Java: Constructing a 2D Matrix Array According to User Input Using a Scanner

I am trying to read elements of a 2-d 3x3 matrix using a scanner.
The input would look something like this:

3
11 2 4
4 5 6
10 8 -12

I am current geting the error:

Scanner scan = new Scanner(System.in);
int a = scan.nextInt();
scan.next();

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

for (int i = 0; i < a; i++) {
 
    String number = scan.nextLine();
    String[] arrRowItems1 = number.split(" ");
    List<Integer> list = new ArrayList<>(); 

    for (int j = 0; j < a; j++) {
        int arrItem = Integer.parseInt(arrRowItems1[j]); 
        list.add(arrItem);
    }

    array.add(list);
}

scan.close();

How do I go about doing this problem, so that a the end a 2d 3x3 matrix array is constructed according to user input? Thank You.

You could read the input line-by-line as a string, check it for validity, parse it and fill it in an int[][]-Matrix.
This way, the program keeps asking for input until it got three valid lines of numbers:

Scanner scan = new Scanner(System.in);
int dim = 3; //dimensions of the matrix
int line = 1; //we start with line 1
int[][] matrix = new int[dim][dim]; //the target matrix

while(line <= dim) {
	System.out.print("Enter values for line " + line + ": "); //ask for line
	String in = scan.nextLine(); //prompt for line
	String[] nums; //declare line numbers array variable
	
	//check input for validity
	if (!in.matches("[\\-\\d\\s]+")
			| (nums = in.trim().split("\\s")).length != dim) {
		System.out.println("Invalid input!");
		continue;
	}
	
	//fill line data into target matrix
	for (int i = 0; i < nums.length; i++) {
		matrix[line - 1][i] = Integer.parseInt(nums[i]);
	}
	
	line++; //next line
}

scan.close(); //close scanner (!)

//test output
for (int i = 0; i < matrix.length; i++) {
	System.out.println(Arrays.toString(matrix[i]));
}

The last for-loop is only for printing the result, just to check if it works:

Enter values for line 1: 11 2 4
Enter values for line 2: 4 5 6
Enter values for line 3: 10 8 -12
[11, 2, 4]
[4, 5, 6]
[10, 8, -12]

BTW by changing the value for dim (dimension) you could even get a 5x5 or whatever matrix!
I hope this helped!