Java: Copy attributes from one object instance to another?

Question

Say you have

public class Car{
  private Engine m_eng;
  public Car(){

  }

  public Engine getEngine(){
    return m_eng;
  }

  public void setEngine(Engine engine){
    m_eng = engine;
  }
}

public class Engine{
  private String m_name;
  public Engine(){};
  public Engine(String name){ m_name = name;}

  public String getName(){
    return m_name;
  }
  public void setName(String name){
    m_name = name;
  }
}

public static void main(String[] args){
  Engine eng1 = new Engine("abc");
  Car car1 = new Car();
  car1.setEngine(eng1);
  Car car2 = new Car();
  car2.setEngine(car1.getEngine());
}

Question: are the engine of car1 and car2 referring to the same instance of Engine, or when I do car2.setEngine(car1.getEngine()), it automatically make a deep copy of car1.getEnginer() and set to car2 ?

It is the same instance. As @SotiriosDelimanolis said, you can easily test it using `==`. — Luiggi Mendoza, Feb 03 '14 at 16:08
If you've not done it intentionally, it certainly isn't a deepcopy in Java. — Rohit Jain, Feb 03 '14 at 16:09
Java allways pass by value, i.e. by parameter it makes a copy of the reference "pointing" to the same value. There is a lot of discussion wheter this should be called passByValue or passByReferende. The Java Language Reference calls it pass by value. You can read a good article on this issue under [Java is Pass by Value and Not Pass by Reference](http://architects.dzone.com/articles/java-pass-value-and-not-pass-0) — wolfrevo, Feb 03 '14 at 16:24
@wolfrevo while this is true, this does not apply for what OP is looking for. Refer to my answer to have a better understanding on the *copy* term used in this question. — Luiggi Mendoza, Feb 03 '14 at 16:27

score 6 · Accepted Answer · answered Feb 03 '14 at 16:11

6

car1--------------->eng1

car2.setEngine(car1.getEngine());

results in

car1--------------->eng1 <------------------car2

thereby pointing to same engine instance

answered Feb 03 '14 at 16:11

user1339772

783
5
19

This is the only answer that explains in some detail why the engine in both cars is the same object reference. – Luiggi Mendoza Feb 03 '14 at 16:14

score 4 · Answer 2 · answered Feb 03 '14 at 16:10

4

Yeah sure these will be same instance.

answered Feb 03 '14 at 16:10

Ashish Ratan

2,838
1
24
50

score 3 · Answer 3 · answered Feb 03 '14 at 16:08

3

There is no deep copy. Both Car instances reference the same instance of Engine.

answered Feb 03 '14 at 16:08

Peter Bratton

6,302
6
39
61

score 3 · Answer 4 · edited May 23 '17 at 12:06

As noted by other people, when you do

car2.setEngine(car1.getEngine())

The engine in car2 will be the same object reference as in car1. This is easily tested by using ==:

System.out.println(car2.getEngine() == car1.getEngine()); //prints "true"

when I do car2.setEngine(car1.getEngine()) it automatically make a deep copy of car1.getEngine() and set to car2?

Be careful here, since when executing that statement there's no copy of the object reference, it is not a deep copy nor a shallow copy, it is the same object reference. This means, if you modify the state of the engine in one of the cars, then the engine in the other car gets modified (since is the same object reference):

public static void main(String[] args){
    Engine eng1 = new Engine("abc");
    Car car1 = new Car();
    car1.setEngine(eng1);
    Car car2 = new Car();
    car2.setEngine(car1.getEngine());
    //additional code to show the last statement
    car2.getEngine().setName("foo");
    System.out.println(car2.getEngine().getName()); //prints "foo"
    System.out.println(car1.getEngine().getName()); //prints "foo" too
    System.out.println(eng1.getName());  //prints "foo" since it is the same object reference used from the beginning
}

Check here to know about how to make copies of object references: Java: recommended solution for deep cloning/copying an instance

Girish · Answer 5 · 2014-02-03T16:19:05.827

There is no deep copy. both reference are refering to same object try to use == operator to compare two objects.

 Engine eng1 = new Engine("abc");
  Car car1 = new Car();
  car1.setEngine(eng1);   //here you have set the reference eng1 which is pointing to the object abc in heap
  Car car2 = new Car();
  car2.setEngine(car1.getEngine());// here you are getting the reference of the object which is in the heap and setting it in car2 Object

Java: Copy attributes from one object instance to another?

5 Answers5