How to implement a Map with array list as keys

Question

I have HashMap with ArrayList as key and value as Integer, how can I get value from specific key.

Map< List<Object>,Integer> propositionMap=new HashMap<List<Object>,Integer>();  

my key are:[Brand, ID], [Launch, ID], [Model, ID], [Brand, UserModelNoMatch], [ProducerPrice, UserModelMatch], [ProducerPrice, ID]]
my values are:[3, 5, 4, 2, 1, 6]

In my program in several time in different place i need to find a specific value for the specific key. i do not want to use for loop evry time to get value. how can i do that?

this is a bad idea. Using collections as keys is rarely a good idea — SoWhat
– SoWhat, Commented Aug 6, 2013 at 10:43
Seeing as how you use it, you really might want to make separate classes for Brand, Launch, Model and ProducerPrice. — bas
– bas, Commented Aug 6, 2013 at 10:45
I guess you have some severe issue with your data model if you want to map a whole list of objects to an Integer. Is this an index? Elaborate on your underlying problem, probably there is a better solution than putting lists in maps as a key. — Matthias
– Matthias, Commented Aug 6, 2013 at 10:46
@SomeshMukherjee Using mutable collections as keys is a bad idea. — Alexey Romanov
– Alexey Romanov, Commented Aug 6, 2013 at 10:57

Alexey Romanov · Accepted Answer · 2013-08-06 11:04:38Z

4

Putting aside that this is a bad idea (as described in the comments), you don't need to do anything special:

List<Object> list = new ArrayList<Object>();
// add objects to list

Map<List<Object>,Integer> propositionMap = new HashMap<List<Object>,Integer>();  
propositionMap.put(list, 1);
Integer valueForList = propositionMap.get(list); // returns 1

You can get the same value when constructing a list independently:

List<Object> list2 = new ArrayList<Object>();
// add the same objects (by equals and by hashcode) to list2 as to list

Integer valueForList = propositionMap.get(list2); // returns 1

But you need to be careful not to change the list after you use it as a key in the map!

list.add(new Object());
Integer valueForList = propositionMap.get(list); // likely returns null

Again, it's very likely a bad idea.

edited Aug 6, 2013 at 11:04

answered Aug 6, 2013 at 10:56

Alexey Romanov

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Comments

bas · Accepted Answer · 2013-08-06 10:57:34Z

Seeing as how you want the same behavior, I strongly recommend using an interface with classes.

public interface Proposition
{
    public int getID();
}

public class Brand implements Proposition
{
    private int id;

    public Brand(int _id_)
    {
        this.id = _id_;
    }

    public int getID()
    {
        return this.id;
    }
}

public class Launch implements Proposition
{
    private int id;

    public Launch(int _id_)
    {
        this.id = _id_;
    }

    public int getID()
    {
        return this.id;
    }
}

public class ProducerPrice implements Proposition
{
    private int id;
    private int UserModelMatch;

    public ProducerPrice(int _id_, int _UserModelMatch_)
    {
        this.id = _id_;
        this.UserModelMatch = _UserModelMatch_;
    }

    public int getID()
    {
        return this.id;
    }

    public int getUserModelMatch()
    {
        return this.UserModelMatch;
    }
}

And then using a hashmap for proposition objects

Map<Integer, Proposition> propositionMap = new HashMap<Integer, Proposition>();

Proposition newprop = new ProducerPrice(6, 1);
propositionMap.put(newprop.getID(), newprop);

Proposition someprop = propositionMap.get(6);

if (someprop instanceof ProducerPrice)
{
    ProducerPrice myprodprice = (ProducerPrice)someprop;
    // rest of logic here
}

blackSmith · Accepted Answer · 2013-08-06 11:03:45Z

0

You can get the value as usual way :

propositionMap.get(arrayListN)

until you modify the list itself after adding.

answered Aug 6, 2013 at 11:03

blackSmith

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