算法题——实现类似parseInt的方法

Scanner sc = new Scanner(System.in);
String str = "";
while(true){
    System.out.println("请输入");
    String str1 = sc.nextLine();
    if(str1.length() < 1 || str1.length() > 10 || str1.charAt(0) == '0') {
        System.out.println("格式有误");
        continue;
    }
    str = str1;
    break;
}
int shu = 0;
for (int i = 0; i < str.length(); i++) {
    int ge = str.charAt(i) - 48;
    shu = shu * 10 + ge;
}
System.out.println(shu);