java_cup
Class lexer

java.lang.Object
  java_cup.lexer

public class lexer

extends java.lang.Object

This class implements a small scanner (aka lexical analyzer or lexer) for the JavaCup specification. This scanner reads characters from standard input (System.in) and returns integers corresponding to the terminal number of the next Symbol. Once end of input is reached the EOF Symbol is returned on every subsequent call.

Symbols currently returned include:

    Symbol        Constant Returned     Symbol        Constant Returned
    ------        -----------------     ------        -----------------
    "package"     PACKAGE               "import"      IMPORT 
    "code"        CODE                  "action"      ACTION 
    "parser"      PARSER                "terminal"    TERMINAL
    "non"         NON                   "init"        INIT 
    "scan"        SCAN                  "with"        WITH
    "start"       START                 "precedence"  PRECEDENCE
    "left"        LEFT		  "right"       RIGHT
    "nonassoc"    NONASSOC		  "%prec        PRECENT_PREC  
      [           LBRACK                  ]           RBRACK
      ;           SEMI 
      ,           COMMA                   *           STAR 
      .           DOT                     :           COLON
      ::=         COLON_COLON_EQUALS      |           BAR
    identifier    ID                    {:...:}       CODE_STRING
    "nonterminal" NONTERMINAL
  

All symbol constants are defined in sym.java which is generated by JavaCup from parser.cup.

In addition to the scanner proper (called first via init() then with next_token() to get each Symbol) this class provides simple error and warning routines and keeps a count of errors and warnings that is publicly accessible.

This class is "static" (i.e., it has only static members and methods).

Field Summary

protected static int	absolute_position Character position in current line.
protected static java.util.Hashtable	char_symbols Table of single character symbols.
protected static int	current_line Current line number for use in error messages.
protected static int	current_position Character position in current line.
protected static int	EOF_CHAR EOF constant.
static int	error_count Count of total errors detected so far.
protected static java.util.Hashtable	keywords Table of keywords.
protected static int	next_char First character of lookahead.
protected static int	next_char2 Second character of lookahead.
protected static int	next_char3 Second character of lookahead.
protected static int	next_char4 Second character of lookahead.
static int	warning_count Count of warnings issued so far

Constructor Summary

private

lexer() The only constructor is private, so no instances can be created.

Method Summary

protected static void	advance() Advance the scanner one character in the input stream.
static Symbol	debug_next_token() Debugging version of next_token().
protected static Symbol	do_code_string() Swallow up a code string.
protected static Symbol	do_id() Process an identifier.
static void	emit_error(java.lang.String message) Emit an error message.
static void	emit_warn(java.lang.String message) Emit a warning message.
protected static int	find_single_char(int ch) Try to look up a single character symbol, returns -1 for not found.
protected static boolean	id_char(int ch) Determine if a character is ok for the middle of an id.
protected static boolean	id_start_char(int ch) Determine if a character is ok to start an id.
static void	init() Initialize the scanner.
static Symbol	next_token() Return one Symbol.
protected static Symbol	real_next_token() The actual routine to return one Symbol.
protected static void	swallow_comment() Handle swallowing up a comment.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

next_char

protected static int next_char

First character of lookahead.

next_char2

protected static int next_char2

Second character of lookahead.

next_char3

protected static int next_char3

Second character of lookahead.

next_char4

protected static int next_char4

Second character of lookahead.

EOF_CHAR

protected static final int EOF_CHAR

EOF constant.

See Also:

Constant Field Values

keywords

protected static java.util.Hashtable keywords

Table of keywords. Keywords are initially treated as identifiers. Just before they are returned we look them up in this table to see if they match one of the keywords. The string of the name is the key here, which indexes Integer objects holding the symbol number.

char_symbols

protected static java.util.Hashtable char_symbols

Table of single character symbols. For ease of implementation, we store all unambiguous single character Symbols in this table of Integer objects keyed by Integer objects with the numerical value of the appropriate char (currently Character objects have a bug which precludes their use in tables).

current_line

protected static int current_line

Current line number for use in error messages.

current_position

protected static int current_position

Character position in current line.

absolute_position

protected static int absolute_position

Character position in current line.

error_count

public static int error_count

Count of total errors detected so far.

warning_count

public static int warning_count

Count of warnings issued so far

Constructor Detail

lexer

private lexer()

The only constructor is private, so no instances can be created.

Method Detail

init

public static void init()
                 throws java.io.IOException

Initialize the scanner. This sets up the keywords and char_symbols tables and reads the first two characters of lookahead.

Throws:

java.io.IOException

advance

protected static void advance()
                       throws java.io.IOException

Advance the scanner one character in the input stream. This moves next_char2 to next_char and then reads a new next_char2.

Throws:

java.io.IOException

emit_error

public static void emit_error(java.lang.String message)

Emit an error message. The message will be marked with both the current line number and the position in the line. Error messages are printed on standard error (System.err).

Parameters:

message - the message to print.

emit_warn

public static void emit_warn(java.lang.String message)

Emit a warning message. The message will be marked with both the current line number and the position in the line. Messages are printed on standard error (System.err).

Parameters:

message - the message to print.

id_start_char

protected static boolean id_start_char(int ch)

Determine if a character is ok to start an id.

Parameters:

ch - the character in question.

id_char

protected static boolean id_char(int ch)

Determine if a character is ok for the middle of an id.

Parameters:

ch - the character in question.

find_single_char

protected static int find_single_char(int ch)

Try to look up a single character symbol, returns -1 for not found.

Parameters:

ch - the character in question.

swallow_comment

protected static void swallow_comment()
                               throws java.io.IOException

Handle swallowing up a comment. Both old style C and new style C++ comments are handled.

Throws:

java.io.IOException

do_code_string

protected static Symbol do_code_string()
                                throws java.io.IOException

Swallow up a code string. Code strings begin with "{:" and include all characters up to the first occurrence of ":}" (there is no way to include ":}" inside a code string). The routine returns a String object suitable for return by the scanner.

Throws:

java.io.IOException

do_id

protected static Symbol do_id()
                       throws java.io.IOException

Process an identifier. Identifiers begin with a letter, underscore, or dollar sign, which is followed by zero or more letters, numbers, underscores or dollar signs. This routine returns a String suitable for return by the scanner.

Throws:

java.io.IOException

next_token

public static Symbol next_token()
                         throws java.io.IOException

Return one Symbol. This is the main external interface to the scanner. It consumes sufficient characters to determine the next input Symbol and returns it. To help with debugging, this routine actually calls real_next_token() which does the work. If you need to debug the parser, this can be changed to call debug_next_token() which prints a debugging message before returning the Symbol.

Throws:

java.io.IOException

debug_next_token

public static Symbol debug_next_token()
                               throws java.io.IOException

Debugging version of next_token(). This routine calls the real scanning routine, prints a message on System.out indicating what the Symbol is, then returns it.

Throws:

java.io.IOException

real_next_token

protected static Symbol real_next_token()
                                 throws java.io.IOException

The actual routine to return one Symbol. This is normally called from next_token(), but for debugging purposes can be called indirectly from debug_next_token().

Throws:

java.io.IOException