Java Game Engine

A lightweight 2D game engine written in pure Java.
Originally it grew inside the Immune Till Death project but is now extracted as a standalone library.

This repository contains the engine sources only. The code below shows how the engine can be used to build a game similar to the original project.

Features

• Basic rendering loop using GFrame and GPanel
• Flexible object system (GObject and subclasses)
• Event based input handling (GEvent/GEventHandler)
• Collision detection through spatial partitioning
• Simple particle and animation utilities
• Minimal dependencies (only the Java standard library)

Building

Compile the sources with javac:

find src -name "*.java" > sources.txt
javac @sources.txt

This produces .class files next to the .java sources. You can also import the project into your favourite IDE and build it there.

Creating your own game

1. Create a subclass of GContent that will hold the objects of your world. In the original project this class was named World.
2. Create game entities by extending GEntity or GPlayer. Entities may override handle, draw, and the collision methods from GObject.Collidable.
3. Register event listeners using GEventHandler.register(...).
4. Create a GFrame and set your GContent implementation as its content.
5. Start the main thread and place your initial objects.

Below is an excerpt adapted from the old Immune Till Death game. It demonstrates how a world, a player and some viruses were spawned and how the event system was used.

import eu.nurkert.gamengine.GFrame;
import eu.nurkert.gamengine.logic.GContent;
import eu.nurkert.gamengine.logic.GLocation;
import eu.nurkert.gamengine.logic.events.GEventHandler;
import eu.nurkert.gamengine.visual.GTexture;

public class Main implements GEventHandler.GEventListener {
    static World world;
    static Player player;
    static GFrame frame;

    public static void main(String[] args) {
        GEventHandler.register(new Main());

        frame = new GFrame("ImmuneTillDeath", new GContent.GDefaultContent());
        world = new World();
        frame.setContent(world);
        frame.setBackground(new GTexture("/textures/background.png"));

        // Spawn some enemies
        for (int i = 0; i < 50; i++) {
            Virus virus = new Random().nextBoolean() ?
                new Covid19(new GLocation(new Random().nextInt(1000) - 500,
                                          new Random().nextInt(1000) - 500,
                                          world), 7) :
                new Virus(new GLocation(new Random().nextInt(1000) - 500,
                                        new Random().nextInt(1000) - 500,
                                        world), 5);
            world.place(virus);
        }

        player = new Player(new GLocation(0, 0, world),
                            frame.getPanel().getContent().getViewCenter());
        world.setPlayer(player);
        world.place(player);

        world.place(new PlayerUI(player));
    }
}

Each entity overrides the handle method to update its behaviour. The World class invokes place to add objects and uses a Timer to spawn additional items over time. User input is received through KeyPressedGEvent and MouseClickGEvent which are triggered automatically by the GFrame.

For detailed examples inspect the files under src/eu/nurkert/gamengine and adapt the code above to your own needs. Textures referenced in the example must be available on the classpath.

Directory Overview

• src/eu/nurkert/gamengine – the engine source code
• images/ – artwork used in the README banner

Contributing

Feel free to fork the project and experiment. Pull requests that improve documentation or provide bug fixes are welcome.