Java：将属性从一个对象实例复制到另一个对象实例

Question

说你有

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());
}

问题：car1和car2的引擎是引用相同的Engine实例，还是当我执行car2.setEngine(car1.getEngine()) ，它会自动生成car1.getEnginer（）的深层副本并设置为car2？

Answer 1

CAR 1 ---------------> ENG1

car2.setEngine(car1.getEngine());

结果是

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

从而指向相同的引擎实例

Answer 2

是的，确定这些将是同一个实例。

Answer 3

没有深刻的副本。 两个Car实例都引用相同的Engine实例。

Answer 4

正如其他人所说，当你这样做时

car2.setEngine(car1.getEngine())

该engine在car2将是相同的对象的引用作为car1 。 使用==可以轻松测试

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

当我执行car2.setEngine(car1.getEngine()) 它会自动生成car1.getEngine()的深层副本并设置为car2 ？

这里要小心，因为在执行该语句时没有对象引用的副本 ，它不是深拷贝也不是浅拷贝 ，它是相同的对象引用 。 这意味着，如果您修改其中一辆车的引擎状态，那么另一辆车中的引擎会被修改（因为它是相同的对象引用）：

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
}

请在此处了解如何制作对象引用的副本： Java：深度克隆/复制实例的推荐解决方案

Answer 5

没有深刻的副本。 两个引用都引用同一个对象尝试使用==运算符来比较两个对象。

 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