为什么会在运行期间报数组越界＞﹏＜

import java.util.Arrays;
import java.util.Scanner;

public class BinarySearch {
public static int rank(int key,int[] a){
int lo = 0, hi = a.length - 1;
while(lo <= hi){
int mid = (lo + hi) / 2;
if(a[mid] > key)
hi = mid - 1;
else if(a[mid] < key)
lo = mid + 1;
else
return mid;
}
return -1;
}
public static void main(String[] args){
int[] a = new int[20];
for(int i = 0; i < 20; i++){
a[i] = (int) (Math.random() * 100);

    }
    Arrays.sort(a);
    for(int cc : a)
        System.out.println(cc);
    //BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    Scanner in = new Scanner(System.in);

        int b = 0;
        if(in.hasNextInt())
            b = in.nextInt();
        int address = rank(b,a);
        if(address >= 0){
            System.out.println("索引为：" + address);
        }
        else{
            System.out.println("目标不存在");
        }



}

}

运行半天 没测出问题~~