How can I divide the array?

I have a block of code like the one below. As for the else condition, I want to add what I add to the totalDates array as if it were a single array.

public ResponseEntity<?> findByHourlyAggregated(@PathVariable String hours) {

    HashMap<Date, List<Post>> map = new HashMap<Date, List<Post>>();
    Date startDate = new Date();
    Date date = new Date();
    Timestamp endDate = null;
    Date endDate = new Date();
    List<Post> postAll = (List<Post>) findAll();
    Instant afterHours = null;
    List<Post> totalDates = new ArrayList<>();
    List<Date> allDates = new ArrayList<Date>();
    Date firstDate;
    firstDate = postAll.get(0).getDateCreated();
    for (Post p : postAll) {      
        startDate = p.getDateCreated(); //2020-08-11 10:42:08.0 , time of incoming data
        date = new Date(firstDate.getTime() + TimeUnit.HOURS.toMillis(Integer.parseInt(hours))); // Tue Aug 11 12:42:08 EET 2020 
        endDate = new Timestamp(date.getTime()); // 2020-08-11 12:42:08.0, add the parameter entered with the incoming data
        
        if (firstDate.getTime() <= endDate.getTime() && startDate.getTime() <= endDate.getTime()) {
            totalDates.add(p);
            
        }
        else {
            map.put(firstDate, totalDates);
            firstDate.setTime(endDate.getTime()); // value of new values when values in range are exhausted
            date = new Date(firstDate.getTime() + TimeUnit.HOURS.toMillis(Integer.parseInt(hours))); // Adds 2 hours
            endDate = new Timestamp(date.getTime());
        }
    }
    return new ResponseEntity<>(map, HttpStatus.OK);
}

I separate the date value from the Post data according to the value from the parameter. For example, when the value 2 is entered, I need to assign values between 10-12 to one array and values between 12-14 to the other array. As a result, I want to get a print out.

{
"2020-08-11T10:42:08.000+03:00": [
{
"id": 1,
"postBody": "Post 1",
"author": "michael",
"dateCreated": "2020-08-11T10:42:08.000+03:00"
},
{
"id": 2,
"postBody": "Post 2",
"author": "jam",
"dateCreated": "2020-08-11T10:45:21.000+03:00"
},
{
"id": 3,
"postBody": "Post 3",
"author": "jim",
"dateCreated": "2020-08-11T10:57:33.000+03:00"
},
{
"id": 4,
"postBody": "Post 4",
"author": "dwight",
"dateCreated": "2020-08-11T11:02:35.000+03:00"
},
{
"id": 5,
"postBody": "Post 5",
"author": "scott",
"dateCreated": "2020-08-11T11:52:51.000+03:00"
}
]
}
2020-08-12T12:42:08.000+03:00": [
{
...
},
{
...
}], 
....

I think you should initialize totalDates after map.put statement in else block. Then posts will be divided into different totalDates list object.

map.put(firstDate, totalDates);
totalDates = new ArrayList<>(); // This will create a new ArrayList instance

Update 1

Consider changing your HashMap

HashMap<Date, List<Post>> map = new HashMap<Date, List<Post>>();

to

HashMap<String, List<Post>> map = new HashMap<String, List<Post>>();

The keys in the map should be immutable. Using String as key would ensure that. You can then change the put logic to -

map.put(firstDate.toString(), totalDates);

Hope it helps.