How to sort the month in Map Hashmap Java

Question

I got trouble when put month as key on Java hashmap. Can someone help me?

I want to display : {January=0.0, February=0.0, March=0.0, April=0.0}

But, the result from the code is : {March=0.0, January=0.0, February=0.0, April=0.0}

This is my code..

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    HashMap<String, Double> capitalCities = new HashMap<String,Double>();
    capitalCities.put("January", 0.0);
    capitalCities.put("February", 0.0);
    capitalCities.put("March", 0.0);
    capitalCities.put("April", 0.0);
    System.out.println(capitalCities); 
  }
}

I'm so struggling about it, please help

By the way, Java may be getting a new **“sequenced” collections interfaces**. See a brief preview introduction in yet another good [video by Nicolai Parlog](https://youtu.be/xBBuShS0ERs?t=91) on YouTube. — Basil Bourque, Aug 27 '22 at 03:57

Dawood ibn Kareem · Answer 1 · 2022-08-26T04:45:41.777

6

There are two problems with your approach, as far as I can see.

The first is that a HashMap doesn't sort its entries in any particular order. It's not specified which order the entries will appear in, if you iterate through the Map.

To overcome this, you should use a different kind of Map. A TreeMap should fit your requirement, as it stores its entries in order.

The second problem is that by default, String values will be sorted alphabetically, but you actually want to sort the months in calendar order. You can get around this by using the Month enumeration in the java.time package, as the key for your Map.

If you fix both these issues, the code will look something like this.

Map<Month,Double> capitalCities = new TreeMap<>();
capitalCities.put(Month.JANUARY, 0.0);
capitalCities.put(Month.FEBRUARY, 0.0);

and so on.

Addendum: Alexander Ivanchenko's solution using EnumMap is better than this one.

edited Aug 26 '22 at 04:45

answered Aug 26 '22 at 02:37

Dawood ibn Kareem

77,785
15
98
110

2

Another approach is to use a `LinkedHashMap` and rely on inserting the months in order. Then the original code works as is. – tgdavies Aug 26 '22 at 02:41
1

@tgdavies You could post your own answer. – Dawood ibn Kareem Aug 26 '22 at 02:44
@BasilBourque I prefer my original wording. Do you mind if I undo your edit? – Dawood ibn Kareem Aug 26 '22 at 04:32
Do as you feel best. I do recommend making some mention of`NavigableMap`. And I added more edits, linking to Javadoc. Perhaps you could add another edit rather than rollback. – Basil Bourque Aug 26 '22 at 04:36

score 4 · Answer 2 · answered Aug 26 '22 at 04:37

4

As @Dawood ibn Kareem has pointed out in his answer enum Month from java.time package is better option than using a plain String.

I want to add that since Java 5 we have a special purpose implementation of the Map interface - EnumMap, which maintain its entries sorted according to the natural order of enum used as a key.

Here's an example:

Map<Month, Double> valueByMonth = new EnumMap<>(Month.class);
valueByMonth.put(Month.JULY, 9.3);
valueByMonth.put(Month.APRIL, 4.8);
valueByMonth.put(Month.FEBRUARY, 3.9);
valueByMonth.put(Month.JANUARY, 1.0);
valueByMonth.put(Month.MARCH, 4.5);
        
valueByMonth.forEach((k, v) -> System.out.println(k + " -> " + v)); // printing map's contents

Output:

JANUARY -> 1.0
FEBRUARY -> 3.9
MARCH -> 4.5
APRIL -> 4.8
JULY -> 9.3

answered Aug 26 '22 at 04:37

Alexander Ivanchenko

25,667
5
22
46

Thank you. I didn't know about `EnumMap` until now. This solution is better than mine. – Dawood ibn Kareem Aug 26 '22 at 04:41
@DawoodibnKareem There's also a specialized version of `Set` - [`EnumSet`](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/EnumSet.html#method-summary), sometimes it might be handy. It's `allOf()` method can be used as a substitution of `values()`, and there are few more interesting methods like `range()`, which creates a set of enum-members that are within the range of two specified elements. – Alexander Ivanchenko Aug 26 '22 at 04:47

How to sort the month in Map Hashmap Java

2 Answers2