What collection to use instead of 2D array in Java?

Question

I want to use a collection in place of 2D array so that I don't need to give its size at the time of declaration and I can add as many elements as I want dynamically.

Answer 1

The problem with List> is you have to redimension each row if you want to redimension your matrix.

If you want to use a sparse matrix, or maybe an infinite matrix you can do something like:

class SparseMatrix<X> {
  private Map<Coord, X> values = new HashMap<Coord, X>();

  public SparseMatrix() {
  }

  public X get(int x, int y) {
     return values.put(new Coord(x,y)); // null if there's no value
  }

  public void set(int x, int y, X value) { // you can use null (like in a List)
     values.set(new Coord(x,y), value);
  }

  private static class Coord {
    int x; int y;
    public Coord(int x, int y) {
       this.x = x;
       this.y = y;
    }

    @Override
    public boolean equals(Object other) {
       if (other instance of Coord) {
          Coord o = (Coord) other;
          return o.x == x && o.y == y;
       }
       return false;
    }

    @Override
    public int hashCode() {
       return o.x + o.y; // or some more clever implementation :)
    }

  }
}

Edit: Apache Commons HashCodeBuilder is a great tool for generating hash-codes.

Answer 2

The easiest way is to use nested collections... say (assuming your values are Strings) List<List<String>> which can then be used like this:

List<List<String>> fakeArray = new ArrayList<List<String>>();

// Pretend you fill it with values between these calls
String retrieve = fakeArray.get(0).get(0);

Edit: This was originally a Map<String,List<String>> which really doesn't make sense in this context.

However, you may want to see if Google Collections or Apache Commons Collections have something more specialized that you can use.

Answer 3

What do you want to be able to do with it? I would probably simply use a Collection<Collection<Element>> (where Collection might be replaced by List).

Or you might create your own class with metods to iterate over rows or columns or all elements as needed.

Answer 4

I'm personally using the Vector class for that purpose, though different requirements may eventually dictate the use of other, more specialized classes.

Answer 5

java.util.ArrayList is my preferred choice.

http://java.sun.com/j2se/1.5.0/docs/api/java/util/ArrayList.html

Answer 6

Import java.util.ArrayList;

ArrayList is what you want, you don't need to set up its size at creation time and you can add elements dynamically using the add method.

Answer 7

It depends on what you're trying to do, but I would recommend ArrayList. It's faster than Vector. Unless you care about synchronization! If you want it as a 2-dimensional list, then you create an ArrayList and each element of this list would be another ArrayList.

Answer 8

You could do a trial with an ArrayList having ArrayLists as items. If that doesn't do what you want, it will give instight in what you need to built yourself.

Answer 9

It depends on the way you want to use the data structure. Your options are:

• Two lists; it's your job to synchronize between them.
• A map; instead of a key-value relationship, your map entries will simply be tuples of objects.
• A list of 2-cell object arrays; each item in the list will be an object array of size 2.

EDIT: I completely misread the question; I thought it was about a 2D array of width 2.

Having properly read the question (I hope :-)), I agree with those who said list-of-lists.