inserting many ArrayList inside a single structure

Question

as title says i want to know if exist any structure that can allows something like this:

key = 1 -> arrayList1 , arrayList2 , arrayList3...

someStructure.add(1,arrayList<1>);
someStructure.add(1,arrayList<2>);
someStructure.add(1,arrayList<3>);

IMPORTANT

the key has to remain the same for each list.

Answer 1

You can roll your own by using a Map

Map<YourKeyClass, List<Data>> someStructure
     = new HashMap<YourKeyClass, List<Data>>();

To your desired situation, looks like you need:

Map<YourKeyClass, List<List<Data>>> someStructure
     = new HashMap<YourKeyClass, List<List<Data>>>();

Here's a kickoff example of the structure:

class MyMapOfList<K, V> {
    Map<K, List<V>> innerMap;

    public MyMapOfListOfList() {
        innerMap = new HashMap<K, V>();
    }

    public void put(K key, V value) {
        List<V> list = innerMap.get(key);
        if (list == null) {
            list = new ArrayList<V>();
            innerMap.put(key, list);
        }
        list.add(value);
    }
}

And you may use it like this:

MyMapOfList<String, List<Data>> myMapOfList;

If you don't like to add/maintain the Lists internally in the Map, you could use a structure from third party library like MultiMap from Guava, which behaves more like you want/need.

More info:

• What does it mean to "program to an interface"?

Answer 2

A HashMap<Key,Value> would do it.

SSCCE:

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class Number {
    public static void main(String[] args) {
        try {

            Map mMap = new HashMap();
            mMap.put("PostgreSQL", "Free Open Source Enterprise Database");
            mMap.put("DB2", "Enterprise Database , It's expensive");
            mMap.put("Oracle", "Enterprise Database , It's expensive");
            mMap.put("MySQL", "Free Open SourceDatabase");

            Iterator iter = mMap.entrySet().iterator();

            while (iter.hasNext()) {
                Map.Entry mEntry = (Map.Entry) iter.next();
                System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
            }

            mMap.put("Oracle", "Enterprise Database , It's free now ! (hope)");

            System.out.println("One day Oracle.. : " + mMap.get("Oracle"));

        } catch (Exception e) {
            System.out.println(e.toString());
        }
    }
}  

Source: http://www.mkyong.com/java/how-to-use-hashmap-tutorial-java/