Nested Loops in Java Explained — How They Work, When to Use Them, and Common Traps

public class SingleLoopDemo {
    public static void main(String[] args) {

        // This loop starts at rowNumber = 1
        // It keeps running as long as rowNumber is 5 or less
        // After each iteration, rowNumber increases by 1
        for (int rowNumber = 1; rowNumber <= 5; rowNumber++) {

            // This line runs ONCE per iteration
            System.out.println("Row number: " + rowNumber);
        }

        // Execution reaches here only AFTER the loop finishes completely
        System.out.println("Loop is done.");
    }
}

public class MultiplicationTable {
    public static void main(String[] args) {

        System.out.println("--- 5x5 Multiplication Table ---\n");

        // Outer loop: controls the current ROW (1 through 5)
        for (int row = 1; row <= 5; row++) {

            // Inner loop: controls the current COLUMN (1 through 5)
            // This entire inner loop runs completely before row increases by 1
            for (int column = 1; column <= 5; column++) {

                int product = row * column;

                // %-4d means: print an integer, left-padded to 4 characters wide
                // This keeps columns neatly aligned regardless of digit count
                System.out.printf("%-4d", product);
            }

            // After the inner loop finishes all 5 columns, move to a new line
            // This happens once per outer loop iteration (i.e., once per row)
            System.out.println();
        }
    }
}

public class StarPatterns {
    public static void main(String[] args) {

        int totalRows = 5;

        // ── Pattern 1: Right-angle triangle (growing) ──
        System.out.println("Pattern 1: Right-angle triangle");

        for (int currentRow = 1; currentRow <= totalRows; currentRow++) {

            // Inner loop runs from 1 up to currentRow
            // When currentRow is 3, we print exactly 3 stars — dynamic!
            for (int starCount = 1; starCount <= currentRow; starCount++) {
                System.out.print("* ");
            }

            System.out.println(); // Move to next line after each row
        }

        System.out.println(); // Blank line between patterns

        // ── Pattern 2: Inverted triangle (shrinking) ──
        System.out.println("Pattern 2: Inverted triangle");

        for (int currentRow = totalRows; currentRow >= 1; currentRow--) {

            // Now outer loop counts DOWN, inner loop still goes from 1 to currentRow
            // Row 5 prints 5 stars, Row 4 prints 4 stars, etc.
            for (int starCount = 1; starCount <= currentRow; starCount++) {
                System.out.print("* ");
            }

            System.out.println();
        }

        System.out.println();

        // ── Pattern 3: Solid square ──
        System.out.println("Pattern 3: Solid 5x5 square");

        for (int currentRow = 1; currentRow <= totalRows; currentRow++) {

            // Inner loop always runs exactly totalRows times — a fixed square
            for (int currentColumn = 1; currentColumn <= totalRows; currentColumn++) {
                System.out.print("* ");
            }

            System.out.println();
        }
    }
}

public class CinemaSeatingChart {
    public static void main(String[] args) {

        int totalRows = 3;
        int seatsPerRow = 4;

        // Declare a 2D array: 3 rows, 4 columns
        // Think of this as a grid with 3 rows of 4 seats each
        int[][] seatNumbers = new int[totalRows][seatsPerRow];

        int seatCounter = 1;

        // ── Step 1: Fill the grid with seat numbers ──
        // Outer loop moves through each ROW of the array
        for (int row = 0; row < totalRows; row++) {

            // Inner loop moves through each COLUMN (seat) within the current row
            for (int seat = 0; seat < seatsPerRow; seat++) {

                // Assign a unique seat number to this position in the grid
                seatNumbers[row][seat] = seatCounter;
                seatCounter++; // Move to next seat number
            }
        }

        // ── Step 2: Print the seating chart ──
        System.out.println("=== Cinema Seating Chart ===");
        System.out.println("Row\t| Seat 1\t| Seat 2\t| Seat 3\t| Seat 4");
        System.out.println("--------|-----------|-----------|-----------|----------");

        // Use nested loops again to READ the grid we just filled
        for (int row = 0; row < totalRows; row++) {

            // Print the row label (Row 1, Row 2, Row 3 — adding 1 so it's not zero-indexed)
            System.out.print("Row " + (row + 1) + "\t|");

            for (int seat = 0; seat < seatsPerRow; seat++) {

                // Read and print the seat number at this [row][seat] position
                System.out.print(" Seat " + seatNumbers[row][seat] + "\t|");
            }

            System.out.println(); // New line after all seats in the row are printed
        }

        System.out.println("\nTotal seats: " + (totalRows * seatsPerRow));
    }
}