java不使用库函数将字符串转为double类型

需要考虑哪些问题？

• 参数校验
• 正负数
• 小数点
• 科学计数法
• 指数
• 指数正负

codeing

public static double StringToDouble(String str) {

        //数值正负号，和e指数正负号
        boolean zeroFlag = true;
        boolean eZeroFlag = true;
        //小数点位置和e位置
        int posBeforePoint = 0;
        int posBeforeE = 0;
        //小数点位数
        double posPoint = 1.0;
        //小数点钱数字，小数点后数字，而之后数字
        double beforePoint=0.0,afterPoint=0.0,afterE = 0.0;
        int length = str.length();
        char[] tempStr;
        if (str == null || length == 0) {
            System.out.println("参数不合法");
            return 0.0;
        }

        if (str.startsWith("+") || str.startsWith("-")) {
            tempStr = str.substring(1).toCharArray();
            length--;
            if (str.startsWith("-")) {
                zeroFlag = false;
            }
        }else{
            tempStr = str.toCharArray();
        }
        //遍历数组，处理
        for (int i = 0; i < tempStr.length; i++) {
            //处理数字
            if (tempStr[i] >= '0' && tempStr[i] <= '9') {
                //e之前
                if (posBeforeE == 0) {
                    //0之前
                    if (posBeforePoint == 0) {
                        beforePoint = beforePoint * 10 + (tempStr[i]-48);
                    }else{
                        posPoint = posPoint * 0.1;
                        afterPoint = afterPoint + (tempStr[i]-48) * posPoint;
                    }
                }else{
                    //e之后先考虑大小，后面添加符号
                    afterE = afterE * 10 + (tempStr[i]-48);
                }

            } else if (tempStr[i] == '.') {
                posBeforePoint++;
                if (posBeforePoint > 1) {
                    System.out.println("参数不合法");
                    return 0.0;
                }
            }else if(tempStr[i] == 'E' || tempStr[i] == 'e'){
                posBeforeE++;
                if (posBeforeE > 1) {
                    System.out.println("参数不合法");
                    return 0.0;
                }
            } else if (tempStr[i] == '+' || tempStr[i] == '-') {
                if (tempStr[i] == '-') {
                    eZeroFlag = false;
                }
            }else{
                System.out.println("参数不合法");
                return 0.0;
            }
        }

        //e的指数为负数
        if (!eZeroFlag) {
            afterE = afterE * (-1.0);
        }

        //正数
        if (zeroFlag) {
            return (beforePoint + afterPoint) * Math.pow(10, afterE);
        }else{
            return (beforePoint + afterPoint) * Math.pow(10, afterE)*(-1.0);
        }
    }

运行结果

public static void main(String[] args) {
        double res = StringToDouble("3.4211e-2");
        System.out.println(res);
}

输出：

0.034211