IslandMatrixCalculator.java

public class IslandMatrixCalculator {

    public int calculateNumberOfIslands(int[][] matrix) {
        int islands = 0;
        if (matrix.length == 0) return islands;

        for (int i=0; i<matrix.length; i++) {
            for (int j=0; j<matrix[0].length; j++) {
                if (matrix[i][j] == 1) {
                    removeIsland(matrix, i, j);
                    islands++;
                }
            }
        }

        return islands;
    }

    private void removeIsland(int[][] matrix, int i, int j) {
        if (matrix[i][j] == 1) {
            matrix[i][j] = 0;

            removeIsland(matrix, i, Math.max(0, j-1)); // [i, j-1]
            removeIsland(matrix, Math.max(0, i-1), j); // [i-1, j]
            removeIsland(matrix, i, Math.min(matrix[0].length-1, j+1)); // [i, j+1]
            removeIsland(matrix, Math.min(matrix.length-1, i+1), j); // [i+1, j]
        }
    }
}