Inheritance - Absolute Java - page 465

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
Chapters 8 and 13 .

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 .
In most situations, this will not matter. However, there are situations in which it
does. Some library methods assume your class's definition of equals has the following
heading, the same as in the class Object :

public boolean equals(Object otherObject)

We need to change the type of the parameter for the equals method in the
class Employee from type Employee to type Object . A first try might produce
the following:

public boolean equals(Object otherObject)
{
Employee otherEmployee = (Employee)otherObject;
return (name.equals(otherEmployee.name)
&& hireDate.equals(otherEmployee.hireDate));
}
We needed to type cast the parameter otherObject from type Object to type
Employee . If we omit the type cast and simply proceed with otherObject , the
compiler will give an error message when it sees the following:

otherObject.name
The class Object does not have an instance variable named name .
This first try at an improved equals method does override the definition of equals
given in the class Object and will work well in many cases. However, it still has
a shortcoming.
Our definition of equals now allows an argument that can be any kind of object
whatsoever. What happens if the method equals is used with an argument that is not
an Employee ? A run-time error will occur when the type cast to Employee is executed.