Before moving forward, it is very important to understand how to find a relationship between different classes. How do we know that a student can be inherited from Person or that Circle can be inherited from the Shape class? If the designed hierarchy is correct, the child class must be a type of the parent class and both classes must have an is-a relationship between them. Take a look at the following examples:

• Student is a person
• Circle is a shape
• Football player is a basketball player
• German shepherd is a dog
• Sofa is a bedroom

Most of these examples are correct but a few of them are completely false. Student is a person is correct because students have all the attributes and behaviors that a person has. Students also have some unique features as well. Circle is a shape is also correct, because every shape has a size and a color, so circle can be inherited from shape. Sofa is a bedroom does not make any sense, however, nor does Football player is a basketball player, because both games are completely different.

If the inheritance and the classes are well designed, every inherited class is a type of its parent class. If class B is inherited from A, then B is a type of A. As a circle is inherited from the Shape class, a circle is a type of shape. If class C is inherited from B and B is inherited from A, then C contains all features that both A and B have. A cylinder is inherited from a circle and a circle is inherited from a shape. A cylinder is therefore a type of shape as well as a circle.

An important thing to remember is that inheritance always works in one direction. A student is inherited from the Person class, which means that every student is a person. This is one-directional relationship; it won't work the other way around, because every person is not a student. Similarly, all dogs are animals, but not all animals are dogs. For this reason, a dog can be inherited from the animal class, but an animal cannot be extended from the dog class.