我可以在Java中从父实例化相同子类型的类吗？

Question

Is there is the way to instantiate class of the same child type from parent in Java?

Eg We have:

class Animal {
    Animal clone() {
        ...
    } 
}

class Rabbit extends Animal {
    ...
}

class Fox extends Animal {
    ...
}

Could I write something in animal clone method to get Rabbit if I call it from Rabbits instance and Fox if calling from Fox? I know that I may write two separate clone methods in Rabbit and Fox but if there is a way to write common method in Animal? Thanks!

UPD: As I stated in comment I need something like PHP:

function clone (): Animal
{
    return eval("return new " . get_class($this) . " ();");
}

Answer 1

You can do it...but you need to use reflection heavily.

However, the right thing to do would be giving each child class its own clone method and keep the cloner() abstract method in the parent class. As you would not want the parent to be aware of all child class implementations.

(PS. clone method is already present in Object class)

abstract class Animal{

//..
//..

Animal cloner();


}


abstract class Rabbit extends Animal{

//..
//..

Rabbit cloner(){
//impl
}


}



abstract class Fox extends Animal{

//..
//..

Rabbit cloner(){
//impl
}


}

Answer 2

You could write something like this:

Animal clone1() {
    if (this instanceof Rabbit){
      System.out.println("Returning Rabbit instance");
      return new Rabbit();
    }
    elseif (this instanceof Fox){
      System.out.println("Returning Fox instance");
      return new Fox();
    }else
      return null;
}

or if want to use reflection and do not want to change the super class every time new subclass is added. You could use Dozer Mapper, which will map to a new/clone object. See below code:

class Animal {

  Animal clone2(){
    Mapper mapper = new DozerBeanMapper();
    return mapper.map(this, this.getClass());
  }

}

class Rabbit extends Animal {
  int testVar = 20;
}

public class App {
  public static void main(String[] args) throws CloneNotSupportedException, IOException {
    Rabbit rabbit = new Rabbit();
    System.out.println("Object: " + rabbit.hashCode());
    System.out.println("Object var: " + rabbit.testVar);

    Rabbit rabbit2 = (Rabbit) rabbit.clone2();
    System.out.println("Clone object: " + rabbit2.hashCode());
    System.out.println("Object var: " + rabbit2.testVar);
  }

}

Run and check the output both have same field but but are separate object. Note: you need to add dozer mapper dependency/jar in your project. For maven check: https://mvnrepository.com/artifact/net.sf.dozer/dozer/5.3.1

But why would you want to do that? If you want to create clone, prototype pattern is much cleaner approach. Prototype pattern : http://www.newthinktank.com/2012/09/prototype-design-pattern-tutorial/

Answer 3

For what you ask you only need to let JVM know you want this class and it's sub-classes to be cloneable:

class Animal implements Cloneable

and use Object::clone() for Animal, Fox and Rabbit.

With this you only get a shallow copy of the object - meaning the clone and the original object will share the same instances of the non-primary fields.

For every object that requires a deep copy you want to implement a dedicated clone() method.

More about cloning here .