If a class does not implement any or all of an interface method, then it should be declared as abstract.
An abstract class is made up of one or more abstract methods that are declared but left unimplemented.
Abstract classes cannot be instantiated, but they can be subclassed.
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract.
A class containing one or more abstract methods must be declared as abstract. However, a class can also be declared as abstract even if it has no abstract methods.
The final and abstract class modifiers are not used in combination.
An abstract method is a method that is not defined in the class in which it is declared. Instead, the method definition is deferred to one or more subclasses. An abstract method only has a semicolon after the method name and parenthetical argument list. The subclasses provide the body of the abstract method.
Responses to “7 key points about abstract classes and methods.”