Basics

// Create a 3-row, 4-column grid of integers
int[][] grid = new int[3][4];

         col 0  col 1  col 2  col 3
row 0  [   0      0      0      0   ]
row 1  [   0      0      0      0   ]
row 2  [   0      0      0      0   ]

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

[  1  2  3 ]
[  4  5  6 ]
[  7  8  9 ]

int[][] matrix = {
    {10, 20, 30},
    {40, 50, 60},
    {70, 80, 90}
};

System.out.println(matrix[0][0]);  // 10 (row 0, col 0)
System.out.println(matrix[1][2]);  // 60 (row 1, col 2)
System.out.println(matrix[2][1]);  // 80 (row 2, col 1)

matrix[1][1] = 99;  // change center element
// Row 1 is now: [40, 99, 60]

int[][] grid = new int[3][4];  // 3 rows, 4 columns

int rows = grid.length;        // 3 (number of rows)
int cols = grid[0].length;     // 4 (number of columns in first row)

grid  →  [ row0, row1, row2 ]
              ↓      ↓      ↓
           [0,0,0,0] [0,0,0,0] [0,0,0,0]

int[][] table = {
    {1, 2},
    {3, 4},
    {5, 6}
};

table → [ ref0, ref1, ref2 ]
            ↓       ↓       ↓
         [1, 2]  [3, 4]  [5, 6]

public class SeatingChart {
    public static void main(String[] args) {
        String[][] seats = {
            {"Alice", "Bob",   "Charlie"},
            {"Diana", "Eve",   "Frank"},
            {"Grace", "Henry", "Ivy"}
        };
        
        // Access specific seat
        System.out.println("Row 1, Seat 2: " + seats[1][2]);  // Frank
        
        // Move a student
        String temp = seats[0][1];         // Bob
        seats[0][1] = seats[2][0];         // Grace moves to Bob's seat
        seats[2][0] = temp;                // Bob moves to Grace's seat
        
        // Print the chart
        System.out.println("Rows: " + seats.length);      // 3
        System.out.println("Cols: " + seats[0].length);    // 3
    }
}

int[][] jagged = new int[3][];
jagged[0] = new int[2];   // row 0 has 2 elements
jagged[1] = new int[4];   // row 1 has 4 elements
jagged[2] = new int[1];   // row 2 has 1 element