Inheritance - Java - page 450

The Class Object
In Java, every class is a descendent of the class Object . So, every object of every class is of
type Object , as well as being of the type of its class.

The class Object is in the package java.lang , which is always imported automati-
cally. So, you do not need any import statement to make the class Object available to
your code.
The class Object does have some methods that every Java class inherits. For exam-
ple, every object inherits the methods equals and toString from some ancestor class,
which either is the class Object or a class that itself inherited the methods ultimately
from the class Object . However, the inherited methods equals and toString will not
work correctly for (almost) any class you define. You need to override the inherited
method definitions with new, more appropriate definitions.
It is important to include definitions of the methods toString and equals in the
classes you define, since some Java library classes assume your class has such methods.
There are no subtleties involved in defining (actually redefining or overriding) the
method toString . We have seen good examples of the method toString in many of
our class definitions. The definition of the overridden method equals does have some
subtleties and we will discuss them in the next subsection.
Another method inherited from the class Object is the method clone , which is
intended to return a copy of the calling object. We discuss the method clone in Chap-
ters 8 and 13.

toString

equals

clone

The Right Way to Define equals
Earlier we said that the class Object has an equals method, and that when you define
a class with an equals method you should override the definition of the method
equals given in the class Object . However, we did not, strictly speaking, follow our
own advice. The heading for the method equals in our definition of the class
Employee (Display 7.2) is as follows:

public boolean equals(Employee otherEmployee)

On the other hand, the heading for the method equals in the class Object is as follows:

public boolean equals(Object otherObject)

The two equals methods have different parameter types, so we have not overridden
the definition of equals . We have merely overloaded the method equals . The class
Employee has both of these methods named equals .