What does polymorphism mean in Java?

In object-oriented programming, polymorphism in Java refers to an object’s ability to respond to the same method call differently based on its specific class.

Specifically, polymorphism mainly includes two aspects of meanings:

1. Subclass objects can be assigned to parent class reference variables, meaning that a subclass can be upcasted to a parent class. This way, by using a parent class reference variable, one can call methods that have been overridden by the subclass, allowing for a uniform operation on different subclass objects.
2. Declare an abstract method in the parent class, and implement this method in the subclass. When using a reference variable of the parent class to point to an object of the subclass and calling the abstract method, the corresponding method will be executed based on the actual type of the subclass object (run-time type).

Polymorphism makes programs more flexible, extensible, and enhances the readability and maintainability of the code.