How do I implement nested arraylist in java?

Question

I want to create a custom collection something like :

{"2011-10-27": {object1 , object2 , ...}  , "2011-10-28" : {object n , object n+1 , ..}}

And for this matter I think arraylist can satisfy my requirement but I don't know how I'm able to implement a nested arraylist in java.

If you think for my purpose arraylist is not suitable what kind of collection set do I have to use in java?

regards.

Answer 1

Consider using a Map , in which you can store key-value pairs. See the Map javadoc.

Answer 2

Looks like what you are looking for is a Map :

Map<Date, Set<Object>> map = new HashMap<Date, Set<Object>>();

map.put(new Date(), new HashSet<Object>(Arrays.asList(1,2,3)));

Answer 3

What you're showing looks more like a Map of Lists:

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

then you can populate each one with:

List list = new ArrayList();
list.add(object1);
map.put("2001-01-10",list);

Answer 4

I think what you're looking for is a Map storing a String as the key, with each key mapping to an ArrayList .

        Map<String,ArrayList> mp=new HashMap<String, ArrayList>();
        ArrayList<String>newarray = new ArrayList<String>();
        newarray.add("hello");
        mp.put("2011-10-97", newarray);

Depending on what kind of object you want to store in the ArrayList , you can change the type accordingly, I've left it as a String .

Answer 5

You can use a HashMap coupled to an ArrayList :

HashMap<String, ArrayList<Object>> myMap = new  HashMap<String, ArrayList<Object>>();
ArrayList<Object> myList = new ArrayList<Object>();
myList.add(anObject);
myMap.put("2011-10-27", myList);

You can also use a Date instead of String for the HashMap key.

In fact, instead of using HashMap and ArrayList , you can use any combination of classes implementing the Map interface and the List or Set interface