MyDog is a reference variable name, and its type is Dog.

= is an assignment.

New can be understood as "generation".

Dog () calls the parameterless constructor of the Dog class. If it is not defined in the Dog class, the system defaults to a parameterless constructor.

Define a myDog variable with type: Dog and value: a Dog object.

ABC x = new ABC()； This is ok, provided that there is abc class.

ABC x = new BCD()； This will be divided into two situations.

1. If abc is the parent class of bcd, or bcd implements the interface abc, it will be fine.

2. Otherwise, you can't.