Defensive Copy Part 2: Defensive Copy using Copy Constructors

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Part 1 | Part 2
In part 1 you read what is defensive copy and how to implement it. In this part you will see how to implement defensive copy using copy constructors.

Here we have added a copy constructors to Engine class which will be used to implement defensive copy mechanism in Car class.

Engine.java

class Engine {
    private String engineLocation;

    /*Simple Constructor*/   
    Engine(String engineLocation){
    this.engineLocation=engineLocation;
    }
   
    /*Copy Constructor */
   Engine(Engine engine){
    this.engineLocation = engine.getEngineLocation();
  }

  public void setEngineLocation(String engineLocation){
        this.engineLocation=engineLocation;
  }
  
    public String getEngineLocation(){
        return engineLocation;
    }
   
}

Car.java

class Car {

    private String make;
    private Engine engine;

    Car(String make,Engine otherEngine){
        this.make=make;
       /*Using copy constructor of Engine*/
        this.engine = new Engine(otherEngine);
    }


  public void setMake(String make){
        this.make=make;
    }
 
    public String getMake(){
        return make;
    }
 
    public void setEngine(Engine otherEngine){
        /*Using copy constructor of Engine*/
        this.engine = new Engine(otherEngine);
    }
 
    public Engine getEngine(){
           /*Using copy constructor of Engine*/
         Engine engineTemp = new Engine(engine); /*'engine' is a member of Car class*/
         return engineTemp;
    }
}

Client.java

public class Client{
public static void main(String args[]){
   Engine engine= new Engine("Front");
   Car car = new Car("Maruti",engine);

  System.out.println("Car Engine Location Before ="+car.getEngine().getEngineLocation());
  engine.setEngineLocation("Back");
  System.out.println("Car Engine Location After ="+car.getEngine().getEngineLocation());
 }   
}

Output:

Car Engine Location Before =Front
Car Engine Location After =Front

Note that here Car class did not get affected.

Now change your Client class as following to see the effect of Setter Method:

Client.java

public class Client{
public static void main(String args[]){
  Engine engine= new Engine("Front");
  Car car = new Car("Maruti",engine);

  Engine engine2= new Engine("Front");
  car.setEngine(engine2);

 System.out.println("Car Engine Location Before ="+car.getEngine().getEngineLocation());

 engine2.setEngineLocation("back");

 System.out.println("Car Engine Location After ="+car.getEngine().getEngineLocation());

}
}

Output:

Car Engine Location Before =Front
Car Engine Location After =Front

Notice that engine location of engine of car did not get changed upon changing engine location on engine2.

For seeing the effect of Getter Method change Client class as:

Client.java

public class Client{
public static void main(String args[]){
  Engine engine= new Engine("Front");
 Car car = new Car("Maruti",engine);


 Engine engine2= car.getEngine();
 System.out.println("Car Engine Location Before ="+car.getEngine().getEngineLocation());
 engine2.setEngineLocation("Back");
 System.out.println("Car Engine Location After ="+car.getEngine().getEngineLocation());

}

}

Output:

Car Engine Location Before =Front
Car Engine Location After =Front

As you are seeing in each of the output above, car object is not getting effected now.

Part 1 | Part 2

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