Object-Oriented Concepts - SCJP Sun Certified Programmer for Java Platform SE6 - page 408

Using Object as a Parameter
If you need to write a method that takes in any type of argument, use Object as the data
type of the parameter. This situation is quite common in the Java API. For example, the
writeObject method of ObjectOutputStream takes in an Object :
public final void writeObject(Object obj)
Because of polymorphism, every object in Java is of type Object . Therefore, any
reference can be passed into the writeObject method. Of course, as we saw in
Chapter 4, “API Contents,” the Object passed in to writeObject needs to be of type
Serializable or an exception is thrown. The writeObject method uses the instanceof
operator to determine if the argument passed in implements Serializable . The code
looks similar to the following:
if(!(obj instanceof Serializable)) {
throw new NotSerializableException(obj.getClass());
}
Using Polymorphic Parameters
I developed an application that required an event to be logged every time a customer
preference is changed. For example, a customer can choose whether or not to receive
emails with promotions, special offers, and news items, and these preferences are stored
in a database. The corresponding Java objects to represent the various preferences all
extend a class named Preference . The event logging method is defi ned as
public void preferenceChanged(Preference pref) {
logger.writeUTF(pref.toString());
}
The logger variable is a DataOutputStream that writes to a fi le. Instead of defi ning
multiple overloaded preferenceChanged methods, this single method can log
any Preference that is changed. If a new type of preference comes along, the
preferenceChanged method can remain unchanged as long as the new preference
extends the Preference class.