计算器后台逻辑实现——表达式分析器

public class Parser {
final int NONE = 0;
final int DELIMITER = 1;
final int VARIABLE = 2;
final int NUMBER = 3;

final int SYNTAX = 0;
final int UNBALPARENS = 1; //unbalparens
final int NOEXP = 2;
final int DIVBYZERO = 3;

final String EOE = "\0";

private String exp;
private int expIdx;
private String token;
private int tokType;

public double evaluate(String expstr) throws ParserException
{
double result;
exp = expstr;
expIdx = 0;

getToken();
if(token.equals(EOE))
handleError(NOEXP);

result = evalExp2();

if(!token.equals(EOE))
handleError(SYNTAX);

return result;
}

private double evalExp2() throws ParserException
{
char op;
double result;
double partialResult;

result = evalExp3();

while ((op=token.charAt(0))=='+' || op=='-') {
getToken();
partialResult = evalExp3();
switch(op) {
case '-':
result = result - partialResult;
break;
case '+':
result = result + partialResult;
break;
}
}
return result;
}

private double evalExp3() throws ParserException
{
char op;
double result;
double partialResult;

result = evalExp4();

while((op=token.charAt(0))=='*'||op=='/'||op=='%'){
getToken();
partialResult = evalExp4();
switch(op){
case '*':
result = result * partialResult;
break;
case '/':
if(partialResult ==0.0)
handleError(DIVBYZERO);
result = result / partialResult;
break;
case '%':
if(partialResult == 0.0)
handleError(DIVBYZERO);
result = result % partialResult;
break;
}
}
return result;
}

private double evalExp4() throws ParserException
{
double result;
double partialResult;
double ex;
int t;

result = evalExp5();

if(token.equals("^")){
getToken();
partialResult = evalExp4();
ex = result;
if (partialResult == 0.0){
result = 1.0;
} else {
for (t=(int)partialResult-1;t>0;t--)
result = result * ex;
}
}
return result;
}

private double evalExp5() throws ParserException
{
double result;
String op;

op="";
if((tokType==DELIMITER)&& token.equals("+")||token.equals("-")) {
op = token;
getToken();
}
result = evalExp6();

if(op.equals("-"))
result = -result;

return result;
}

private double evalExp6() throws ParserException
{
double result=0.0;

if(token.equals("(")){
getToken();
result = evalExp2();
if(!token.equals(")"))
handleError(UNBALPARENS);
getToken();
} else
result = atom();

return result;
}

private double atom() throws ParserException {
double result = 0.0;

switch(tokType) {
case NUMBER:
try {
result = Double.parseDouble(token);
}catch (NumberFormatException exc) {
handleError(SYNTAX);
}
getToken();
break;
default:
handleError(SYNTAX);
break;

}
return result;
}

private void handleError(int error) throws ParserException {
String[] err = {
"Syntax Error",
"Unbalanced Parentheses",
"No Expression Present",
"Division by Zero"
};

throw new ParserException(err[error]);
}

private void getToken() {
tokType = NONE;
token = "";

if (expIdx == exp.length()){
token = EOE;
return;
}
while (expIdx < exp.length() && Character.isWhitespace(exp.charAt(expIdx)))
++expIdx;

if(expIdx == exp.length()){
token = EOE;
return;
}

if (isDelim(exp.charAt(expIdx))) {
token += exp.charAt(expIdx);
expIdx++;
tokType = DELIMITER;
} else if (Character.isLetter(exp.charAt(expIdx))){
while(!isDelim(exp.charAt(expIdx))){
token += exp.charAt(expIdx);
expIdx++;
if(expIdx>=exp.length())
break;
}
tokType = VARIABLE;
} else if (Character.isDigit(exp.charAt(expIdx))){
while(!isDelim(exp.charAt(expIdx))){
token+=exp.charAt(expIdx);
expIdx++;
if(expIdx>=exp.length())
break;
}
tokType = NUMBER;
} else {
token = EOE;
return;
}
}

private boolean isDelim(char c){
if ((" +-/*%^=()").indexOf(c)!= -1)
return true;
return false;
}
}