Wzorzec projektowy adapter

MapMatrix.java

/**
 * @author Mateusz Angulski <mateusz@angulski.pl>.
 */
public class MapMatrix implements Matrix {
    private final int size;

    private final HashMap<String, Integer> matrix;

    public MapMatrix(int size) {
        this.size = size;
        this.matrix = new HashMap<>(size * size);
    }

    public int get(int x, int y) {
        this.assertBoundaries(x, y);
        return this.matrix.getOrDefault(key(x, y), 0);
    }

    public void set(int x, int y, int value) {
        this.assertBoundaries(x, y);
        this.matrix.put(key(x, y), value);
    }

    private void assertBoundaries(int x, int y) {
        if (x < 0 || x > size || y < 0 || y > size) {
            throw new IllegalArgumentException(key(x, y));
        }
    }

    public int size() {
        return size;
    }

    private String key(int x, int y) {
        return x + "," + y;
    }
}