字符串转int

将string类型转换成int型：

可以用integer类里面的valueof，但我们这里用parseint

String str="123";
Integer.parseInt(str);
Integer.parseInt(str,2);

其实对于valueof,其实他底层也是用的parseint来进行转换，因此我们来看一下parseint的底层代码看看他是怎么将字符类型转换成数据类型：

public static int parseInt(String s, int radix)
            throws NumberFormatException
{
    /*
     * WARNING: This method may be invoked early during VM initialization
     * before IntegerCache is initialized. Care must be taken to not use
     * the valueOf method.
     */

    if (s == null) {
        throw new NumberFormatException("null");
    }

    if (radix < Character.MIN_RADIX) {//最小2进制
        throw new NumberFormatException("radix " + radix +
                                        " less than Character.MIN_RADIX");
    }

    if (radix > Character.MAX_RADIX) {//最大36进制
        throw new NumberFormatException("radix " + radix +
                                        " greater than Character.MAX_RADIX");
    }

    int result = 0;
    boolean negative = false;//判断正负
    int i = 0, len = s.length();
    int limit = -Integer.MAX_VALUE;
    int multmin;
    int digit;

    if (len > 0) {
        char firstChar = s.charAt(0);
        if (firstChar < '0') { // Possible leading "+" or "-"
            if (firstChar == '-') {
                negative = true;
                limit = Integer.MIN_VALUE;
            } else if (firstChar != '+')
                throw NumberFormatException.forInputString(s);

            if (len == 1) // Cannot have lone "+" or "-"
                throw NumberFormatException.forInputString(s);
            i++;
        }
        multmin = limit / radix;
        while (i < len) {
            // Accumulating negatively avoids surprises near MAX_VALUE
            digit = Character.digit(s.charAt(i++),radix);
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            }
            if (result < multmin) {
                throw NumberFormatException.forInputString(s);
            }
            result *= radix;
            if (result < limit + digit) {
                throw NumberFormatException.forInputString(s);
            }
            result -= digit;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}

写个方法来将string转换成十进制的数字：（肯定没有底层写的那么完美啦）

private static int toInt(String str,int radix) throws ParseException {
    Objects.nonNull(str);
    if(str=="")throw new NumberFormatException("str is null");
    int firstChar=str.charAt(0);
    boolean negative=false;
    int i=0;
    int len=str.length();
    if(firstChar<'0'){
        if(firstChar=='-')
            negative = true;
        else if(firstChar!='+')
        throw new NumberFormatException("input number is illegal");
        if(len==1)
        throw new NumberFormatException("str has illegal character");
    i++;
}
int num=0;
    while(i<len){
        int n=str.charAt(i++);
        if(n<'0'||n>'9')
            throw new NumberFormatException("middle num illegal");
        num=num*radix+(n-48);

    }
    return negative?-num:num;
}

没有考虑数值类型的范围2^-32~2³¹ ，因此超出范围可能还是有问题……