Sorting a HashMap based on date, How to reconstruct it back?

Question

I am trying to sort a Hash Map based on date. (Only Month and Day, year not relevant in sorting but relevant in key). My Hash Map:

static HashMap<Date, List<String>> Hash = new HashMap<Date, List<String>>(); 

static List<String> li = new ArrayList<String>(); 

Logic:

public static void main(String[] args){

        li.add("1"); 
        li.add("2"); 
        li.add("3"); 
        li.add("4"); 
        li.add("5"); 

        Date D1 = funx("1999-04-04"); 
        Date D2 = funx("1999-01-03"); 
        Date D3 = funx("1999-03-03"); 
        Date D4 = funx("1999-08-05"); 
        Date D5 = funx("1999-12-25"); 

        Hash.put(D1, li); 
        Hash.put(D2, li); 
        Hash.put(D3, li); 
        Hash.put(D4, li); 
        Hash.put(D5, li); 



        for (Map.Entry<Date, List<String>> entry : Hash.entrySet()) {

            Date key = entry.getKey(); 
            List value = entry.getValue(); 

            System.out.println("The key: "+key);
            System.out.println("The Comparison: "+compx(key));

        }
    }




    public static Date funx(String S){

        String DateStr = S;
        Date d = null;
        try {
            d = new SimpleDateFormat("yyyy-MM-dd").parse(DateStr);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        java.sql.Date d1 = new java.sql.Date(d.getTime());

        return d1; 
    }


    public static int compx(Date o){

        Calendar cal1 = Calendar.getInstance();
        cal1.setTime(getCurrentDate());
        Calendar cal2 = Calendar.getInstance();
        cal2.setTime(o);



        int month1 = cal1.get(Calendar.MONTH)+1; 
        int month2 = cal2.get(Calendar.MONTH)+1;

        System.out.println("Current Month: "+month1);
        System.out.println("Comparing With Month: "+month2); 

        if(month1 < month2) 
            return -1;
        else if(month1 == month2) {
            System.out.println("Current Month"+cal1.get(Calendar.DAY_OF_MONTH)); 
            System.out.println("Is ot be subtracted"+cal2.get(Calendar.DAY_OF_MONTH)); 
            return cal1.get(Calendar.DAY_OF_MONTH) - cal2.get(Calendar.DAY_OF_MONTH);
        }
        else return 1; 

    }

    public static Date getCurrentDate() {
        String formattedDate = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(Calendar.getInstance().getTime());
        String DateStr = formattedDate;
        Date d = null;
        try {
            d = new SimpleDateFormat("yyyy-MM-dd").parse(DateStr);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        java.sql.Date dx = new java.sql.Date(d.getTime());
        return dx;
    }

}

Now I get the return values, that is for comparison but how to reconstruct the Map again so that it is sorted?

Answer 1

For sorting, use TreeMap with custom Comparator to consider month and day,

  Map<Date, List<String>> Hash= new TreeMap<>(new Comparator<Date>() {
        @Override
        public int compare(Date o1, Date o2) {
            Calendar first = Calendar.getInstance();
            first.setTime(o1);
            Calendar second = Calendar.getInstance();
            second.setTime(o2);
            int month1 = first.get(Calendar.MONTH);
            int month2 = second.get(Calendar.MONTH);
            if (month1 != month2) {
                return month1 - month2;
            } else {
                int day1 = first.get(Calendar.DATE);
                int day2 = second.get(Calendar.DATE);
                return day1 - day2;
            }
        }
    });

Updated with Full Code:

  public static void main(String[] args) {
    List<String> li = new ArrayList<String>();
    li.add("1"); 
    li.add("2"); 
    li.add("3"); 
    li.add("4"); 
    li.add("5"); 
    Map<Date, List<String>> Hash = new TreeMap<>(new Comparator<Date>() {
        @Override
        public int compare(Date o1, Date o2) {
            Calendar first = Calendar.getInstance();
            first.setTime(o1);
            Calendar second = Calendar.getInstance();
            second.setTime(o2);
            int month1 = first.get(Calendar.MONTH);
            int month2 = second.get(Calendar.MONTH);
            if (month1 != month2) {
                return month2 - month1;
            } else {
                int day1 = first.get(Calendar.DATE);
                int day2 = second.get(Calendar.DATE);
                return day2 - day1;
            }
        }
    });

    Date D1 = funx("1999-04-04");
    Date D2 = funx("1999-01-03");
    Date D3 = funx("1999-03-03");
    Date D4 = funx("1999-08-05");
    Date D5 = funx("1999-12-25");

    Hash.put(D1, li);
    Hash.put(D2, li);
    Hash.put(D3, li);
    Hash.put(D4, li);
    Hash.put(D5, li);

    for (Map.Entry<Date, List<String>> entry: Hash.entrySet()) {
        System.out.println( entry.getKey() +" "+entry.getValue());
    }        
}

public static Date funx(String S) {

    String DateStr = S;
    Date d = null;
    try {
        d = new SimpleDateFormat("yyyy-MM-dd").parse(DateStr);
    } catch (ParseException e) {
        e.printStackTrace();
    }
    java.sql.Date d1 = new java.sql.Date(d.getTime());

    return d1;
}

Output:

1999-12-25 [1, 2, 3, 4, 5]
1999-08-05 [1, 2, 3, 4, 5]
1999-04-04 [1, 2, 3, 4, 5]
1999-03-03 [1, 2, 3, 4, 5]
1999-01-03 [1, 2, 3, 4, 5]

Answer 2

HashMaps are not inherently ordered, so taking out the elements, ordering them, and putting them back in a HashMap will remove the ordering. You can use TreeMap (which is ordered), or use a List or some other ordered mechanism. You could even do something more complicated depending on your requirements like a List of keys (to maintain order) and a Map of key, object (for fast retrieval).