public class CountIslands {

public int countIslands(int[][] matrix) {

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

return 0;

}

int count = 0;

for (int r = 0; r < matrix.length; r++) {

for (int c = 0; c < matrix[0].length; c++) {

// If a land cell is found, perform DFS to explore the full

// island, and include this island in our count.

if (matrix[r][c] == 1) {

dfs(r, c, matrix);

count++;

}

}

}

return count;

}

private void dfs(int r, int c, int[][] matrix) {

// Mark the current land cell as visited.

matrix[r][c] = -1;

// Define direction vectors for up, down, left, and right.

int[][] dirs = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};

// Recursively call DFS on each neighboring land cell to continue

// exploring this island.

for (int[] d : dirs) {

int nextR = r + d[0];

int nextC = c + d[1];

if (isWithinBounds(nextR, nextC, matrix) && matrix[nextR][nextC] == 1) {

dfs(nextR, nextC, matrix);

}

}

}

private boolean isWithinBounds(int r, int c, int[][] matrix) {

return 0 <= r && r < matrix.length && 0 <= c && c < matrix[0].length;

}

}