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.