java面试题，将String字符串转换成数字

题目要求：将String字符串转换成数字，不能用java自带的方法转换字符串，要求自己写一个atoi(String s)，如果输入的不是数字则返回0。

import java.util.Scanner;

/**
 * Created by Dell on 2014/7/14.
 *
 * 面试题
 * 将字符串转换成数字，不用java自带的方法
 */
public class MianShi_01 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("str = ");
        String str = input.nextLine();
        System.out.println(new MianShi_01().atoi(str));
    }

    public int atoi(String s) {
        int[] num = new int[10];
        int j = 0;
        String tmp = null;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(0) == ‘-‘ || Character.isDigit(s.charAt(i))) {
                if (s.charAt(0) == ‘-‘) {
                    tmp = "-";
                }
                switch (s.charAt(i)) {
                    case ‘0‘:
                        num[j] = 0;
                        j++;
                        continue;
                    case ‘1‘:
                        num[j] = 1;
                        j++;
                        continue;
                    case ‘2‘:
                        num[j] = 2;
                        j++;
                        continue;
                    case ‘3‘:
                        num[j] = 3;
                        j++;
                        continue;
                    case ‘4‘:
                        num[j] = 4;
                        j++;
                        continue;
                    case ‘5‘:
                        num[j] = 5;
                        j++;
                        continue;
                    case ‘6‘:
                        num[j] = 6;
                        j++;
                        continue;
                    case ‘7‘:
                        num[j] = 7;
                        j++;
                        continue;
                    case ‘8‘:
                        num[j] = 8;
                        j++;
                        continue;
                    case ‘9‘:
                        num[j] = 9;
                        j++;
                        continue;
                }
            }
            else {
                return 0;
            }
        }
        int NUM = (int) Math.pow(10, j);
        int a = 0;
        for (int i = 0; i < num.length; i++) {
            NUM = NUM / 10;
            a += num[i] * NUM;
        }
        if (tmp != null) {
            return -a;
        }
        return a;
    }
}

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