IDG | 四则运算表达式计算

import java.util.LinkedList;
import java.util.List;
import java.util.Stack;

public class ExpCal {
public static double calc(String exp) {
if (exp == null || exp.length() <= 0) {
throw new IllegalArgumentException();
}

char[] c = exp.toCharArray();
Stack<Character> s = new Stack<Character>();
List<String> reversePolishNotation = new LinkedList<String>();
for (int i = 0; i < c.length; ++i) {
if (c[i] == '+' || c[i] == '-' || c[i] == '*' || c[i] == '/') {
while (!s.isEmpty() && compOp(s.peek(), c[i]) >= 0) {
reversePolishNotation.add(String.valueOf(s.pop()));
}
s.push(c[i]);
} else {
StringBuilder sb = new StringBuilder();
while (i < c.length && c[i] >= '0' && c[i] <= '9') {
sb.append(c[i++]);
}
reversePolishNotation.add(sb.toString());
--i;
}
}
while (!s.isEmpty()) {
reversePolishNotation.add(String.valueOf(s.pop()));
}

Stack<Double> num = new Stack<Double>();
for (String e : reversePolishNotation) {
if (e.equals("+")) {
num.push(num.pop() + num.pop());
} else if (e.equals("-")) {
double a = num.pop();
double b = num.pop();
num.push(b - a);
} else if (e.equals("*")) {
num.push(num.pop() * num.pop());
} else if (e.equals("/")) {
double a = num.pop();
double b = num.pop();
num.push(b / a);
} else {
num.push(Double.parseDouble(e));
}
}

return num.pop();
}

private static int compOp(char a, char b) {
return getPri(a) - getPri(b);
}

private static int getPri(char c) {
switch (c) {
case '+':
case '-':
return 1;
case '*':
case '/':
return 2;
default:
return 0;
}
}

public static void main(String[] args) {
System.out.println(calc("4*3+2*5-8/8-2*6/3+2/1-4"));
}
}