#100DaysOfCode – Day 8: Mastering 2D Arrays in Java
Up to this point, we’ve worked with 1D arrays — great for storing lists of items. But what if you want to represent something like:
That’s when we switch to 2D arrays — a grid-like structure made of rows and columns.
What Is a 2D Array?
In Java, a 2D array is basically an array of arrays. Imagine a table with rows and columns — that's what a 2D array looks like.
Each element is located using two indices:
How to Declare a 2D Array
Method 1: Declaration with values
int[][] matrix = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
Think of it like this:
sql
Row 0 → 1 2 3
Row 1 → 4 5 6
Row 2 → 7 8 9
Method 2: Declare and assign later
int[][] data = new int[2][3]; // 2 rows, 3 columns
data[0][0] = 10;
data[0][1] = 20;
// and so on...
How to Access Elements
To access a specific cell:
System.out.println(matrix[1][2]); // Output: 6 (row 1, column 2)
Remember: Indexing starts from 0 — not 1!
Looping Through a 2D Array (Nested Loops)
for (int i = 0; i < matrix.length; i++) { // Rows
for (int j = 0; j < matrix[i].length; j++) { // Columns
System.out.print(matrix[i][j] + " ");
}
System.out.println(); // Newline after each row
}
This prints:
1 2 3
4 5 6
7 8 9
matrix.length → number of rows matrix[i].length → number of columns in row i
Real-World Example: Seat Booking System
String[][] seats = {
{"A1", "A2", "A3"},
{"B1", "B2", "B3"}
};
System.out.println("Your seat: " + seats[1][2]); // Output: B3
Imagine building a cinema app — this is exactly how you’d store seat info!
Bonus: Sum All Elements in a 2D Array
int[][] nums = {
{1, 2, 3},
{4, 5, 6}
};
int sum = 0;
for (int i = 0; i < nums.length; i++) {
for (int j = 0; j < nums[i].length; j++) {
sum += nums[i][j];
}
}
System.out.println("Total sum: " + sum);
Output: Total sum: 21
Day 8 Challenges:
1. Create a 3x3 grid and fill it with numbers from 1 to 9
2️. Build a tic-tac-toe board using char[][]
3️. Write code to find the maximum value in a 2D array
4️. Calculate row-wise and column-wise sums
Summary:
Today you unlocked a powerful way to work with structured, tabular data in Java. 2D arrays are everywhere — in games, data dashboards, spreadsheets, and more. You now know how to declare, access, loop through, and solve real problems using them.
Tomorrow, we’ll go deeper into array-based algorithms — like searching, row/column sums, and diagonal logic.
Let’s level up our code — one array at a time.
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