Java: Read the mass data and find the number of islands

Last update on May 13 2025 12:58:50 (UTC/GMT +8 hours)

Count Islands from Grid

There are 10 vertical and horizontal squares on a plane. Each square is painted in blue and green. Blue represents the sea, and green represents the land. When two green squares are in contact with the top and bottom, or right and left, they are ground. The area created by only one green square is called an "island". For example, the figure below shows five islands.
Write a Java program to read the mass data and find the number of islands.

Input:
A single data set is represented by 10 rows of 10 numbers representing green squares as 1 and blue squares as zeros.
Output: For each data set, output the number of islands.

Visual Presentation:

Sample Solution:

Java Code:

// Importing the Scanner class from java.util package
import java.util.Scanner;

// Main class named "test"
public class test {

    // 2D array to represent the map
    public static boolean[][] map;

    // Array representing possible moves: down, right, up, left
    public static int[][] move = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};

    // Recursive method to perform depth-first search (DFS) on the map
    public static void fds(int i, int j){
        // Marking the current cell as visited
        map[i][j] = false;

        // Checking neighbors in all four directions
        for(int k = 0; k < 4; k++){
            int i2 = i + move[k][0];
            int j2 = j + move[k][1];

            // Recursively applying DFS to unvisited neighbors within the map boundaries
            if(0 <= i2 && i2 < 10 && 0 <= j2 && j2 < 10 && map[i2][j2]){
                fds(i2, j2);
            }
        }
    }

    // Main method to execute the program
    public static void main(String[] args) {
        // Prompting the user to input 10 rows of 10 numbers representing the map
        System.out.println("Input 10 rows of 10 numbers representing green squares (island) as 1 and blue squares (sea) as zeros");

        // Creating a Scanner object to read input from the console
        Scanner sc = new Scanner(System.in);

        // Initializing the map array
        map = new boolean[10][10];

        // Reading input to populate the map
        for(int i = 0; i < 10; i++){
            char[] s = sc.next().toCharArray();
            for(int j = 0; j < 10; j++){
                // Converting characters to boolean values (1 as true, 0 as false)
                map[i][j] = s[j] == '1';
            }
        }

        // Variable to store the number of islands
        int x = 0;

        // Iterating through each cell on the map
        for(int i = 0; i < 10; i++){
            for(int j = 0; j < 10; j++){
                // If the cell represents an island and is not visited, perform DFS
                if(map[i][j]){
                    fds(i, j);
                    // Incrementing the island count
                    x++;
                }
            }
        }

        // Displaying the number of islands
        System.out.println("Number of islands:");
        System.out.println(x);
    }
} 

Sample Output:

Input 10 rows of 10 numbers representing green squares (island) as 1 and blue squares (sea) as zeros
1100000111
1000000111
0000000111
0010001000
0000011100
0000111110
0001111111
1000111110
1100011100
1110001000
Number of islands:
5

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to count the number of islands in a grid using depth-first search with diagonal connectivity.
• Write a Java program to count islands in a grid that wraps around its edges (toroidal array).
• Write a Java program to compute the area of each island in a grid and output the island with the maximum area.
• Write a Java program to count the number of distinct islands in a grid, considering islands with identical shapes as the same.

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• Java Basic Part-II Programming Exercises Home ↩
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