Java：抽象类的对象

Question

I got confused about the code stated below, where Player is an abstract class:

private void cbxTeamAItemStateChanged(java.awt.event.ItemEvent evt) {                                          
        if (evt.getStateChange() == ItemEvent.SELECTED) {
            Player p = (Player) evt.getItem();
            if(P.getTipo().contains("Defense")){
                lblDefense1.setText("Skill:");
            }
        }
}

If in Java I can not create an Object of an abstract class, why is this working?

Isn't this line a instance of Player ?

Player p = (Player) evt.getItem();

Answer 1

The variable p holds a reference to an instance of a subclass of the abstract class Player. Since the Player is an abstract class, no instance of it can be created. However, instances of the subclass of the abstract Player class can still be assigned to the variable p. See https://www.computingnotes.net/java/abstract-class-in-java/

Answer 2

You're confusing variable with instance. p is a Player variable, and evt.getItem() returns a concrete instance of a type that extends Player (or implements Player if Player were an interface). What exact sub-type of Player isn't important since all that you need to know is that it adhere's to the Player abstract class contract and thus has all the necessary Player behaviors.

Answer 3

You can have objects that have an abstract class as their type even though you can't directly construct abstract classes. evt.getItem() has a specific, non-abstract type that it returns under the hood, and that type implements Player with some specific code. If, someday, they wanted to change the name of that class, they could, and it wouldn't affect this code as long as it continued to implement all the methods in Player . Basically, this abstracts the specifics of that object away from you and only assures you that somewhere, someone implemented all the methods that a Player ought to have in the object.

Answer 4

You can not create instance of abstract class but you can up cast child to it's parent. In your case evt.getItem() is returning the child ( ie TennisPlayer ) and you are casting it to super class Player which is valid.

Consider following example,

class Mango extends Fruit {}

abstract class Fruit {}

public class Test {

    public static void main(String[] args) {
        Mango mango = new Mango();
        //Note that mango is also fruit
        Fruit fruit = (Fruit) mango;
    }

}

Note it is valid to up cast child to it's parent because ( ie ) every Mango is surely a Fruit .