Java: Sorting a Map by the value, when value is an arraylist [duplicate]

Question

I'm trying to sort a map by value. The data structure is as follows:

Map<String, ArrayList<Object>>

The ArrayList contains a set of properties for a set of objects whose name corresponds to the string key value in the map.

The idea is that I want to sort these objects by property. So, let's say that transparency was a property of these objects, and transparency is stored in ArrayList.get(3) for all objects.

How would I be able to sort it in a way such that:

I'm able to output:

Transparency     Object Name
   Value            Name 1
   Value            Name 2
   Value            Name 3 
   Value            Name 4

I've tried a few different sorts, and nothing seems to be working. Any ideas?

Edit: More information.

The objects inside the arrayList are all either Double type or String type, and I am defining the comparison between the two values through their default comparison. Essentially, I've organized the data like this:

The name of the object whose properties are contained in a list is the key for the map. The value given by the key is the list of properties for that object. I want to compare property(i), where i is user input, of Object A with property(i) of Object B, and then output them using System.out.print() or printf()

Your question is unclear. How do you define the comparison between two value arraylist objects. Please provide a more explicit example of the contents of your map. i.e. Key=xxx,Value={a,b,c}; Key=yyy,Value={x,y,z}; etc... and then show how you want the results arranged, and the rules for the comparison. — Jim Garrison
– Jim Garrison, Commented Sep 18, 2013 at 20:10

Aaron · Accepted Answer · 2013-09-19 16:49:51Z

1

How about implementing the Comparator interface using a class which accepts an index value a a config parameter to use when the compare method is invoked? And sort on the map.entrySet() ?

e.g :

   import java.util.*;

public class AClass {

    static Map<String, List<Object>> myMap = new HashMap<>();
    static final int ALPHA_ZULU = 0;
    static final int SOME_DOUBLE = 1;

    public static void main(String[] args) {

        List<Object> l1 = new ArrayList<>();
        l1.add("CHARLIE");
        l1.add(new Double(1));
        List<Object> l2 = new ArrayList<>();
        l2.add("ZULU");
        l2.add(new Double(9));
        List<Object> l3 = new ArrayList<>();
        l3.add("ALPHA");
        l3.add(new Double(12));
        List<Object> l4 = new ArrayList<>();
        l4.add("XRAY");
        l4.add(new Double(4));

        myMap.put("one", l1);
        myMap.put("two", l2);
        myMap.put("three", l3);
        myMap.put("four", l4);


        List<Map.Entry<String, List<Object>>> meList = new ArrayList<>(myMap.entrySet());
        Collections.sort(meList, new MyComaprator(ALPHA_ZULU));

        for(Map.Entry me : meList) {
            System.out.println(me.getKey() + " = " + me.getValue());
        }

    }






}


class MyComaprator implements Comparator<Map.Entry<String,List<Object>>> {

    final int compareIndex;

    public MyComaprator(int compareIndex) {
        this.compareIndex = compareIndex;
    }

    @Override
    public int compare(Map.Entry<String, List<Object>> me1, Map.Entry<String, List<Object>> me2) {

        Object item1 = me1.getValue().get(compareIndex);
        Object item2 = me2.getValue().get(compareIndex);
        int compareResult = 0;

        if(item1 instanceof String && item2 instanceof String) {
            compareResult = ((String)item1).compareTo((String)item2);
        } else if(item1 instanceof Double && item2 instanceof Double) {
            compareResult = (int)((double)item1 - (double)item2);
        } else {
            // invalid comparison perhaps?
        }

        return compareResult;
    }
}

edited Sep 19, 2013 at 16:49

answered Sep 18, 2013 at 21:00

Aaron

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2 Comments

wadda_wadda Over a year ago

I've tried something similar to this, but I ran in to the issue that this isn't accepted as an override. My exact code header is: public <E extends Comparable<? super >> int compare(Map.Entry<String, ArrayList<E>> map1, Map.Entry<String, ArrayList<E>> map2) { ... }

Aaron Over a year ago

@user130173 I've edited the example i provided with a working example. Construct the MyComparator with ALPHA_ZULU (0), and it will sort by index 0 of the value list. Use 1 to sort by the 1 index. If you're not on Java7 you'll need to copy the generic types in the diamond operator.

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