在 Java 中使用抽象 class 的名称创建一个新的 object？

Question

I'm working on some exam revision work and created an Employee abstract class and two other classes (AcademicEmployee and ProfessionalEmployee) which extend off of the Employee class.

I have a public static void main set up in the Employee class to run these lines:

ProfessionalEmployee boss = new ProfessionalEmployee("The Big Man", 5000.0, null, 10);
AcademicEmployee e1 = new AcademicEmployee("Ryan", 1000.0, boss, 'A');

I know you can't instantiate an abstract class and the actual object getting created is whatever comes after the "new" keyword but why am I allowed to do this:

Employee boss = new ProfessionalEmployee("The Big Man", 5000.0, null, 10);

I'm thinking it works in the way of "Data type" then "Reference name" = new...

but I'm just confused on why I'm allowed to set the data type to Employee. Sorry if this is a strange question, I'm just new to Java:)

Answer 1

This is a concept of Polymorphism in java. Since both of your class ProfessionalEmployee and AcademicEmployee extends Employee , both can be assigned to Employee reference object.

Polymorphism allows you define one interface or class and have multiple implementations.

Answer 2

Your question reads,

Creating a new object in Java with the class name of an abstract class?

But where have you created a new object like that? You have created two Object s as below,

 new AcademicEmployee("Ryan", 1000.0, boss, 'A');

new ProfessionalEmployee("The Big Man", 5000.0, null, 10);

And, both AcademicEmployee and ProfessionalEmployee cannot be abstract which otherwise you will get a compile-time error.

Since AcademicEmployee and ProfessionalEmployee is an Employee (since you have extended/implemented it), you are allowed to assign!

This is explained in simple words here