Single number problem Java

SingleNumberProblem.java

import java.util.HashMap;
import java.util.Collections;
import java.util.Map;

public class Main
{
    public interface RepetitionChecker {
        int SingleNumber(int[] sequence);
    }

    public static class OnlyRepeatTwice implements RepetitionChecker{
        public int SingleNumber(int[] sequence){
            int once = 0;

            for(int idx=0; idx < sequence.length ; idx++)
                once = (once ^ sequence[idx]);

            return once;
        }
    }

    public static class OnlyRepeatThreeTimes implements RepetitionChecker{
        public int SingleNumber(int[] sequence){
            int once = 0;
            int twice = 0;

            for(int idx=0; idx < sequence.length ; idx++){
                once = canBeSettedInFlag(sequence[idx], once, twice);
                twice = canBeSettedInFlag(sequence[idx], twice, once);
            }

            return once;
        }
        
        private int canBeSettedInFlag(int e, int destinationFlag, int anotherFlag){
            destinationFlag = (destinationFlag ^ e) & ~ anotherFlag;
            return destinationFlag;
        }
    }


    public static class RepetitionFactory {
        private final Map<Integer, RepetitionChecker> checkers;

        RepetitionFactory() {
            Map<Integer, RepetitionChecker> tempMap = new HashMap<>();
            tempMap.put(2, new OnlyRepeatTwice());
            tempMap.put(3, new OnlyRepeatThreeTimes());
            this.checkers = Collections.unmodifiableMap(tempMap);
        }

        public RepetitionChecker getInstance(int repeats){
            return checkers.get(repeats);
        }
    }
    
    public static void main(String[] args) {
        int[] sequence = {1,2,4,2,1};
        System.out.println("re=" +  new RepetitionFactory().getInstance(2).SingleNumber(sequence));
        
        int[] sequence2 = {10,20,20,30,20,10,10};
        System.out.println("re=" +  new RepetitionFactory().getInstance(3).SingleNumber(sequence2));
    }
}