UVA - 10789 - Prime Frequency

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws IOException {
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        StringBuffer sb = new StringBuffer("");
        String m="";
        boolean [] notprimes=sievePrime(2000);
        int cases=Integer.parseInt(br.readLine());
        for(int i=0;i<cases;i++) {
            int[] arr=new int[128];
            m=br.readLine();
            for(int j=0;j<m.length();j++){
                 arr[m.charAt(j)]++;
            }
            boolean flag=true;
            sb.append("Case ").append((i+1)).append(": ");
            for(int j=0;j<arr.length;j++){
                 if(!notprimes[arr[j]]){
                     sb.append((char)j);
                     flag=false;
                 }
            }
            if(flag)
                sb.append("empty");
            sb.append("\n");
        }
        System.out.print(sb);
    }
   
   
    static boolean[] sievePrime(int x) {
        boolean[] notPrime = new boolean[x + 1];
        notPrime[0] = true;
        notPrime[1] = true;
        for (int i = 2; i * i < x + 1; i++) {
            if (!notPrime[i]) {
                for (int j = i; i * j < x + 1; j++) {
                    notPrime[i * j] = true;
                }
            }
        }
        return notPrime;
    }
}