Coupling in Java with Example

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OOPS Tutorial

1. Overview

In this article, we will learn the important object-oriented concept of Coupling. 
Coupling refers to the degree to which one class knows about another class. If one class uses another class, that is coupling.

2. Intent/Definition

Coupling refers to the degree to which one class knows about another class. If one class uses another class, that is coupling. Low dependencies between “artifacts” (classes, modules, components). There shouldn’t be too much dependency between the modules, even if there is a dependency it should be via the interfaces and should be minimal.

Key Points

  • While creating a complex application in java, the logic of one class will call the logic of another class to provide the same service to the clients.
  • If one class calling another class logic then it is called collaboration.
  • When one class is collaborating with another class then there exists a tight coupling between the two classes.
  • If one class wants to call the logic of a second class then the first-class need an object of second class it means the first class creates an object of the second class.

3. Two types of Coupling

  1. Tight-coupling
  2. Loose-coupling

3.1 Tight Coupling

If one class is tightly coupled with another class then it is tight-coupling.

Implementation Example

This is an example of tight coupling. Here to start the journey, the Traveler class is creating Car object to interact with it using the move() method.
In the above example the traveler object is tightly coupled with the car object because in place car object if you want to use bike object then, we need to make changes in Traveller class. Hence tight coupling should be avoided.
Step 1: Create a Car class.
public class Car  {
    @Override
    public void move() {
         System.out.println("Car is moving");
    }
}
Step 2: Create Traveler class which holds the reference of Car class. Traveler class is tightly coupled with Car class.
class Traveler {
    Car c = new Car();
    public void startJourney() {
         c.move();
    }
}

3.2 Loose Coupling

  1. Low dependencies between “artifacts” (classes, modules, components).
  2. There shouldn’t be too much dependency between the modules, even if there is a dependency it should be via the interfaces and should be minimal.
  3. Avoid tight-coupling for collaboration between two classes (if one class wants to call the logic of a second class, then the first-class needs an object of second class it means the first class creates an object of the second class).
  4. Strive for loosely coupled design between objects that interact.
  5. Inversion Of Control (IoC) / Dependency Injection (DI) - With DI objects are given their dependencies at creation time by some third party (i.e. Java EE CDI, Spring DI…) that coordinates each object in the system. Objects aren’t expected to create or obtain their dependencies—dependencies are injected into the objects that need them. The key benefit of DI—loose coupling.

Implementation Example

This is an example of loose coupling. In this class, Traveler class is not tightly coupled with Car or Bike implementation. Instead of applying a dependency injection mechanism, the loose coupling implementation is achieved to allow the start journey with any class which has implemented the Vehicle interface.
Step 1: Vehicle interface to allow loose coupling implementation.
interface Vehicle {
      public void move();
}
Step 2: The Car class implements the Vehicle interface.
class Car implements Vehicle {
    @Override
    public void move() {
         System.out.println("Car is moving");
    }
}
Step 3: Bike class implements Vehicle interface.
class Bike implements Vehicle {
    @Override
    public void move() {
         System.out.println("Bike is moving");
    }
}
Step 4: Now create a Traveler class that holds the reference to the Vehicle interface.
class Traveler {
    private Vehicle v;
    public Vehicle getV() {
          return v;
    }
    public void setV(Vehicle v) {
         this.v = v;
    }

    public void startJourney() {
         v.move();
    }
}
Step 5: Test class for loose coupling example - Traveler is an example of loose coupling.
public static void main(String[] args) {
    Traveler traveler = new Traveler();
    traveler.setV(new Car()); // Inject Car dependency
    traveler.startJourney(); // start journey by Car
    traveler.setV(new Bike()); // Inject Bike dependency
    traveler.startJourney(); // Start journey by Bike
}

GitHub Repository

The source code of this post is available on the GitHub: Object-Oriented Design Guide.
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