First see Entity Component System.

An entity without components doesn't do much, but has some important functionality:

• Entities have a position in the world, which is measured in metres. This is used for rendering and physics.
• Entities have a scale which determines its size. This is used for rendering and physics.
• Entities have an event system to communicate between components and entities. See Event System.

The entity lifecycle can be seen in the figure below.

Note that create() is not called when the entity is instantiated through new Entity(), but when the entity is added into the world. This means we know that an entity doesn't do anything while we are still adding components to it, or before we've placed it in the game.

Entities can also be disabled or enabled during gameplay. While an entity is disabled, it won't draw on the screen or run any updates. This is very useful when an entity isn't needed right now but will be later, since it won't have to be created twice. You can also create entities ahead of time but leave them disabled until they're necessary.

To make an entity, just instantiate one and use addComponent() to add components to it. You can also use getComponent() to access the components on an entity. The entity then has to be registered with the Entity Service to be created.

public static void spawnPlayer()
  // Create entity
  Entity player = new Entity()
    .addComponent(new TextureRenderComponent("images/player.png"))
    .addComponent(new PlayerMovementComponent())
    .addComponent(new InventoryComponent())
    .addComponent(new CombatComponent());

  // Access components
  player.getComponent(PlayerMovementComponent.class).setSpeed(3f);

  // Create in the world
  ServiceLocator.getEntityService().register(player);
}