Hollow Pyramid 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 pyramid of stars.
 * Input   : One integer n (number of rows).
 * Output  : n lines.
 * Approach: Print the first row, the last row, and two stars in the middle rows.
 * Time    : O(n^2)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method returns the text repeated "count" times. Java 11 has String.repeat for this.
static String repeat(String text, int count) {
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < count; i++) {
        result.append(text);
    }
    return result.toString();
}

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

        // Step 1: Repeat once for each row. Row numbers go from 1 to rows.
        for (int row = 1; row <= rows; row++) {
            // Step 2: Print the spaces before the first star.
            String line = repeat(" ", rows - row);

            if (row == 1) {
                // Step 3: The first row has only one star.
                line = line + "*";
            } else if (row == rows) {
                // Step 4: The last row is full of stars. It has 2n-1 stars.
                line = line + repeat("*", 2 * rows - 1);
            } else {
                // Step 5: Middle rows have a star, some spaces inside, and a star. Example: row 3 has 3 spaces inside.
                line = line + "*" + repeat(" ", 2 * row - 3) + "*";
            }

            // Step 6: Print the finished line.
            System.out.println(line);
        }
    }
}