Java Reference
In-Depth Information
private static int numberOfCats;
Cat() {
numberOfCats++;
}
public static final int getNumberOfCats() {
return numberOfCats;
}
@Override
protected void sayWhatIAm() {
System.out.println("I am a cat");
super.sayWhatIAm();
}
}
If a test program then called a
Cat
object's
sayWhatIAm ()
method, the output would be as shown in
Listing 6-3.
Listing 6-3. Output of a Cat object's sayWhatIAm method
I am a cat
I am a mammal
This is a trivial example, but I hope it gives you a sense of how you can use the characteristics of
both a base class and a child class to provide meaningful information and other functionality for your
users. The trick is knowing which objects to put at which level. One rule of thumb is to put each object as
high in your class hierarchy as you can. That's whyI define the speak method at the
Mammal
level but
implement it at the individual animal level.
Static Members
Classes can have
static members
, including fields, methods, and other classes (a class within a class is
called an
inner class
). A static member of a class is often called a
class member
. The important thing to
know about class members is that only one instance of that member ever exists. If our
Cat
class has a
static method, we might have ten different instances of the
Cat
class, but there would only ever be one of
that static method. That becomes an issue when each of the members of the
Cat
class want to use that
method. The instances end up waiting for each other.
Static members have their uses, though. When you want to be certain that only one of something
exists for all the objects that instantiate a particular class, the
static
keyword is how you do it. One
obvious use of the
static
keyword is on the
main
method. Imagine if every instance of a program class
could start a new program. We would quickly swamp the operating system with programs. A more
common and useful use of static members is to implement counters. Suppose we want to keep track of
how many
Cat
objects we create. The
Cat
class could then include a static field called
numberOfCats
, and
the constructors for the class would increment that field every time we create a
Cat
object. That code
would look something like the
Cat
class in Listing 6-4.
