(I'm new to java, and I'm still not sure whether it passes variables by reference or by value.)
Java passes everything by value, without exception. However, Objects are referred to with references, and those references are passed around by value. It's exactly like passing pointers around in C++, except you can't explicitly dereference them or do pointer arithmetic on them. Java will
never copy an object behind your back, so if you pass an object like an Array into a method, it will always refer to the same array.
As for how you get entities to cooperate, that's generally handled by the subsytem concerned with those entities either doing a query of some sort that retrieves the entities involved, or being handed a list of entities by some other subsystem that already contains all the participants. Components shouldn't be dealing with entities, they should only be passed the relevant items involved, which is usually other components of the same type.
For example, the Spatial component of an entity may have a "collides" method that should receive another Spatial, not the entity that contains the Spatial. It's up to the Collision subsystem to detect collisions between the Spatials of nearby entities, and call the "damage" methods of the entities that do collide (or hand off the colliding entities to another subsystem that handles it).
Also consider that standard OO composition mechanisms may be more appropriate than entity systems for your particular program. In fact I'd go so far as to say Entity Systems are highly overrated things that no one should be using unless they have specific knowledge of the exact benefits they gain from using them over a traditional OO approach.
