Sorting HashMap and its nested HashMap

Question

I'm trying to sort nested hashMaps , where both HashMaps need to be sorted.

The first hashMap should be sorted by key. Second one should be sorted by value. So far I manage to sort the keys of the first hashMap by making an array of the keys and sort it .

My hashmap looks like this

 HashMap<String,HashMap<String,Integer>> carOwners = new 
                                    HashMap<String,HashMap<String, Integer>>();

an Example is let say we have first String is Name , car brand, qnty.

Example: David:

Audi>5

Bmw>4

Izabel:

VW>10

MB>4

So Basicly first we sort the names, and then we sort the nested hash by value. How can this be done ... cant find any useful information :(>

You can not sort an HashMap, please consider using LinkedHashMap instead — TuyenNTA
– TuyenNTA, Commented Jun 28, 2017 at 22:37
By nature of HashMap they have no ordering. What do you mean by sorting them? — Chris
– Chris, Commented Jun 28, 2017 at 22:38
@TuyenNguyen LinkedHashMap is a HashMap. And technically you can't sort a LinkedHashMap either. — shmosel
– shmosel, Commented Jun 28, 2017 at 22:39
"Seriossly can't sort HashMap?" - Yes. Seriously. "It is the only way I figured out to solve my problem ..." - well you need need to rethink your problem. There are always alternatives. For example, don't use HashMap as your only data structure. — Stephen C
– Stephen C, Commented Jun 28, 2017 at 22:52

Alex Mamo · Accepted Answer · 2017-06-28 22:42:49Z

3

For sorting a Map you can use the following class: TreeMap. As the official documentation says,

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

If you want to sort the elements in insertion order, please use LinkedHashMap.

This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).

For sorting a Map by value, please see this post. It's also for Java7 and Java8.

Hope it helps.

edited Jun 28, 2017 at 22:42

answered Jun 28, 2017 at 22:38

Alex Mamo

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1 Comment

Alex Mamo Over a year ago

Have you figured it out?

Zabuzard · Accepted Answer · 2017-06-29 04:18:00Z

Make the first map as TreeMap and for the second Map, sort it by values. Refer this post

Below is the code snippet for your problem.

public static void main(String[] args) {
    Map<String, HashMap<String, Integer>> carOwners = new TreeMap<String, HashMap<String, Integer>>();
    HashMap<String, Integer> nameQuantity = new HashMap<String, Integer>();
    nameQuantity.put("Audi", 5);
    nameQuantity.put("BMW", 4);
    carOwners.put("David", sortByValue(nameQuantity)); 
    nameQuantity = new HashMap<String, Integer>();
    nameQuantity.put("VW", 10);
    nameQuantity.put("MB", 4);
    carOwners.put("Izabel", sortByValue(nameQuantity)); 
    for (Map.Entry<String, HashMap<String, Integer>> carOwnerEntry : carOwners.entrySet()) {
        System.out.println(carOwnerEntry.getKey());
        HashMap<String, Integer> nameQty = carOwnerEntry.getValue();
        for (Map.Entry<String, Integer> nameQtyEntry : nameQty.entrySet()) {
            System.out.println(nameQtyEntry.getKey() + " " + nameQtyEntry.getValue());
        }
    }

public static <K, V extends Comparable<? super V>> HashMap<K, V> sortByValue(Map<K, V> map) {
    List<Map.Entry<K, V>> list = new LinkedList<Map.Entry<K, V>>(map.entrySet());
    Collections.sort(list, new Comparator<Map.Entry<K, V>>() {
        public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) {
            return (o1.getValue()).compareTo(o2.getValue());
        });
    HashMap<K, V> result = new LinkedHashMap<K, V>();
    for (Map.Entry<K, V> entry : list) {
        result.put(entry.getKey(), entry.getValue());
    }
    return result;
}

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