How to group some objects by certain field in Java?

Question

I've been trying to group of facets by translated value, but what I always get is only one last object on the List (no dataset). Here is what I tried:

HashMap<String, List<Facet>> map = new HashMap<>();

for (Facet facet : getFacets()) {
        map.put(facet.getTranslatedValue(), new ArrayList<com.schneider.gss.model.Facet>());
        map.get(facet.getTranslatedValue()).add(facet);
    }

Can you suggest anything?

MulltiMap is the way you want to go. The reason thats happening is you are replacing your ArrayList everytime you add. To fix your implementation, you need to check if there is already an ArrayList where that key is. If there is one, add the Facet to the list, if there is not, create the arraylist and add the Facet like you are currently doing — Michael
– Michael, Commented Sep 22, 2014 at 13:34

user3769706 · Accepted Answer · 2014-09-22 13:34:15Z

2

Change your for loop as below

for (Facet facet : getFacets()) {
        if(map.get(facet.getTranslatedValue()) == null) {
            map.put(facet.getTranslatedValue(), new ArrayList<com.schneider.gss.model.Facet>());
        }
        map.get(facet.getTranslatedValue()).add(facet);
    }

answered Sep 22, 2014 at 13:34

user3769706

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Comments

Compass · Accepted Answer · 2014-09-22 13:32:54Z

1

You're overwriting your list each time you get an identical translated value with a new ArrayList. Instead, you should check if it exists:

HashMap<String, List<Facet>> map = new HashMap<>();

for (Facet facet : getFacets()) {
    //get the list
    ArrayList<com.schneider.gss.model.Facet> list = map.get(facet.getTranslatedValue());
    //if list doesn't exist, create it
    if(list == null) {
        map.put(facet.getTranslatedValue(), new ArrayList<com.schneider.gss.model.Facet>());
    }
    //then add to list
    map.get(facet.getTranslatedValue()).add(facet);
}

answered Sep 22, 2014 at 13:32

Compass

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Comments

Kamil.H · Accepted Answer · 2014-09-23 05:23:27Z

0

in Guava there is class Multimap (or ArrayListMultimap) which does exactly what you need

answered Sep 23, 2014 at 5:23

Kamil.H

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