12.1 Fill in the blanks in each of the following statements:

1. If a class contains at least one abstract method, it must be declared as a(n)             class.

2. Classes from which objects can be instantiated are called             classes.

3.             involves using a base-class variable to invoke methods on base-class and derived-class objects, enabling you to “program in the general.”

4. Methods in a class that do not provide implementations must be declared using keyword            .

5. Casting a reference stored in a base-class variable to a derived-class type is called            .

12.2 State whether each of the statements that follows is true or false. If false, explain why.

1. It’s possible to treat base-class objects and derived-class objects similarly.

2. All methods in an abstract class must be declared as abstract methods.

3. Attempting to invoke a derived-class-only method through a base-class variable is an error.

4. If a base class declares an abstract method, a derived class must implement that method.

5. An object of a class that implements an interface may be thought of as an object of that interface type.