四则运算的java实现（中缀转后缀）

package basic;

import java.util.Stack;

class Arithmetic2 {
    public static void main(String[] args) {
        System.out.println("!!!!!!!!");
        // 中缀 => 后缀表达式
        String s = "(+0.2 +  (+3 + 0.5) / (-2 * 2) - 3 ) * 2"; // 中缀
        String strTemp = "";
        int index = -1;
        boolean lastIsNum = false; //前一个字符是否是数字
        String[] str = new String[s.length()]; // 后缀
        Stack<String> operationStack = new Stack<String>();
        Stack<Double> resultStack = new Stack<Double>();
        for (int i = 0; i < s.length(); i++) {
            str[i] = "";
        }
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == ' ') {
                continue;
            }
            if (ch >= '0' && ch <= '9' || ch == '.') {
                if (lastIsNum) {
                    str[index] += String.valueOf(ch);
                } else {
                    str[++index] += String.valueOf(ch);
                }
                lastIsNum = true;
            }
            if (ch == '+' || ch == '-') {
                if (i == 0 || s.substring(i - 1, i).equals("(")) {
                    lastIsNum = true;
                    str[++index] += String.valueOf(ch);
                } else {
                    lastIsNum = false;
                    while (!operationStack.isEmpty() && !operationStack.peek().equals("(")) {
                        str[++index] += operationStack.pop();
                    }
                    operationStack.push(String.valueOf(ch));
                }
            }
            if (ch == '*' || ch == '/') {
                lastIsNum = false;
                operationStack.push(String.valueOf(ch));
            }
            if (ch == '(') {
                lastIsNum = false;
                operationStack.push(String.valueOf(ch));
            }
            if (ch == ')') {
                lastIsNum = false;
                while (!operationStack.isEmpty() && !operationStack.peek().equals("(")) {
                    str[++index] += operationStack.pop();
                }
                operationStack.pop();
            }
        }
        if (!operationStack.isEmpty()) {
            str[++index] += operationStack.pop();
        }
//        System.out.println(new ArrayList(Arrays.asList(str)));
        index = -1;
        try {
            while (index < s.length() && !str[++index].equals("")) {
                strTemp = str[index];
                if (isNum(strTemp)){
                    resultStack.push(Double.valueOf(strTemp));
                } else {
                    double num2 = resultStack.pop();
                    double num1 = resultStack.pop();
                    if (strTemp.equals("+")) {
                        resultStack.push(num1 + num2);
                    }
                    if (strTemp.equals("-")) {
                        resultStack.push(num1 - num2);
                    }
                    if (strTemp.equals("*")) {
                        resultStack.push(num1 * num2);
                    }
                    if (strTemp.equals("/")) {
                        resultStack.push(num1 / num2);
                    }
                }
            }

        } catch (Exception e) {
//            e.printStackTrace();
            System.out.println("the expression is incorrect");
            System.exit(1);
        }
        System.out.println(resultStack.pop());
    }

    public static boolean isNum(String str){
        return str.matches("^([+-]?\\d+)(\\.\\d+)?$");//浮点数
    }
}