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Print Hollow Square in Java

intermediate· Patterns · Star Patterns

Problem

A hollow square only needs stars on its border — the first row, the last row, and the first and last position of every row in between — with every other position left blank.

Given a size n, print an n by n square where only the border is stars and the interior is blank.

Input

n = 5

Output

* * * * *
*       *
*       *
*       *
* * * * *

Java Program

Java

public class HollowSquare {
    public static void main(String[] args) {
        int n = 5;
        for (int i = 0; i < n; i++) {
            StringBuilder row = new StringBuilder();
            for (int j = 0; j < n; j++) {
                if (row.length() > 0) row.append(" ");
                boolean border = (i == 0 || i == n - 1 || j == 0 || j == n - 1); // true only on the outer edge
                row.append(border ? "*" : " ");
            }
            System.out.println(row);
        }
    }
}

▶ Run Code

Output

* * * * *
*       *
*       *
*       *
* * * * *

Core Logic

Visiting every position in the grid, just like the solid square, but only printing a star at the border positions and a blank everywhere else, hollows out the interior.

How It Works

1A position is on the border when its row is the first or last row, or its column is the first or last column — i == 0 || i == n - 1 || j == 0 || j == n - 1.
2Every position in the grid is still visited, unlike a hollow triangle where the interior skip depends on the row shape.
3Border positions print "*"; every other position prints a single space, keeping every row the same printed width.
4The same space-separator convention used by the solid square keeps the columns visually aligned.

For n = 5, row 0 and row 4 are entirely stars, while rows 1 through 3 only have stars in their first and last column.

💡

Key Point: The interior positions still have to print a blank character, not be skipped — omitting them instead of printing a space would collapse the row's width and break the square's alignment.

Complexity

Time Complexity: O(n²)Space Complexity: O(1)

Why: The nested loop still visits all n² positions to decide star-versus-blank, even though most interior positions print nothing but a space.

Key Concepts

nested for loopborder checkconditional character

Approach 2: Java 8

Java

import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class HollowSquareStream {
    public static void main(String[] args) {
        int n = 5;
        IntStream.range(0, n)
            .mapToObj(i -> IntStream.range(0, n)
                .mapToObj(j -> (i == 0 || i == n - 1 || j == 0 || j == n - 1) ? "*" : " ")
                .collect(Collectors.joining(" ")))
            .forEach(System.out::println);
    }
}

▶ Run Code

Output

* * * * *
*       *
*       *
*       *
* * * * *

Core Logic

Since every position's border-or-interior status depends only on its own row and column, mapping each column index directly to a star or a space reproduces the outline without visiting neighboring cells.

How It Works

1IntStream.range(0, n) generates one stream element per row i.
2For each row, an inner IntStream.range(0, n) generates that row's column indices j.
3.mapToObj(j -> (i == 0 || i == n - 1 || j == 0 || j == n - 1) ? "*" : " ") tests the same border condition the loop version does, per position.
4Collectors.joining(" ") joins that row's characters before forEach prints it.

For row 2 (an interior row), only j = 0 and j = 4 satisfy the border condition, so the joined row is a star, three blank positions, and a closing star.

💡

Key Point: A per-cell conditional formula like this maps onto a stream just as cleanly as a per-cell numeric one — the border test reads its own i and j only, with no dependency on any other cell's result.

Complexity

Time Complexity: O(n²)Space Complexity: O(1)

Why: The nested streams still evaluate the border condition once per grid position, a total proportional to n², without collecting the full square.

Print Hollow Square in Java

Problem

Java Program

Output

Core Logic

Key Concepts

Approach 2: Java 8

Output

Core Logic

Key Concepts

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