public class Demo {
   public static void main(String[] args) {
       int num = parseInt("123456");
       System.out.println(num);
   }

   public static int parseInt(String str) {
       //声明一个字符数组
       char[] chars = new char[str.length()];
       //声明一个整型数组（用来装字符转换成整型数字结果）
       int[] ints = new int[str.length()];

       for (int i = 0; i < str.length(); i++) {
           //分解字符串，装入字符数组
           chars[i] = str.charAt(i);
           //把字符数组里的字符通过-'0'转换成整型数字，装入整型数组
           ints[i] = chars[i] - '0';
       }

       //把整型数组转换成整型
       int num = 0;
       for (int i = 0; i < ints.length; i++) {
           num = num * 10 + ints[i];
       }

       return num;
   }
}