How To - multi dimensional ArrayList of 2 ArrayLists ????

Posted on 2008-10-29
Last Modified: 2012-06-22

What's the (best) way/syntax to create an Array/ArrayList/List that consists of 2 ArrayLists?  

List<someObject> oldObject = new ArrayList<someObject>();
List<someObject> newObject = new ArrayList<someObject>();
// do stuff
// I want to return a Array/ArrayList/List containing
// List<someObject> oldObject, List<someObject> newObject
// something like this:  someObject[][] = new someObject[oldObject.size][newObject.size];
Anybody know how to do that?
Question by:pdlarue
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LVL 24

Expert Comment

ID: 22835134
     ArrayList[] a2 = new ArrayList[2];
      a2[0] = new ArrayList<String>();
      a2[1] = new ArrayList<String>();

JAVA refuses to create a generic 2 dimensional List-family,

List<String>[] = new ArrayList<String>()[];  // Is not possible.

LVL 24

Expert Comment

ID: 22835139
I meant:

List<String>[] = new ArrayList<String>()[2];  // Is not possible.


Author Comment

ID: 22836837
OK.  Is it possible to create a multi-dimensional array that contains 2 ArrayLists?
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Author Comment

ID: 22836985
Is it possible to use multidimensional HashMap?  Something like this:

myHashMap.put (key, object1, object2);
LVL 24

Expert Comment

ID: 22838504
Think: what would be the result of

        myHashMap.get(someKey) ??

It IS possible on the contrary to create a HashMap of HashMap's:

       HashMap<String, HashMap<String, MyObject>> h2 = new HashMap<String, HashMap>();

now you can:

       h2.put(thisKey, new HashMap<String, MyObject>());


Author Comment

ID: 22843315
OK.  Your answers are great.  I know if anyone can point me in the right direction you can.  I may not be phrasing the question quite right.  This is the result I want.  Do you know of any way to do this?

newCustomer[0], oldCustomer[0]
newCustomer[1], oldCustomer[1]
newCustomer[2], oldCustomer[2]
newCustomer[3], oldCustomer[3]
newCustomer[4], oldCustomer[4]

   OR   (if HashMap is necessary)

"0",newCustomer[0], oldCustomer[0]
"1",newCustomer[1], oldCustomer[1]
"2",newCustomer[2], oldCustomer[2]
"3",newCustomer[3], oldCustomer[3]
"4",newCustomer[4], oldCustomer[4]

I am trying to bundlie this up in an Array or HashMap or something from the server-side code and send to the client-side code.  The gui developer is using Flex, which has the ability to get multiple objects in the same iteration (just like the first group of objects listed above).  Since this is a comparison between the old object and the new object (to include properties) the ListArrays will be the same size.

If you can tell me how to do this I'll bump up the points to 500.  Thank you for your help.
LVL 24

Expert Comment

ID: 22843928
A HashMap stores relations from 1 key to only 1 object.

You can of course put the next 'hidden' class at the bottom of your source:

class CustomerPair
     Customer oldC;
     Customer newC;

     public CustomerPair(Customer o, Customer n)
          oldC = o;
          newC = n;

now you can use code like this (ONLY IN THIS CLASS ABOVE THE HIDDEN CLASS!):

HashMap<String, CustomerPair> links = new HashMap<String, CustomerPair>();

// and:

links.put("Jensen", new CustomerPair(new Customer(p), new Customer(q)));

// Not very beautiful but it's a workable solution.
// And, please, don't praise me too much; I'm just an old programmer ..................

LVL 24

Accepted Solution

sciuriware earned 500 total points
ID: 22843947
And, because one day you want the data back:

   CustomerPair t = links.get("Jensen");
   if(t != null)
         Customer oldJensen = t.oldC;


Author Comment

ID: 22846500
Thank you.  I'll give it a try.
LVL 24

Expert Comment

ID: 22847937
Thanks, you gave me a promotion.


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