String str = "1";
System.out.println(Integer.valueOf(str));//1
System.out.println(Integer.parseInt(str));//1

public static int parseInt(String s, int radix) throws NumberFormatException {
        /* 警告：在初始化IntegerCache之前，VM初始化期間可能會提前調用此方法。
         * 必須注意不要使用valueOf方法。
         * WARNING: This method may be invoked early during VM initialization
         * before IntegerCache is initialized. Care must be taken to not use
         * the valueOf method.
         */
        //字符串為空則拋出NumberFormatException
        if (s == null) {
            throw new NumberFormatException("null");
        }
        //傳的進制參數(shù)小于2，拋出NumberFormatException，
        //并且提示進制小于最小進制
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +
                    " less than Character.MIN_RADIX");
        }
        //傳的進制參數(shù)大于36，拋出NumberFormatException，
        //并且提示進制大于最大進制
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix +
                    " greater than Character.MAX_RADIX");
        }
        int result = 0;
        //negative 用來判斷結果是否為負數(shù)
        boolean negative = false;
        //獲取字符串長度
        int i = 0, len = s.length();
        //limit = -2147483647
        int limit = -Integer.MAX_VALUE;
        //用于在添加下一位數(shù)字的前判斷是否溢出的值
        int multmin;
        //需要追加的數(shù)字
        int digit;
        //字符長度大于0
        if (len > 0) {
            //判斷字符串是否有符號
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    //第一個符號是負號，所以結果是負數(shù)
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    //不為負數(shù)或正數(shù)，拋出NumberFormatException
                    throw NumberFormatException.forInputString(s);
                //長度為1，拋出NumberFormatException
                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            //計算 multmin ，注意負數(shù)和正數(shù)的limit是不一樣的,
            //負數(shù)的limit = -2147483648，正數(shù)的limit = -2147483647
            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 {
            //字符不為空，但是字符長度等于0，拋出NumberFormatException
            throw NumberFormatException.forInputString(s);
        }
        //根據(jù)正負數(shù)的標識來判斷結果取正還是取反
        return negative ? result : -result;
    }