1.Integer.parseInt()

"123";
        int b = Integer.parseInt(aString);

        System.out.println(b + "");

2.转换为字符串数组再运算

package com.lizi.demo;

public class String2Int {

    public static void main(String[] args) {
        String aString = "123";
        int b = Integer.parseInt(aString);

        System.out.println(b + "");

        // 方法二
        // 1.字符串转换成数组 string.toCharArray();

        // 2.遍历字符型数组，每一位与'0'作减法，存入数值型数组
        // Ingeter[] ints = new Integer[s.length];
        // ints[i] = c[i] - '0';

        // 3.遍历相减得到的数组，每位*(10^(length-1))
        String2Int string2Int = new String2Int();
        string2Int.StringToInt(aString);
    }

    public void StringToInt(String string) {
        char[] c = string.toCharArray();
        Integer[] intLength = new Integer[c.length];
        for (int i = 0; i < c.length; i++) {
            intLength[i] = c[i] - '0';
        }
        Double result = 0D;
        for (int i = 0; i < intLength.length; i++) {
            result += (intLength[i] * Math.pow(10, (intLength.length - 1 - i)));
        }
        Integer n = result.intValue();
        System.out.println(n + "");
    }

}