GameOfLife.java

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class GameOfLife {

    public static void main(String[] args) throws InterruptedException {
        String glider = """
                ........................*...........
                ......................*.*...........
                ............**......**............**
                ...........*...*....**............**
                **........*.....*...**..............
                **........*...*.**....*.*...........
                ..........*.....*.......*...........
                ...........*...*....................
                ............**......................
                """;
        int width = 720;
        int height = 360;
        int period = 50;
        int rows = 18;
        int cols = 36;
        Board board = new Board(rows, cols);
        BoardCanvas boardCanvas = new BoardCanvas(board);
        JFrame frame = new JFrame("Conway's Game of Life");
        frame.add(boardCanvas);
        frame.setSize(width, height);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        String[] lines = glider.split("\n");
        for (int r = 0; r < lines.length; r++) {
            for (int c = 0; c < lines[r].length(); c++) {
                if (lines[r].charAt(c) == '*') {
                    board.alive(r, c);
                }
            }
        }
        while (true) {
            boardCanvas.repaint();
            Thread.sleep(period);
            board.next(); // advance to next gen
        }
    }

    public static class Board {
        final int rows;
        final int cols;

        boolean[][] grid; // [row][col], grid[0][1] = grid[row=0][col=1]

        Board(int rows, int cols) {
            this.rows = rows;
            this.cols = cols;
            grid = new boolean[rows][cols];
        }

        void alive(int row, int col) {
            set(row, col, true);
        }

        void set(int row, int col, boolean value) { // set(0, 1, true)
            grid[row][col] = value;
        }

        int countLiveNeighbors(int row, int col) {
            int count = 0;
            for (int r = row - 1; r <= row + 1; r++) {
                for (int c = col - 1; c <= col + 1; c++) {
                    if (r >= 0 && c >= 0 && r < rows & c < cols) {
                        if (r != row || c != col) {
                            if (grid[r][c]) {
                                count++;
                            }
                        }
                    }
                }
            }
            return count;
        }

        void next() {
            boolean nextGrid[][] = new boolean[rows][cols];
            for (int r = 0; r < rows; r++) {
                for (int c = 0; c < cols; c++) {
                    int liveNeighbors = countLiveNeighbors(r, c);
                    if (grid[r][c]) {
                        if (liveNeighbors == 2 || liveNeighbors == 3) {
                            nextGrid[r][c] = true;
                        }
                    } else {
                        if (liveNeighbors == 3) {
                            nextGrid[r][c] = true;
                        }
                    }
                }
            }
            grid = nextGrid;
        }
    }

    public static class BoardCanvas extends JPanel {
        final Board board;

        BoardCanvas(Board board) {
            this.board = board;
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);
            int boardWidth = getWidth();
            int boardHeight = getHeight();
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, boardWidth, boardHeight);

            g.setColor(Color.BLACK);
            int cellWidth = boardWidth / board.cols;
            for (int x = 0; x <= boardWidth; x += cellWidth) {
                g.drawLine(x, 0, x, boardHeight);
            }
            int cellHeight = boardHeight / board.rows;
            for (int y = 0; y <= boardHeight; y += cellHeight) {
                g.drawLine(0, y, boardWidth, y);
            }

            for (int r = 0; r < board.rows; r++) {
                for (int c = 0; c < board.cols; c++) {
                    if (board.grid[r][c]) {
                        g.fillRect(c * cellWidth, r * cellHeight, cellWidth, cellHeight);
                    }
                }
            }
        }
    }
}

Gosper glider gun from: https://people.sc.fsu.edu/~jburkardt/m_src/exm/lexicon.txt