migratoryBirds * output int arr element that occus frequence is most high and

/**
 * migratoryBirds
 * output int arr element that occus frequence is most high and rank at the lowest down layer
 * constriants:
 * arrList numberSize: 5<=n<=2*10to5
 * it is guaranteed that each arr element is 1,2,3,4,5
 * @param arr
 * @return
 */
public static int migratoryBirds(List<Integer> arr){
    if (arr==null){
        return 0;
    }
    if (arr.isEmpty()){
        return 0;
    }
    int targetSize=1;
    for (int i = 0; i < 5; i++) {
        targetSize*=10;
    }
    if (arr.size()<5 || arr.size()>targetSize){
        return 0;
    }
    ArrayList<Integer> integers = new ArrayList<>();
    integers.add(1);
    integers.add(2);
    integers.add(3);
    integers.add(4);
    integers.add(5);
    for (Integer integer : arr) {
        if (!(integers.contains(integer))){
            return 0;
        }
    }
    HashMap<Integer, Integer> integerIntegerHashMap = new HashMap<>();
    for (Integer integer : arr) {
        if (integerIntegerHashMap.containsKey(integer)){
            integerIntegerHashMap.put(integer,integerIntegerHashMap.get(integer)+1);
        }else {
            integerIntegerHashMap.put(integer,1);
        }
    }
    ArrayList<CustomerIntStatistic> customerIntStatistics = new ArrayList<>();
    integerIntegerHashMap.forEach((k,v)->{
        CustomerIntStatistic customerIntStatistic = new CustomerIntStatistic();
        customerIntStatistic.setId(UUID.randomUUID().toString());
        customerIntStatistic.setArrIntEle(k);
        customerIntStatistic.setArrIntEleOccusFrequence(v);
        customerIntStatistics.add(customerIntStatistic);
    });
    Collections.sort(customerIntStatistics, new Comparator<CustomerIntStatistic>() {
        @Override
        public int compare(CustomerIntStatistic o1, CustomerIntStatistic o2) {
            if (o1.getArrIntEleOccusFrequence()>o2.getArrIntEleOccusFrequence()){
                return 1;
            } else if (o1.getArrIntEleOccusFrequence()<o2.getArrIntEleOccusFrequence()) {
                return -1;
            } else if (o1.getArrIntEleOccusFrequence()==o2.getArrIntEleOccusFrequence()) {
                if (o1.getArrIntEle()<o2.getArrIntEle()){
                    return 1;
                } else if (o1.getArrIntEle()>o2.getArrIntEle()) {
                    return -1;
                }
            }
            return 0;
        }
    });
    return customerIntStatistics.get(customerIntStatistics.size()-1).getArrIntEle();
}