Problem in understanding the identifiers in Java

general
, , , ,
1

I am facing a serious problem now a days.

import java.io. * ;
import java.util. * ;
import java.text. * ;
import java.math. * ;
import java.util.regex. * ;
public class Solution {
    public static void main(String[] args) {
       Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    int d,q,x,y;
    ArrayList[] set = new ArrayList[n];
    for(int i=0;i < n;i++){
        d = in.nextInt();
        set[i] = new ArrayList();
        for(int j=0;j < d;j++){  
            set[i].add(in.nextInt());                
        }
    }
    q=in.nextInt();
    for(int i=0;i < q;i++){
        x=in.nextInt();
        y=in.nextInt();
        try{
            System.out.println(set[x-1].get(y-1));
        } catch(Exception e){
            System.out.println("ERROR!");
        }
    }
}
}

Above solution is correct and now I am giving the same solution with the different variable name of ArrayList and different Scanner object name I am getting a runtime error.

import java.io. * ;
import java.util. * ;
import java.text. * ;
import java.math. * ;
import java.util.regex. * ;
public class Solution {
    public static void main(String[] args) {
       Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int d,x,y,q;
    ArrayList[] a = new ArrayList[n];
    for(int i=0;i <  n;i++){
        d = sc.nextInt();
        a[i] = new ArrayList();
        for(int j=0;j < d;j++){  
            a[i].add(sc.nextInt());                
        }
    }
    q=sc.nextInt();
    for(int i=0;i < q;i++){
        x=sc.nextInt();
        y=sc.nextInt();
        try{
            System.out.println(a[x-1].get(y-1));
        } catch(Exception e){
            System.out.println("ERROR!");
        }
    }
}
}

and this Solution sometime gives Runtime Error and some time it gives correct output.
I really need to know the reason behind this issue.