dfs would mark every connected land 1; then count++. 

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private int m;
private int n;

public int numIslands(char[][] grid) {
    if (grid.length == 0 || grid[0].length == 0) return 0;
    m = grid.length; 
    n = grid[0].length;
    int count = 0;
    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < n; ++j) {
            if (grid[i][j] == '1') {
                dfs(grid, i, j);
                count++;
            }
        }
    }
    return count;
}

public void dfs(char[][] grid, int i, int j) {
    if (i < 0 || j < 0 || i >= m || j >= n || grid[i][j] == '0') return;
    grid[i][j] = '0';
    dfs(grid, i-1, j); dfs(grid, i+1, j); dfs(grid, i, j-1); dfs(grid, i, j+1); 
}

If count is outside dfs, i.e. outside inner for loop, then each row would make count++: a count == grid.length: meaningless, not dfs.

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private int m;
private int n;

public int numIslands(char[][] grid) {
    if (grid.length == 0 || grid[0].length == 0) return 0;
    m = grid.length; 
    n = grid[0].length;
    int count = 0;
    for (int i = 0; i < m; ++i) {
        for (int j = 0; j < n; ++j) {
            if (grid[i][j] == '1') {
                dfs(grid, i, j);
            }
            count++;
        }
    }
    return count;
}

public void dfs(char[][] grid, int i, int j) {
    if (i < 0 || j < 0 || i >= m || j >= n || grid[i][j] == '0') return;
    grid[i][j] = '0';
    dfs(grid, i-1, j); dfs(grid, i+1, j); dfs(grid, i, j-1); dfs(grid, i, j+1); 
}