- The methods that are declared but not implemented are called abstract methods.
- An abstract class can contain abstract methods and fully implemented methods both.
- All of the methods in an interface are implicitly abstract, so the
abstractmodifier is not used with interface methods.
- If an abstract class contains only abstract method declarations, it should be declared as an interface instead.
- Unlike interfaces, abstract classes can contain fields that are not
final, and they can contain implemented methods.