对HashMap及其嵌套的HashMap进行排序

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 :(>

Answer 1

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.

Answer 2

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;
}