求值的过程即手算的过程。
逐字符扫描表达式，遇到运算数将其入栈，遇到运算符时就作用于从栈中弹出的两个操作数再并将结果入栈。扫描完整个表达式时，栈中的数即为表达式的结果。

package stack;
import java.util.Stack;

/**
 * 后缀表达式求值
 */
public class Stack01 {
    public static int start = 0;
    public static void calc(String s) {
        Stack stack = new Stack();
        String token;
        while ((token = getToken(s)) != null) {
            int res=0;
            try {
                //运算数
                res = Integer.parseInt(token);
            } catch (NumberFormatException e) {
                //运算符
                int num2 = (int)stack.pop();
                int num1 = (int)stack.pop();
                res = op(num1,num2,token);
            } finally {
                stack.push(res);
            }
        }
        System.out.println(stack.peek());

    }

    private static int op(int num1, int num2, String oprand) {
        int res;
        switch (oprand) {
            case "+":
                res = num1 + num2;
                break;
            case "-":
                res = num1 - num2;
                break;
            case "*":
                res = num1 * num2;
                break;
            case "/":
                res = num1 / num2;
                break;
            default:
                res = 0;
        }
        return res;
    }

    private static String getToken(String s) {
        int len = s.length();
        if (start == len) {
            return null;
        }
        if (start == len-1) {
            int temp = start;
            start++;
            return s.substring(temp);
        }
        int end = start;
        char ch = s.charAt(end);
        while (ch != ' ') {
            end++;
            ch = s.charAt(end);
        }
        String token = s.substring(start, end);
        start = end + 1;
        return token;
    }

    public static void main(String[] args) {
        calc("10 10 + 2 *");
        System.out.println();
    }

}