Is it possible to create a 2D arraylist?

[pahiker]

New here, moderate exposure to Java (not advanced, but not exactly a beginner either).

I've worked with 2D arrays before, but I didn't know if it was possible to create a 2D ArrayList. If so, how would it be defined?

[Norm]

I would think you can create a multidimensional array of any type.
<type>[][] name = new <type>[3][]; // create 2 dim array of 3 elements

[Fubarable]

Try it and see what happens. Please report back on what happens.

[pahiker]

I would think you can create a multidimensional array of any type.
<type>[][] name = new <type>[3][]; // create 2 dim array of 3 elements

That's the thing, I can see the [3][], but I can't wrap my head around how to do the <type>. How would you get it to handle two dimensions? I'd think you'd have to have two <type> definitions, but how would you get them to work together.

[pahiker]

Try it and see what happens. Please report back on what happens.

I have been trying, but just can't seem to figure out how to get it to work.

[m00nchile]

Since this seems a specific enough question, I'll just provide the answer. Creating a 2d arraylist is indeed possible, just not in the way you're used to with arrays. You create an ArrayList, then you add ArrayLists to it, like this:

ArrayList<ArrayList<Integer>> toparr = new ArrayList<ArrayList<Integer>>();
for(int i = 0; i < 10; i++)
  toparr.add(new ArrayList<Integer>());

Accessing the elemenst is also a little different than arrays:

int a = toparr.get(2).get(1);

This would fetch the ArrayList from toparr with the index 2, and then fetch the Integer from the contained ArrayList at index 1.

[pahiker]

Hmm, not what I expected. What I have been doing is using an array as the data in an arraylist.

Object[] arrData = { 12345, "test" }

ArrayList<Object> arrList = new ArrayList<Object>();
toparr.add(arrData);


May not be the exact code, but close.

[masijade]

But, do want a 2D ArrayList or do you want an ArrayList of arrays?

[pahiker]

I'm happy using what I have. This was more a question of curiosity. I couldn't find it addressed anywhere on java.sun, or in any other sites or books that address ArrayList. I tried a couple ways to do it on my own, with no success, so I thought I would ask some "experts" to see if it was even a possibility.

The "inner" array is a fixed-size array in my application (7 entries), but the "outer" array needs to be able to grow (hence the ArrayList). So the array-inside-an-ArrayList works fine.

[masijade]

Then what you should be using is

Object[] arrData = { 12345, "test" }

ArrayList<Object[]> arrList = new ArrayList<Object[]>();
toparr.add(arrData);

to, at least, make it a specific as possible.

But, since what you're working with is a "mixed" array, it would probably make more sense to write a class instead of using an array. I.E. if your "fixed" array is really something that contains, say, an integer, two string, a double, a date, and two more strings then what you should is instead of

Obect[] a = new Object() { 0, "a", "b", 0.0, new Date(), "c", "d" };
... a[0] ... a[1] ...

you should be writing a class

class Member {
  int id;
  String firstName;
  String lastName;
  double fee;
  Date joinDate;
  String preferredGroup;
  String alias;

  Member(int id, String firstName, String lastName, double ...) {
    this.id = id;
    this.firstName = firstName;
    ...
  }

  public int getId() { return id; }

  ...
}

Member m = new Member(0, "a", "b", 0.0, new Date(), "c", "d");
... m.getId() ... m.getFirstName() ...

// and use ArrayList<Member>

[Norm]

how to do the <type>

I meant the <type> to be meta. It should be replaced with any valid java type. For example:
int[][] a2DimInt = new int[2][];
ArrayList[][] a2DimAL = new ArrayList[2][];
String[][] a2DimStr = new String[2][];
etc

[pahiker]

Hmm, interesting. I'll give that some thought. Thanks.

[JosAH]

Maybe a bit of Goedelization can be of any help: Goedelization is the mapping of more than one number to just one number. If no more than 2D matrixes are need the following mapping will do: given i and j the Goedelization could be (i << 16) | j for i and j < 2^16. A Map can come in handy too:

Map<Integer, Double> map= new HashMap<Integer, Double>

... and the following methods complete this trick:

public Double get(int i, int j) {
   return map.get((i << 16) | j);
}

public void set(int i, int j, double d) {
   map.put((i << 16) | j, d);
}

When the get( ... ) method returns null there was no element stored at (i, j).

kind regards,

Jos