I wrote an ECS in (Android's version of) Java that does just this for game entity components. Each component type has a big array associated with it; plus there

I wrote an ECS in (Android's version of) Java that does just this for game entity components. Each component type has a big array associated with it; plus there's a big bit array of used/free flags for entities. The EID (entity ID) serves as an index into the arrays of its various components.