How to represent a 2D matrix in Java?

I'll give you an example:

int rowLen = 10, colLen = 20;   
Integer[][] matrix = new Integer[rowLen][colLen];
for(int i = 0; i < rowLen; i++)
    for(int j = 0; j < colLen; j++)
        matrix[i][j] = 2*(i + j); // only an example of how to access it. you can do here whatever you want.

Clear?

You should use Vector for 2D array. It is threadsafe.

Vector<Vector<Double>>  matrix= new Vector<Vector<Double>>();

    for(int i=0;i<2;i++){
        Vector<Double> r=new Vector<>();
        for(int j=0;j<2;j++){
            r.add(Math.random());
        }
        matrix.add(r);
    }
    for(int i=0;i<2;i++){
        Vector<Double> r=matrix.get(i);
        for(int j=0;j<2;j++){
            System.out.print(r.get(j));
        }
        System.out.println();
    }

If this is your matrix indexes

00 01

10 11

You can get specifix index value like this

Double r2c1=matrix.get(1).get(0); //2nd row 1st column

Have a look at Vector

If you need thread safe behavior, use

Vector<Vector<Double>> matrix = new Vector<Vector<Double>>();

If you don't need thread safe behavior, use

ArrayList<ArrayList<Double>> matrix = new ArrayList<ArrayList<Double>>();