Find number of islands from a give 2D matrix of 0s and 1s

For a given 2D matrix of 0s and 1s, find number of islands where an island is defined as number of 1s together. For example for a 2D matrix below:

0,1,1,0

0,1,1,0

0,0,0,0

1,1,0,0

Number of islands is 2.

Solution is going to O(m*n) where m and n are number or rows and columns of the matrix respectively.

    int numIslands(int [][]a) {

        if (a == null || a.length == 0 || a[0].length == 0) {

            return 0;

        }

        int rows = a.length;

        int cols = a[0].length;

        boolean[][]b = new boolean[rows][cols];

        int counter = 0;

        for (int i = 0; i < rows; i++)

            for (int j = 0; j < cols; j++)

                if (a[i][j] == 1 && !b[i][j]) {

                    counter ++;

                    visitIsland(a, b, i, j);

                }

        return counter;

    }

    void visitIsland(int[][]a, boolean[][]b, int i, int j) {

        if (a == null || b == null

                || a.length == 0 || b.length == 0

                || a[0].length == 0 || b[0].length == 0

                || i < 0 || j < 0 || i == a.length || j == a[0].length)

            return;

        b[i][j] = true;

        if (i + 1 < a.length && a[i+1][j] == 1 && !b[i + 1][j])

            visitIsland(a, b, i + 1, j);

        if (j + 1 < a[0].length && a[i][j+1] == 1 && !b[i][j+1])

            visitIsland(a, b, i, j + 1);

        if (i - 1 >= 0 && a[i-1][j] == 1 && !b[i - 1][j])

            visitIsland(a, b, i - 1, j);

        if (j - 1 >= 0 && a[i][j-1] == 1 && !b[i][j-1])

            visitIsland(a, b, i, j-1);

    }