Hollow Square Star 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 : Hollow square of stars.
 * Input   : One integer n (side length).
 * Output  : n lines.
 * Approach: Check if each position is on the border.
 * Time    : O(n^2)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

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

        // Step 1: Repeat once for each row. Row numbers go from 1 to size.
        for (int row = 1; row <= size; row++) {
            StringBuilder line = new StringBuilder();

            // Step 2: Visit each column of the row. Column numbers go from 1 to size.
            for (int column = 1; column <= size; column++) {
                // Step 3: A position is on the border if it is in the first or last row or column.
                boolean isBorder = row == 1 || row == size || column == 1 || column == size;

                // Step 4: Add a star on the border and a space inside.
                line.append(isBorder ? "*" : " ");
            }

            // Step 5: Print the finished line.
            System.out.println(line);
        }
    }
}