Passing interface object in constructor - do I copy the interface object?

Question

Following code:

public interface VehicleAbilities {
   public void activateHyperDrive();
}

public class Car implements VehicleAbilities {

   public void activateHyperDrive() {
       fastenSeatBelt();
       pressTheRedButton();
   }
}

public class Garage {
   VehicleAbilities iVehicle;
   public Garage(VehicleAbilities aVehicle) {
       iVehicle = aVehicle;
   }

   public void fireUpCars() {
       iVehicle.activateHyperDrive();
   }
}

public static void main (String[] args) {
    Car car = new Car();
    Garage garage = new Garage(car);
    garage.fireUpCars();
}

My question is this: Is the car that activateHyperDrive is called on in garage object the same car instance as in main, or is it copied when it´s passed to garage? AFAIK Java is pass-by-value only, so isn´t the object copied? Are there any problems that can come up? Thank you.

Java is always [pass by value](http://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value). — Jerome L, Mar 03 '17 at 09:35
In Java Objects don't get copied unless something explicitly copies them. When you pass a reference to object X by value, the reference is copied, giving you another reference to object X. — khelwood, Mar 03 '17 at 09:39

score 2 · Accepted Answer · answered Mar 03 '17 at 09:36

2

Is the car that activateHyperDrive is called on in garage object the same car instance as in main

Yes, the same object.

is it copied when it´s passed to garage?

It is not copied, just the reference been passed.

AFAIK Java is pass-by-value only, so isn´t the object copied?

Not the whole object, just object reference been passed as a value.

answered Mar 03 '17 at 09:36

Suresh Atta

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So I don´t have two objects in memory? – 最白目 Mar 03 '17 at 09:37
@dan No, you don't. Cause you created only one object – Suresh Atta Mar 03 '17 at 09:37
The object is created when you use the keyword 'new'. You have used only once, so there is only one object. – Bahramdun Adil Mar 03 '17 at 09:46

Passing interface object in constructor - do I copy the interface object?

1 Answers1