Dependency

A dependency between two classes indicates that one class needs to make use of the other, typically for a short period of time.

For example, suppose we have a transport simulation involving electric vehicles. A vehicle will be able to drive for a period of time, until the level of charge on its battery falls below some threshold, at which point it will need to seek out a charging station in order to recharge the battery.

There are two obvious classes here: Vehicle and ChargingStation. However, the relationship between them is transient in nature. A Vehicle object won’t need permanent access to a ChargingStation object. The two objects will interact only during the recharging process.

In UML, we represent relationships of this kind using a dashed line, with a v-shaped arrowhead. The arrow must point from the class that has the dependency, towards the class on which it depends:

classDiagram
  direction LR
  Vehicle ..> ChargingStation
  class Vehicle {
    batteryLevel: Int
    drive()
    recharge()
  }

This diagram tells us that Vehicle depends on ChargingStation. The implementation of the recharge() method will involve some interaction with a ChargingStation object.