java Parser Context Interface:: Circumstances of a syntax error
 * Java Scanner Interface::      Specifying the scanner for the parser
 * Java Action Features::        Special features for use in actions
 * Java Push Parser Interface::  Instantiating and running the a push parser
@@ -12673,6 +12674,7 @@ main (int argc, char *argv[])
 * Java Semantic Values::        %token and %nterm vs. Java
 * Java Location Values::        The position and location classes
 * Java Parser Interface::       Instantiating and running the parser
+* Java Parser Context Interface:: Circumstances of a syntax error
 * Java Scanner Interface::      Specifying the scanner for the parser
 * Java Action Features::        Special features for use in actions
 * Java Push Parser Interface::  Instantiating and running the a push parser
@@ -12917,6 +12919,37 @@ Identify the Bison version and skeleton used to generate this parser.
 @end deftypecv
 
 
+@node Java Parser Context Interface
+@subsection Java Parser Context Interface
+
+The parser context provides information to build error reports when you
+invoke @samp{%define parse.error custom}.
+
+@deftypemethod {YYParser.Context} {YYParser.SymbolKind} getToken ()
+The kind of the lookahead.  Maybe return @code{null} when there is no
+lookahead.
+@end deftypemethod
+
+@deftypemethod {YYParser.Context} {YYParser.Location} getLocation ()
+The location of the lookahead.
+@end deftypemethod
+
+@deftypemethod {YYParser.Context} {int} getExpectedTokens (@code{YYParser.SymbolKind[]} @var{argv}, @code{int} @var{argc})
+Fill @var{argv} with the expected tokens, which never includes
+@code{YYSYMBOL_YYEMPTY}, @code{YYSYMBOL_YYERROR}, or
+@code{YYSYMBOL_YYUNDEF}.
+
+Never put more than @var{argc} elements into @var{argv}, and on success
+return the effective number of tokens stored in @var{argv}.  Return 0 if
+there are more than @var{argc} expected tokens, yet fill @var{argv} up to
+@var{argc}.  When LAC is enabled, may return a negative number on errors,
+such as @code{YYNOMEM} on memory exhaustion.
+
+If @var{argv} is null, return the size needed to store all the possible
+values, which is always less than @code{YYNTOKENS}.
+@end deftypemethod
+
+
 @node Java Scanner Interface
 @subsection Java Scanner Interface
 @c - %code lexer
@@ -12979,6 +13012,40 @@ The return type can be changed using @samp{%define api.value.type
 @{@var{class-name}@}}.
 @end deftypemethod
 
+@deftypemethod {Lexer} {void} reportSyntaxError (@code{YYParser.Context} @var{ctx})
+If you invoke @samp{%define parse.error custom} (@pxref{Bison
+Declarations}), then the parser no longer passes syntax error messages to
+@code{yyerror}, rather it delegates that task to the user by calling the
+@code{reportSyntaxError} function.
+
+Whether it uses @code{yyerror} is up to the user.
+
+Here is a typical example of a reporting function.
+
+@example
+public void yyreportSyntaxError (YYParser.Context ctx)
+@{
+  System.err.print (ctx.getLocation () + ": syntax error");
+  // Report the expected tokens.
+  @{
+    final int TOKENMAX = 5;
+    YYParser.SymbolKind[] arg = new YYParser.SymbolKind[TOKENMAX];
+    int n = ctx.getExpectedTokens (arg, TOKENMAX);
+    for (int i = 0; i < n; ++i)
+      System.err.print ((i == 0 ? ": expected " : " or ")
+                        + ctx.yysymbolName (arg[i]));
+  @}
+  // Report the unexpected token which triggered the error.
+  @{
+    YYParser.SymbolKind lookahead = ctx.getToken ();
+    if (lookahead != null)
+      System.err.print (" before " + ctx.yysymbolName (lookahead));
+  @}
+  System.err.println ("");
+@}
+@end example
+@end deftypemethod
+
 @node Java Action Features
 @subsection Special Features for Use in Java Actions