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 : Centered pyramid of stars.
 * Input   : One integer n (number of rows).
 * Output  : n lines.
 * Approach: Print spaces first, then stars.
 * 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: Count the spaces before the stars. Row 1 needs the most spaces.
            int spaces = rows - row;

            // Step 3: Count the stars. Each row adds 2 stars. Row 1 has 1 star, row 2 has 3 stars.
            int stars = 2 * row - 1;

            // Step 4: Print the spaces and then the stars.
            System.out.println(repeat(" ", spaces) + repeat("*", stars));
        }
    }
}