Java How to Program Fourth Edition - DCC

Chapter 9 Object-Oriented Programming 459
invoke method toString to append the String representation of the Circle. Lines
39–40 append the area of the Circle to output.
Next, the if/else structure at lines 43–48 attempts a dangerous cast in line 44. We cast
point1, which refers to a Point object, to a Circle. If the program attempts to execute this
statement, Java would determine that point1 really refers to a Point, recognize the cast to
Circle as being dangerous and indicate an improper cast with ClassCastException
message. However, we prevent this statement from executing with the if condition
if ( point1 instanceof Circle ) {
that uses operator instanceof to determine whether the object to which point1 refers
is a Circle. This condition evaluates to true only if the object to which point1 refers
is a Circle; otherwise, the condition evaluates to false. Reference point1 does not
refer to a Circle, so the condition fails, and a String indicating that point1 does not
refer to a Circle is appended to output.
If we remove the if test from the program and execute the program, the following
message is generated at execution time:
Exception in thread "main" java.lang.ClassCastException: Point
at InheritanceTest.main(InheritanceTest.java:43)
Such error messages normally include the file name (InheritanceTest.java) and
line number at which the error occurred (43) so you can go to that specific line in the program
for debugging.
9.5 Constructors and Finalizers in Subclasses
When an object of a subclass is instantiated, the superclass’s constructor should be called
to do any necessary initialization of the superclass instance variables of the subclass object.
An explicit call to the superclass constructor (via the super reference) can be provided as
the first statement in the subclass constructor. Otherwise, the subclass constructor will call
the superclass default constructor (or no-argument constructor) implicitly.
Superclass constructors are not inherited by subclasses. Subclass constructors, however,
can call superclass constructors via the super reference.
Software Engineering Observation 9.10
When an object of a subclass is created, first the subclass constructor calls the superclass
constructor (explicitly via super or implicitly), the superclass constructor executes, then
the remainder of the subclass constructor’s body executes. 9.10
If the classes in your class hierarchy define finalize methods, the subclass
finalize method as its last action should invoke the superclass finalize method to
ensure that all parts of an object are finalized properly if the garbage collector reclaims the
memory for the object.
The application of Fig. 9.7–Fig. 9.9 shows the order in which superclass and subclass
constructors and finalizers are called. For the purpose of this example, class Point and
class Circle are simplified.
Class Point (Fig. 9.7) contains two constructors, a finalizer, a toString method
and protected instance variables x and y. The constructor and finalizer each print that
they are executing, then display the Point for which they are invoked. Note the use of
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