java计算器

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package excel;

import java.util.NoSuchElementException;
import java.util.StringTokenizer;

public class Calculator {
private StringTokenizer T;

private static final int MC_SYMB = 1;
private static final int MC_NUM = 2;
private static final int MC_NONE = 0;
private static final int MC_EOT = -1;

private int MCode; //Tokencode
private char MSymb; //token value
private double MNum; //token value

private double Result;

/**
* Konstruktor Constructor
*/

public Calculator(String input) throws SyntaxErrorException {
  T = new StringTokenizer(input, "+-*/() /t", true);
  MCode = MC_NONE;
  Result = expr();
  if (MCode != MC_EOT)
   throw new SyntaxErrorException();
}

/***************************************************************************
* returns the calculated value of the expression
**************************************************************************/

public double getResult() {
return Result;
}

/***************************************************************************
* the lexer to prduce a token when MCode is MC_NONE
**************************************************************************/

private boolean lex() throws SyntaxErrorException {
  String Token;
  if (MCode == MC_NONE) {
   MCode = MC_EOT;
   MSymb = '/0';
   MNum = 0.0;
   try {
    do
     Token = T.nextToken();
    while (Token.equals(" ") || Token.equals("/t"));
   } catch (NoSuchElementException e) {
    return false;
   }
   // numeral
   if (Character.isDigit(Token.charAt(0))) {
    int i;
    for (i = 0; i < Token.length()
      && (Character.isDigit(Token.charAt(i)) || Token
        .charAt(i) == '.'); i++);
    if (i != Token.length())
     throw new SyntaxErrorException();
    try {
     MNum = (Double.valueOf(Token)).doubleValue();
    } catch (NumberFormatException e) {
     throw new SyntaxErrorException();
    }
    MCode = MC_NUM;
   }
   // symbol
   else {
    MSymb = Token.charAt(0);
    MCode = MC_SYMB;
   }
  }
  return true;
}

/***************************************************************************
* expr implements the rule <expr>-><term>{ '+' <term>} | <term>{ '-'
* <term>} .
**************************************************************************/

private double expr() throws SyntaxErrorException {
  double tmp = term();
  while (lex() && MCode == MC_SYMB && (MSymb == '+' || MSymb == '-')) {
   MCode = MC_NONE;
   if (MSymb == '+')
    tmp += term();
   else
    tmp -= term();
  }
  if (MCode == MC_SYMB && MSymb == '(' || MCode == MC_SYMB
    && MSymb == ')' || MCode == MC_EOT)
   ;
  else
   throw new SyntaxErrorException();
  return tmp;
}

/***************************************************************************
* term implements the rule <term>-><fact>{ '*' <fact>} | <fact>{ '/'
* <fact>} .
**************************************************************************/

private double term() throws SyntaxErrorException {
  double tmp = fac();
  while (lex() && MCode == MC_SYMB && (MSymb == '*' || MSymb == '/')) {
   MCode = MC_NONE;
   if (MSymb == '*')
    tmp *= fac();
   else
    tmp /= fac();
  }
  return tmp;
}

/***************************************************************************
* fac implements the rule <fact>-><nmeral>| '(' <expr>')' .
**************************************************************************/

private double fac() throws SyntaxErrorException {
double tmp;
boolean minus = false;

  if (lex() && MCode == MC_SYMB && MSymb == '-') {
   MCode = MC_NONE;
   minus = true;
  }
  if (lex() && MCode == MC_SYMB && MSymb == '(') {
   MCode = MC_NONE;
   tmp = expr();
   if (lex() && MCode != MC_SYMB || MSymb != ')')
    throw new SyntaxErrorException();
   MCode = MC_NONE;
  } else if (MCode == MC_NUM) {
   MCode = MC_NONE;
   tmp = MNum;
  } else
   throw new SyntaxErrorException();
  if (minus)
   tmp = -tmp;
  return tmp;
}

}