Java large datastructure for storing a matrix

Question

I need to store a 2d matrix containing zip codes and the distance in km between each one of them. My client has an application that calculates the distances which are then s

Answer 1

Lately I've managed similar requisites for my master thesis.

I ended with a Matrix class that uses a double[], not a double[][], in order to alleviate double deref costs (data[i] that is an array, then array[i][j] that is a double) while allowing the VM to allocate a big, contiguous chunk of memory:

public class Matrix {

    private final double data[];
    private final int rows;
    private final int columns;

    public Matrix(int rows, int columns, double[][] initializer) {
        this.rows = rows;
        this.columns = columns;
        this.data = new double[rows * columns];

        int k = 0;

        for (int i = 0; i < initializer.length; i++) {
            System.arraycopy(initializer[i], 0, data, k, initializer[i].length);
            k += initializer[i].length;
        }
    }

    public Matrix set(int i, int j, double value) {
        data[j + i * columns] = value;
        return this;
    }

    public double get(int i, int j) {
        return data[j + i * columns];
    }
}

this class should use less memory than an HashMap since it uses a primitive array (no boxing needed): it needs only 906304 * 8 ~ 8 Mb (for doubles) or 906304 * 4 ~ 4 Mb (for floats). My 2 cents.

NB I've omitted some sanity checks for simplicity's sake