Spiral Number Pattern

Java 8 has no String.repeat, so we use a small helper method. In Java 11 and newer you can write " ".repeat(count).

/*
 * Problem : Spiral square.
 * Input   : One integer n (side length).
 * Output  : n lines of n numbers.
 * Approach: Walk around the borders and move the borders inward.
 * Time    : O(n^2)
 * Space   : O(n^2)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int size = new Scanner(System.in).nextInt();
        int[][] grid = new int[size][size];

        // Step 1: Set the four borders of the area that is still empty.
        int top = 0;
        int bottom = size - 1;
        int left = 0;
        int right = size - 1;

        // Step 2: The next number to write. It starts at 1.
        int value = 1;

        // Step 3: Repeat while the empty area still has rows and columns.
        while (top <= bottom && left <= right) {
            // Step 3a: Go right along the top row, then move the top border down.
            for (int column = left; column <= right; column++) {
                grid[top][column] = value++;
            }
            top++;

            // Step 3b: Go down along the right column, then move the right border left.
            for (int row = top; row <= bottom; row++) {
                grid[row][right] = value++;
            }
            right--;

            // Step 3c: Go left along the bottom row, if a row is left. Then move the bottom border up.
            if (top <= bottom) {
                for (int column = right; column >= left; column--) {
                    grid[bottom][column] = value++;
                }
                bottom--;
            }

            // Step 3d: Go up along the left column, if a column is left. Then move the left border right.
            if (left <= right) {
                for (int row = bottom; row >= top; row--) {
                    grid[row][left] = value++;
                }
                left++;
            }
        }

        // Step 4: Print every row of the grid with one space between numbers.
        for (int row = 0; row < size; row++) {
            StringBuilder line = new StringBuilder();
            for (int column = 0; column < size; column++) {
                if (column > 0) {
                    line.append(" ");
                }
                line.append(grid[row][column]);
            }
            System.out.println(line);
        }
    }
}