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I am building a fleet unit gps system and currently and i need to figure out how to link my objects.

Here is the scenario:

  1. Each fleet unit may have attached one or more gps devices.
  2. Each fleet unit may have one or more engines.
  3. Each fleet unit's engine may have one or two fuel flow meters connected to one or two gps devices. Depending on the engine type the fuel flow meter can handle both the forward fuel and backward fuel, but sometimes there needs to be two different fuel flow meters, one for the forward fuel and one for the backward fuel. What makes it even worse is that some gps device models have inputs for the both fuel flow meters but others don't, so when two fuel flow meters are needed there needs to be mounted two different gps devices.

So if i have the following objects:

FleetUnit, GpsDevice, Engine, FuelFlowMeter

I want to find a way to link them without having a circular dependency.

If the FleetUnit object have a list of GpsDevice objects and list of Engine objects and GpsDevice object has a list of FuelFlowMeter and FuelFlowMeter has reference to a GpsDevice object and Engine object, I think there are too many circular dependency and the design is not clean.

Can you suggest me how to handle such scenario where few objects are behaving like a graph?

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  • If you don't want relationships in the form of keys (FK) you could add a table called "connections" that maintains how fleetunits, gpsdevice, engines, and fuelflowmeters are connected to each other. In that case the other tables have no keys. Commented Jan 31, 2018 at 18:37
  • I am not talking about the database but the Objects using Java as programing language :) Commented Jan 31, 2018 at 18:59
  • There is nothing wrong with circular references. Most relationships are bidirectional, so that immediately causes a cycle. However, as @Bob says you probably want some notion of directed relationship, such as ownership or containment (where the reverse though equally valid relationship is: is-owned-by or is-contained-by). Commented Feb 1, 2018 at 2:03

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A few things here that might help. Thinking about your problem here, one thing that complicates it is that you have the thing with some GPS and fuel-meters being able to support two engines and others not. If your goal is to keep an inventory of these, those details might matter but my guess is that it doesn't. I think what you really care about is: for each unit, what is the fuel usage for each engine and probably how much work they have done (distance, elevation, etc.)

I point this out because that is a much simpler modelling problem. So to do that you need something like these (rough) entities:

  • FleetUnit
  • Engine
  • Fuel Reading
  • Location Information

One of the big problems I have about how OO is explained is that the examples an explanation either imply or state that they should represent concrete real world things like cats and dogs or trees. This really isn't the case at all and most classes in a java program do not represent real world entities.

If for some reason it's important for your application to know that Fuel Reading x is associated to GPS unit Y, that can be added to the model. The key here is that you should design your classes based on how you need to use them, not based on the concrete entities in your problem domain.

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  • I need to know how objects are related because the data is stored in a database inside tables named the same as the gps device ID. So if gps device 00023 stores the forward fuel consumption of engine-1 and gps device 00024 stores the backward fuel consumption of engine-1 i need to know where to query each from for agiven period of time for example. Commented Jan 31, 2018 at 19:43
  • @Papa-rapa-beo Your object model doesn't need to be isomorphic to the database mode in order to do this. That's where I think you are struggling. The point of OO is hide complexities such as these details and expose a simple interface that makes it easy to solve your problem. You can (and should) store the sources of the data in the model I have proposed. Commented Feb 1, 2018 at 15:17
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You aren't describing a circular dependency so much as a network of relationships. A relationship can be something like "owns", or "has a", or "is part of" or "is attached to".

In your case, the Fleet Unit "has" some GPSDevices and "has" some Engines and "has" some FuelFlowMeters. An Engine "is attached to" some FuelFlowMeters. Each FuelFlowMeter "is attached to" an Engine. Etc.

These kinds of relationships happen all the time when describing physical systems.

If you need to understand how to represent them in software, then you will need to use abstract interfaces to define components, and have component implementation classes work with the interfaces instead of other implementation classes.

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