NZEC Runtime error in java

import java.io.; import java.lang.System.; import java.util.*;

public class Main {

public static void main(String[] args) throws IOException {

    Scanner sc = new Scanner(System.in);
    int T;
    for (T = sc.nextInt(); T != 0; T--) {
        int N;
        N = sc.nextInt();
        Reader ob = new Reader();
        Map<Character, Character> hm= new HashMap<>();
        boolean b=false;
        for (; N != 0; N--) {
            ob.Read();
            char c = ob.c;
            char p = ob.p;
            hm.put(c, p);
            b=true;
        }
       BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
       String userName = br.readLine();
       if(b==true){
       char[] ch=userName.toCharArray();
       for(int i=0,len=ch.length;i<len;i++){
           char ch1=ch[i];
           if(hm.get(ch1)!=null)
              ch[i]=hm.get(ch1);
       }
        userName=String.valueOf(ch);
       }
      String W=Real_password(userName);  
      System.out.println(W); 
    }

}

    static String Real_password(String S)
{
  char[] str=S.toCharArray();
   int Start=0;
   int End=S.length()-1;
  while(str[Start]=='0')
     Start++;
  int pos=S.indexOf('.');
  if(pos!=-1)
  {
    while(str[End]=='0')
        End--;
  }
  if(str[End]=='.')
      End--;
  S=String.valueOf(str);
  return(S.substring(Start,End+1));

}

}

class Reader {

char c, p;
char ch;

public void Read() throws IOException {
    c = (char) System.in.read();
    ch = (char) System.in.read();
    p = (char) System.in.read();
    ch = (char) System.in.read();
}

}

Why am i getting NZEC error in this java code , Runs very well on my PC?? PLZ Help