Defining Classes II - Java - page 252

PITFALL: (continued)

and so it is illegal within the definition of a static method. (A static method has no
this .)
However, it is legal to invoke a static method within the definition of another static
method.
There is one way that you can invoke a nonstatic method within a static method: if
you create an object of the class and use that object (rather than this ) as the calling
object. For example, suppose myMethod() is a nonstatic method in the class MyClass .
Then, as we already discussed, the following is illegal within the definition of a static
method in the class MyClass :

myMethod();

However, the following is perfectly legal in a static method or any method definition:

MyClass anObject = new MyClass();
anObject.myMethod();

The method main is a static method, and you will often see code similar to this in the
main method of a class. This point is discussed in the subsection “You Can Put a main
in Any Class.” ■

TIP: You Can Put a main in Any Class

So far, whenever we have used a class in the main part of a program, that main method
was by itself in a different class definition within another file. However, sometimes it
makes sense to have a main method within a regular class definition. The class can then
be used for two purposes: It can be used to create objects in other classes, or it can be run
as a program. For example, you can combine the class definition RoundStuff and the
program RoundStuffDemo (both in Display 5.1) by placing the main method inside the
definition of the class RoundStuff , to obtain the class definition shown in Display 5.2.
Another example of a class with a main added is given in Display 5.3. Note that
in addition to the static method main , the class has another static method named
toCelsius . The class has both static and nonstatic methods. Note that the static
method toCelsius can be invoked without the class name or a calling object because it
is in another static method (namely main ) in the same class. However, the nonstatic
method toString requires an explicit calling object ( temperatureObject ). Java requires
that a program's main method be static. Thus, within a main method, you cannot
invoke a nonstatic method of the same class (such as toString ) unless you create an
object of the class and use it as a calling object for the nonstatic method.
You do not want to place just any main method in a class definition that is to be used as
a regular class to create objects. One handy trick is to place a small diagnostic program in a
main method that is inside of your class definition. ■