Falling sand simulation made with JavaFX, inspired by the coding train challenge #180 of the coding train

FallingSand.java

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.scene.canvas.*;
import javafx.scene.paint.Color;
import javafx.animation.*;
import javafx.util.Duration;
import javafx.scene.input.MouseButton;
import javafx.scene.input.KeyCode;

public class FallingSand extends Application{
	private static final int WIDTH = 800;
	private static final int HEIGHT = 800;
	private static int CELL_SIZE = 5;

	private int[][] grid;
	private double currentHue;
	private int brushSize = 11;
	private double chance = 0.05;
	private boolean simulationRunning = true;

	@Override
	public void start(Stage stage){
		StackPane pane = new StackPane();
		Canvas canvas = new Canvas(WIDTH, HEIGHT);
		pane.getChildren().add(canvas);
		GraphicsContext gc = canvas.getGraphicsContext2D();
		this.grid = new int[WIDTH/CELL_SIZE][HEIGHT/CELL_SIZE];

		canvas.setOnMouseDragged(e -> {
			int xPos = (int)(e.getX()/CELL_SIZE);
			int yPos = (int)(e.getY()/CELL_SIZE);
			for (int x = -this.brushSize/2; x < this.brushSize/2; x++){
				for (int y = -this.brushSize/2; y < this.brushSize/2; y++){
					if (this.chance >= Math.random() || e.getButton() == MouseButton.SECONDARY){
						int xp = xPos+x;
						int yp = yPos+y;
						if (xp >= 0 && yp >= 0 && xp < this.grid.length && yp < this.grid[0].length){
							if (e.getButton() == MouseButton.PRIMARY){
								if (this.grid[xp][yp] == 0) this.grid[xp][yp] = (int)(this.currentHue += 0.05);
							} else if (e.getButton() == MouseButton.SECONDARY){
								this.grid[xp][yp] = 0;
							}
						}
					}
				}
			}
		});

		canvas.setOnScroll(e -> {
			final int bInc = 2;
			final int cInc = 1;
			final double chanceInc = 0.05;
			final int minB = 3;
			final int minC = 5;

			if (e.getDeltaY() > 0){
				if (e.isControlDown()){
					CELL_SIZE += cInc;
					this.grid = new int[WIDTH/CELL_SIZE][HEIGHT/CELL_SIZE];
				} else if (e.isAltDown()){
					this.chance += chanceInc;
				} else {
					this.brushSize += bInc;
				}
			} else if (e.getDeltaY() < 0){
				if (e.isControlDown()){
					CELL_SIZE -= cInc;
					if (CELL_SIZE >= minC) this.grid = new int[WIDTH/CELL_SIZE][HEIGHT/CELL_SIZE];
				} else if (e.isAltDown()){
					this.chance -= chanceInc;
				} else {
					this.brushSize -= bInc;
				}
			}

			this.brushSize = Math.max(minB, this.brushSize);
			CELL_SIZE = Math.max(minC, CELL_SIZE);
			this.chance = Math.max(0, Math.min(1, this.chance));
		});

		Timeline loop = new Timeline(new KeyFrame(Duration.millis(10), e -> update(gc)));
		loop.setCycleCount(Animation.INDEFINITE);
		loop.play();

		Scene scene = new Scene(pane, WIDTH, HEIGHT);
		scene.setOnKeyPressed(e -> {
			if (e.getCode() == KeyCode.SPACE){
				this.simulationRunning = !this.simulationRunning;
			}
		});

		stage.setScene(scene);
		stage.setResizable(false);
		stage.setTitle("Falling sand");
		stage.show();
	}

	private Integer getElementAt(int x, int y){
		if (x >= 0 && y >= 0 && x < this.grid.length && y < this.grid[0].length){
			return this.grid[x][y];
		} else {
			return null;
		}
	}

	private void update(GraphicsContext gc){
		gc.clearRect(0, 0, WIDTH, HEIGHT);
		gc.setFill(Color.BLACK);
		gc.fillRect(0, 0, WIDTH, HEIGHT);

		for (int i = 0; i < this.grid.length; i++){
			for (int j = 0; j < this.grid[i].length; j++){
				if (this.grid[i][j] > 0){
					Color color = Color.hsb(this.grid[i][j], 1, 1);
					gc.setFill(color);
					gc.fillRect(i*CELL_SIZE, j*CELL_SIZE, CELL_SIZE, CELL_SIZE);
				}
			}
		}

		// Sand logic
		if (this.simulationRunning){
			for (int j = this.grid[0].length-1; j >= 0; j--){
				for (int i = 0; i < this.grid.length; i++){
					if (this.grid[i][j] > 0){
						Integer below = getElementAt(i, j+1);
						if (below != null){
							if (below == 0){
								this.grid[i][j+1] = this.grid[i][j];
								this.grid[i][j] = 0;
							} else {
								int dir = Math.random() > 0.5 ? 1 : -1;
								Integer a = getElementAt(i+dir, j+1);
								Integer b = getElementAt(i-dir, j+1);
								if (a != null && a == 0){
									this.grid[i+dir][j+1] = this.grid[i][j];
									this.grid[i][j] = 0;
								} else if (b != null && b == 0){
									this.grid[i-dir][j+1] = this.grid[i][j];
									this.grid[i][j] = 0;
								}
							}
						}
					}
				}
			}
		}

		gc.setFill(Color.WHITE);
		gc.fillText(String.format("Bursh size: %d\nCELL_SIZE: %d\nChance: %.3f\nSimulation running: %s", this.brushSize, CELL_SIZE, this.chance, this.simulationRunning), 10, 25);
	}

	public static void main(String[] args){
		System.out.println("LB - Add sand\nRB - Remove sand\nAlt-scroll: Chance\nControl-scroll: CELL_SIZE\nScroll: Brush size\nSPACE: pause/resume");
		launch(args);
	}
}