Java：将属性从一个对象实例复制到另一个对象实例

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 ?

Answer 1

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

car2.setEngine(car1.getEngine());

results in

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

thereby pointing to same engine instance

Answer 2

是的，确定这些将是同一个实例。

Answer 3

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

Answer 4

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

Answer 5

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