Improving structure with inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 297

Now consider this code fragment, in which Bicycle is also a subclass of Vehicle :

Vehicle v;
Car c;
Bicycle b;
c = new Car();
v = c; // okay
b = (Bicycle) c; // compile time error!
b = (Bicycle) v; // runtime error!

The last two assignments will both fail. The attempt to assign c to b (even with the cast) will be a
compile-time error. The compiler notices that Car and Bicycle do not form a subtype/supertype
relationship, so c can never hold a Bicycle object—the assignment could never work.

The attempt to assign v to b (with the cast) will be accepted at compile time but will fail at
runtime. Vehicle is a superclass of Bicycle , and thus v can potentially hold a Bicycle ob-
ject. At runtime, however, it turns out that the object in v is not a Bicycle but a Car , and the
program will terminate prematurely.

Casting should be avoided wherever possible, because it can lead to runtime errors, and that is
clearly something we do not want. The compiler cannot help us to ensure correctness in this case.

In practice, casting is very rarely needed in a well-structured object-oriented program. In almost
all cases, when you use a cast in your code, you could restructure your code to avoid this cast
and end up with a better-designed program. This usually involves replacing the cast with a
polymorphic method call (more about this in the next chapter).

8.8 The Object class

All classes have a superclass. So far, it has appeared as if most classes we have seen do not have
a superclass. In fact, while we can declare an explicit superclass for a class, all classes that have
no superclass declaration implicitly inherit from a class called Object .

Object is a class from the Java standard library that serves as a superclass for all objects.
Writing a class declaration such as

Concept:

All classes with no
explicit superclass
have Object as
their superclass.

public class Person
{
...
}

is equivalent to writing

public class Person extends Object
{
...
}

The Java compiler automatically inserts the Object superclass for all classes without an ex-
plicit extends declaration, so it is never necessary to do this for yourself. Every single class
(with the sole exception of the Object class itself) inherits from Object , either directly or
indirectly. Figure 8.10 shows some randomly chosen classes to illustrate this.