abstract
1 . abstract classes
(1)abstract is used to decorate classes
(2) classes decorated by abstract are called abstract classes;
(3) abstract classes cannot create objects independently, but can only declare references;
(4) member variables and member methods can be defined in abstract classes;
(5) there are construction methods in abstract classes, but the construction methods are not used to create objects independently.
2.abstract can be used to modify methods (1) abstract methods are only declared unimplemented (note: there is no {});
(2) access modifier abstract returns the type method name (parameter list);
(3) access modifier and abstract are in no order;
(4) abstract methods can only be defined in abstract classes;
(5) abstract classes can also define non-abstract methods.