What Collections Can Be Used Instead of a 2D Array in Java?

What are the alternative data structures to using a 2D array in Java?

List<List<Integer>> matrix = new ArrayList<>();

Java provides several data structure alternatives to using 2D arrays. These alternatives are often more flexible and can ease operations such as resizing and dynamic data manipulation. Here, we will discuss some of the most effective collection types for replacing 2D arrays.

// Example of creating a List of Lists in Java
ArrayList<ArrayList<Integer>> matrix = new ArrayList<>();
for (int i = 0; i < rows; i++) {
    ArrayList<Integer> row = new ArrayList<>(); // Create a new row
    for (int j = 0; j < columns; j++) {
        row.add(0); // Initialize with zeros
    }
    matrix.add(row); // Add row to matrix
}

Causes

• Need for dynamic resizing
• Simplified data manipulation and access
• Enhanced readability and maintainability of code
• Ease of use with built-in methods for data structures

Solutions

• **ArrayList<ArrayList<T>>**: This creates a list of lists where each inner list can represent a row of your matrix. This allows dynamic resizing and ease of element manipulation. ```java ArrayList<ArrayList<Integer>> matrix = new ArrayList<>(); ``` **2D ArrayList as an Example**: ```java ArrayList<ArrayList<Integer>> matrix = new ArrayList<>(); for (int i = 0; i < numberOfRows; i++) { matrix.add(new ArrayList<>()); } ```
• **HashMap**: For sparse data, using a HashMap can be very effective. You can map coordinates to values, which saves space when dealing with a sparse matrix. ```java Map<Point, Integer> sparseMatrix = new HashMap<>(); sparseMatrix.put(new Point(x, y), value); ```
• **Custom Classes**: Creating a custom class can help encapsulate the 2D data structure more meaningfully, especially when each cell can have multiple attributes.

Mistake: Not accounting for dynamic resizing, leading to IndexOutOfBoundsException.

Solution: Always check the list size and use methods like `add()` and `remove()` for dynamic lists.

Mistake: Confusing maps with lists or forgetting how to access nested collections.

Solution: Use proper syntax when accessing nested elements, like `matrix.get(i).get(j)`.