Issue with Game of Life

Question

I'm working on a Java implementation of Conway's game of life as a personal project for myself. So far it works but the rules are coming out wrong. The expected patterns aren't showing up as well as it should. Is there something wrong with my code?

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Cell extends JComponent implements MouseListener {

  private int row, col;
  private boolean isLiving;

  public Cell(int r, int c) {
    this.row = r;
    this.col = c;
    this.addMouseListener(this);
  }

  public void isAlive(int neighbors) {
    if (this.isLiving) {
      if (neighbors < 2) {
        this.isLiving = false;
      } else if (neighbors == 2 || neighbors == 3) {
        this.isLiving = true;
      } else if (neighbors > 3) {
        this.isLiving = false;
      }
    } else {
      if (neighbors == 3) {
        this.isLiving = true;
      }
    }
  }

  public boolean isLiving() {
    return this.isLiving;
  }

  public void paintComponent(Graphics g) {
    if (this.isLiving) {
      g.fillRect(0, 0, 10, 10);
    } else {
      g.drawRect(0, 0, 10, 10);
    }
  }

  public void mouseClicked(MouseEvent e) {
    this.isLiving = !this.isLiving;
  }

  public void mouseEntered(MouseEvent e) {
  }

  public void mouseExited(MouseEvent e) {
  }

  public void mousePressed(MouseEvent e) {
  }

  public void mouseReleased(MouseEvent e) {
  }
}

Answer 1

I suspect¹ the problem with your code is that it is setting the cells to living or dead as soon as the neighbors are checked. That caused my early variants of this code to fail. That change of state has to be delayed until the entire grid (biosphere) has been checked.

This example shows typical Game of Life behavior.

1. Note that "suspicions ain't answers", so it is best to post an SSCCE.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Random;

public class GameOfLife extends JPanel {

    private final int row, col;
    private boolean isLiving;

    public static Random random = new Random();

    public GameOfLife(int r, int c) {
        this.row = r;
        this.col = c;
        MouseListener listener = new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                isLiving = !isLiving;
                repaint();
            }
        };
        this.addMouseListener(listener);
        isLiving = random.nextBoolean();
    }

    public boolean isAlive(int neighbors) {
        boolean alive = false;
        if (this.isLiving) {
            if (neighbors < 2) {
                alive = false;
            } else if (neighbors == 2 || neighbors == 3) {
                alive = true;
            } else if (neighbors > 3) {
                alive = false;
            }
        } else {
            if (neighbors == 3) {
                alive = true;
            }
        }
        return alive;
    }

    public void setAlive(boolean alive) {
        isLiving = alive;
    }

    public boolean isLiving() {
        return this.isLiving;
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (this.isLiving) {
            g.fillRect(0, 0, getWidth() - 1, getHeight() - 1);
        } else {
            g.drawRect(0, 0, getWidth() - 1, getHeight() - 1);
        }
    }

    public static void main(String[] args) {
        final int s = 40;
        final GameOfLife[][] biosphere = new GameOfLife[s][s];
        final JPanel gui = new JPanel(new GridLayout(s, s, 2, 2));
        for (int ii = 0; ii < s; ii++) {
            for (int jj = 0; jj < s; jj++) {
                GameOfLife cell = new GameOfLife(ii, jj);
                cell.setPreferredSize(new Dimension(10, 10));
                gui.add(cell);
                biosphere[ii][jj] = cell;
            }
        }

        ActionListener al = (ActionEvent ae) -> {
            boolean[][] living = new boolean[s][s];
            for (int ii = 0; ii < s; ii++) {
                for (int jj = 0; jj < s; jj++) {
                    int top = (jj > 0 ? jj - 1 : s - 1);
                    int btm = (jj < s - 1 ? jj + 1 : 0);
                    int lft = (ii > 0 ? ii - 1 : s - 1);
                    int rgt = (ii < s - 1 ? ii + 1 : 0);
                    int neighbors = 0;
                    if (biosphere[ii][top].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[ii][btm].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[lft][top].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[lft][btm].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[lft][jj].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[rgt][jj].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[rgt][top].isLiving()) {
                        neighbors++;
                    }
                    if (biosphere[rgt][btm].isLiving()) {
                        neighbors++;
                    }
                    living[ii][jj] = biosphere[ii][jj].isAlive(neighbors);
                }
            }
            for (int ii = 0; ii < s; ii++) {
                for (int jj = 0; jj < s; jj++) {
                    biosphere[ii][jj].setAlive(living[ii][jj]);
                }
            }
            gui.repaint();
        };

        Timer timer = new Timer(50, al);
        timer.start();

        JOptionPane.showMessageDialog(null, gui);
        timer.stop();
    }
}