java--字符串转int

给定一个String类型的字符串，例如String str = "123";
将他转换为int类型

package exercises2;
import com.sun.org.apache.bcel.internal.generic.RETURN;
import java.util.Objects;


public class TranFor {
    public static void main(String[] args) {
        String str = "123";
        System.out.println(tranForMation(str));
    }

    private static int tranForMation(String str) {
        Objects.requireNonNull(str);
        int length = str.length();
        if (length == 0) {
            throw new NumberFormatException("字符串为空");
        }
        final int radix = 10;
        int index = 0;
        boolean flag = false;
        char firstChar = str.charAt(index);
        if (firstChar == '-') {
            flag = true;
            index++;
        } else if (firstChar == '+') {
            index++;
        } else if (firstChar < 48 || firstChar > 57) {
            throw new NumberFormatException("字符串为空");
        }
        int result = 0;
        while (index < length) {
            char ch = str.charAt(index++);
            if (ch < 48 || ch > 57) {
                throw new NumberFormatException("字符串为空");
            }
            int num = ch - 48;
            result = num+result*radix;
        }
        return flag?-result:result;
    }
}