override them--it cannot simply use the version defined in the superclass. To declare an
abstract method, use this general form:
abstract type name(parameter-list);
As you can see, no method body is present.
Any class that contains one or more abstract methods must also be declared abstract. To
declare a class abstract, you simply use the abstract keyword in front of the class keyword
at the beginning of the class declaration. There can be no objects of an abstract class. That is,
an abstract class cannot be directly instantiated with the new operator. Such objects would
be useless, because an abstract class is not fully defined. Also, you cannot declare abstract
constructors, or abstract static methods. Any subclass of an abstract class must either implement
all of the abstract methods in the superclass, or be itself declared abstract.
Here is a simple example of a class with an abstract method, followed by a class which
implements that method:
// A Simple demonstration of abstract.
abstract class A {
abstract void callme();
// concrete methods are still allowed in abstract classes
void callmetoo() {
System.out.println("This is a concrete method.");
}
}
class B extends A {
void callme() {
System.out.println("B's implementation of callme.");
}
}
class AbstractDemo {
public static void main(String args[]) {
B b = new B();
b.callme();
b.callmetoo();
}
}
Notice that no objects of class A are declared in the program. As mentioned, it is not
possible to instantiate an abstract class. One other point: class A implements a concrete
method called callmetoo( ). This is perfectly acceptable. Abstract classes can include as
much implementation as they see fit.
Although abstract classes cannot be used to instantiate objects, they can be used to create
object references, because Java's approach to run-time polymorphism is implemented through
the use of superclass references. Thus, it must be possible to create a reference to an abstract
class so that it can be used to point to a subclass object. You will see this feature put to use in
the next example.
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