An example of my flappy balloon JavaFX Project

GameCore.java

package core;

import javafx.scene.canvas.GraphicsContext;
import javafx.scene.input.MouseEvent;
import javafx.scene.paint.Color;
import libs.Configs;
import libs.CoreFunc;
import libs.GameText;

import java.util.ArrayList;
import java.util.Random;

public class GameCore implements CoreFunc {

    // Main Game variables should be here
    Background background = new Background();
    Balloon balloon = new Balloon();
    GameText text = new GameText(Color.YELLOW, Color.YELLOW);

    int textX = 10;
    int textY = 50;
    int fontSize = 30;

    int balloonX = 500;
    int balloonY = 300;

    int speed = 2;

    int hole = 160;
    int wallWidth = 50;
    int wallHeight = (Configs.appHeight - hole) / 2;

    Wall wall1 = new Wall();
    int wallX = Configs.appWidth - wallWidth;
    int wallY = 0;

    Wall wall2 = new Wall();
    int wall2X = Configs.appWidth - wallWidth;
    int wall2Y = Configs.appHeight - wallHeight;

    int wall1Height = wallHeight;

    @Override
    public void init(GraphicsContext gc) {
        // initialize objects (initial position, initial size, etc)
        background.resize(Configs.appWidth, Configs.appHeight);
        background.render(gc, 0, 0);

        wall1.resize(wallWidth, wallHeight);
        wall1.render(gc, wallX, wallY);

        wall2.resize(wallWidth, wallHeight);
        wall2.render(gc, wall2X, wall2Y);

        balloon.resize(80, 100);
        balloon.render(gc, balloonX, balloonY);

        System.out.println("Initial wall height: " + wallHeight);

        text.setText(gc, "Score: ", fontSize, textX, textY);
    }

    @Override
    public void animate(GraphicsContext gc, int time, ArrayList input) {
        // any animations and keyboard controls should be here

        if (wallX <= 0) {
            wallX = Configs.appWidth - wallWidth;
            wall2X = Configs.appWidth - wallWidth;
            if (speed < 5)
                speed++;

            hole = 150 + new Random().nextInt(50);
            wall1Height = 50 + new Random().nextInt(300);

            wallHeight = (Configs.appHeight - hole) - wall1Height;
            System.out.println("wall 2 height: " + wallHeight);
            wall2Y = Configs.appHeight - wallHeight;
        }

        wall1.resize(wallWidth, wall1Height);
        wall2.resize(wallWidth, wallHeight);

        // randomize the hole size
        background.render(gc, 0, 0);

        wallX -= speed;
        wall2X -= speed;

        wall1.render(gc, wallX, wallY);
        wall2.render(gc, wall2X, wall2Y);

        balloon.render(gc, balloonX, balloonY);

        if (input.contains("UP"))
            balloonY-=2;
        else if (input.contains("DOWN"))
            balloonY+=2;

        if (balloon.collide(wall1))
            System.exit(0);
        if (balloon.collide(wall2))
            System.exit(0);

        text.setText(gc, "Score: ", fontSize, textX, textY);

    }

    @Override
    public void mouseClick(MouseEvent e) {
        // mouse click event here
    }
    
    @Override
    public void mouseMoved(MouseEvent e) {
        // mouse move event here
    }
}