Java Reference
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moved, then the call Superhero.memberCount() will result in error. The reason is that the
Superhero class will now not contain the method and Java will not check the superclass.
The reason is that polymorphism does not apply to methods of type static .
8.11 Explicit Type Checking
Let us continue our superhero and villain example. Suppose that we have created several
superheroes and villains and have added them to the single ArrayList characters .
ArrayList < FictionalCharacter > characters = new ArrayList <> () ;
Suppose that now we want to count the number of superheroes and the number of
villains in the ArrayList . We can call the memberCount method that was described in the
previous section. However, this method will give us the total number of superheroes and
not the number of superheroes in the ArrayList . Note that in the ArrayList we have a
number of objects of type FictionalCharacter . However, since the FictionalCharacter
class is abstract, these objects must belong to its subclasses. Therefore, we need a way to
determine the type of an object. One way of performing this task is using the instanceof
keyword. Here is an example.
int count = 0;
for (FictionalCharacter el : characters)
if (el instanceof Superhero)
The instanceof keyword checks if the object belongs to the class. It returns true if it does
and false otherwise. The above code iterates through all the elements of the ArrayList .
If an element belongs to the Superhero class, then the counter is incremented by one. The
code computes the number of superheroes in the ArrayList .
The expression o instanceof C returns true exactly when the o object is an
instance of the C class or one of its descendents in the inheritance hierarchy.
Note that the code: el instance of FictionalCharacter will be true for all the ele-
ments el of the ArrayList .
Alternatively, one can use the getClass method to determine the type of an object. The
method returns the class of an object. Here is an example.
for (FictionalCharacter el : characters) {
if ( el . getClass ()==Superhero . class ) {
The getClass method is defined in the Object class. In other words, it can be called
on any object. It returns the runtime type of the object.
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