如何按地图对包含另一个地图的地图进行排序

Question

I have a HashMap of type:

HashMap<String, HashMap<String, Integer>>

This HashMap has values like

{name, {dateasString, numberOfTasks}} 

I want to sort this by numberOfTasks . I can't find the way to do that.

Answer 1

Create your custom Comparator that compares elemetns according to the sum of objects, and use Arrays.sort() or Collections.sort() after populating a list/array with the data:

//populate example data:
final Map<String,Map<String,Integer>> map = new HashMap<String, Map<String,Integer>>();
map.put("x", new HashMap<String, Integer>());
map.get("x").put("t1",1);
map.get("x").put("t2",1);
map.get("x").put("t3",1);
map.put("y", new HashMap<String, Integer>());
map.get("y").put("t1",2);
map.get("y").put("t2",2);
map.get("y").put("t3",2);
map.put("z", new HashMap<String, Integer>());
map.get("z").put("t1",3);
map.get("z").put("t2",3);
map.get("z").put("t3",3);

//populate the data in a list:  
List<String> list = new ArrayList<String>(map.keySet());

//sort the data with a custom comparator:
Collections.sort(list, new Comparator<String>() {
    private int getSum(String s) { 
        int sum = 0;
        for (Integer x : map.get(s).values()) {
            if (x != null) sum += x;
        }
        return sum;
    }
    public int compare(String o1, String o2) {
        return new Integer(getSum(o1)).compareTo(new Integer(getSum(o2)));
    }
});

System.out.println(list);

Note, to improve performance you could use a caching mechanism to avoid recalculating the sum of each element every time it is being compared.

Answer 2

Does the second HashMap have to be a HashMap ? Can you convert the second HashMap into a class specifically for this? Perhaps something like this:

private class TaskList
{
    String dateAsString;
    int numTasks;
    public TaskList(String dateAsString, int numTasks)
    {
        this.dateAsString = dateAsString;
        this.numTasks = numTasks;
    }
    public getDateAsString()
    {
        return dateAsString;
    }
    public getNumTasks()
    {
        return numTasks;
    }
}

Then you can say HashMap<String, TaskList> and access the number of tasks directly and sort them accordingly. Even then I don't think HashMap is right for this.

Answer 3

You can't sort a HashMap , but you can get a sorted Array (or a List) of the keys. How you want to define your sort is up to you - just modify the comparator (or the numberOfTasks function that it calls:)

So something like this?

public static void main() {
    final
    HashMap<String, HashMap<String, Integer>> map = new HashMap<String, HashMap<String, Integer>>();

    String[] keys = (String[]) map.keySet().toArray();

    Arrays.sort(keys, new Comparator<String>() {
        @Override
        public int compare(String k1, String k2) {
            int v1 = numberOfTasks(map.get(k1));
            int v2 = numberOfTasks(map.get(k2));
            return Integer.valueOf(v1).compareTo(Integer.valueOf(v2));
        }
    });

    // 'keys' is now sorted the way you want.

}

public static int numberOfTasks(HashMap<String, Integer> map) {
    int max = 0;
    for (Integer i : map.values()) {
        if (i > max) max = i;
    }
    return max;
}

Answer 4

I doubt there is a way to do this in the Java APIs. You will either have to do it manually by extracting the data in a separate model and sort that or redesign the model such as it will allow sorting on that field easier.

Answer 5

You would have to convert the hashmap to an array. If it were possible to sort a hashmap it would completely defeat the purpose of using one in the first place.

hashMap.keySet().toArray(); // returns an array of keys
hashMap.values().toArray(); // returns an array of values

Arrays.sort(array); // sorts an array

Hope that helps.