Tạo và Truy cập Mảng 2D

A grid of values

• A 2-D array 二维数组 stores values in a grid of rows and columns.
• Declare with two brackets: int[][] grid = new int[3][4]; (3 rows, 4 columns).
• Or an initializer: int[][] g = {{1, 2}, {3, 4}, {5, 6}}; (3 rows of 2).
• Think spreadsheet, game board, or pixel image.

Row then column

• Access an element with two indices: grid[row][col].
• grid[0][0] is the top-left; grid[1][2] is row 1, column 2.
• Row comes first, then column — a common thing to get backwards.
• Both indices start at 0, like every Java array.

A Java 2D array: grid(row)(col), with both indexes starting at 0

Row and column counts

• grid.length is the number of rows.
• grid[0].length is the number of columns (length of one row).
• For a rectangular grid, every row has the same column count.
• Use these two lengths to bound loops over the grid.

Two bounds to watch

• A valid element is grid[r][c] with 0 <= r < grid.length and 0 <= c < grid[0].length.
• Going out of range on either index throws ArrayIndexOutOfBoundsException.
• Mixing up row and column silently reads the wrong cell (or crashes).
• Double-check [row][col] order every time.