Ashley Entity System is an Entity System library that's managed under the LibGDX organization and is and well-suited for game development. It depends on LibGDX utility classes, but can be used with other Java game frameworks not based on LibGDX with some work.
Entity systems provide a different way to manage data and functionality towards large sets of objects without having to make the object classes rich with inheritance.
Utilizing Ashley might be a helpful approach for those looking for an object modeling approach like Unity provides, but with the scope of a framework instead of game engine.
This method is called once when the Application is started. In this method resources should be loaded and variables should be initialized.
The method is called every frame, and is used to display whatever needs to be displayed. It is also used to update any variables/classes that may need to be updated, such as a camera.
This method is called when the application is destroyed, and is used to free any resources, for examples Textures or the SpriteBatch. You will know that an object has to be disposed of if it implements the Disposable interface.
This method is called when the application is paused. Usually when the application looses focus.
This method is called when the application should be resumed. Usually when the application regains focus.
This method is called when the application is resized. This method is normally used to resize a viewport.