java_cup

Class 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).

Version:
last updated: 7/3/96

Author:
Frank Flannery

Field Summary

protected static int
EOF_CHAR
EOF constant.
protected static int
absolute_position
Character position in current line.
protected static 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.
static int
error_count
Count of total errors detected so far.
protected static 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

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(String message)
Emit an error message.
static void
emit_warn(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.

Field Details

EOF_CHAR

protected static final int EOF_CHAR
EOF constant.

Field Value:
-1


absolute_position

protected static int absolute_position
Character position in current line.


char_symbols

protected static 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.


error_count

public static int error_count
Count of total errors detected so far.


keywords

protected static 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.


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.


warning_count

public static int warning_count
Count of warnings issued so far

Method Details

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.


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.


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.


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.


emit_error

public static void emit_error(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(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.


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.


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.


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.


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.


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.


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().


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.