If we define a class as abstract, then it class must contains AT LEAST ONE SINGLE ABSTRACT METHOD THAT MUST BE IMPLEMENTED.

Answer 1

Not really. It's the other way around: a class with at least one single abstract method must be defined as abstract.

Abstract classes can have non-abstract methods and it's not mandatory for them to have abstract methods.

https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html