[antlr-interest] why antlr not accept string as input stream?

[=?gb2312?B?us6zrA==?=]

e.g.

class MyParser extends Parser;
options
{
    k=2;
}

expr: NUMBER (PLUS|SUB) expr | NUMBER;



class MyLexer extends Lexer;
options
{
    k = 2;
}
PLUS: '+';
SUB: '-';
NUMBER: ('0'..'9')+;

WS: ( '\t'|'\f'|' '| ('\r'|'\n'|"\r\n"){newline();})+ {$setType(Token.SKIP);};

=============================================================================
import java.io.*;
import antlr.CommonAST;
import antlr.collections.AST;
import antlr.debug.misc.ASTFrame;
import java.util.*;
class Test{
   public static void main(String[] args) {
       try {
              MyLexer lexer = new MyLexer("12+23-34");
              MyParser parser = new MyParser(lexer);
              parser.expr();
	      
       } catch(Exception e) {
             System.err.println("exception: "+e);
       }
  }
}
=================================================================

Then an error occurs :

C:\Documents and Settings\Administrator\1>javac Test.java
Test.java:9: cannot resolve symbol
symbol  : constructor MyLexer (java.lang.String)
location: class MyLexer
              MyLexer lexer = new MyLexer("12+23-34");;
                              ^
1 error

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