[flexcoders] Extend Array

[smitade]

I'm trying to extend an Array into a 2D matrix. Any suggestions? The following 
code fails but I don't understand why.

        public class MatRix extends Array
        {
                public function MatRix(numRows:int=0,numCols:int=0)
                {
                        super(numRows);
                        for (var i:int=0;i<numRows;i++)
                        {
                                for (var j:int=0;j<numCols;j++)
                                {
                                        this[j]=new Array();
                                }
                        }
                }