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Ap124.java:19: Base is abstract; cannot be instantiated
Base myVar = new Base(); C. A
An abstract class with an abstract method
This program illustrates the use of an abstract class containing an abstract method to achieve polymorphic behavior .
The following code fragment shows an abstract class named Base that contains an abstract method named test .
abstract class Base{
abstract public void test();}//end class Base Extending abstract class and overriding abstract method
The class named A , shown in the following fragment extends the abstract class named Base and overrides the abstract method named test .
class A extends Base{
public void test(){System.out.print("A ");
}//end test()}//end class A Can store a subclass reference as a superclass type
Because the class named A extends the class named Base , a reference to an object instantiated from the class named A can be stored in a reference variable of the declared type Base . No cast is required in this case.
Polymorphic behavior
Furthermore, because the class named Base contains the method named test , (as an abstract method) , when the method named test is called on a reference to an object of the class named A , stored in a reference variable of type Base , the overridden version of the method as defined in the class named A will actually be called. This is polymorphic behavior.
(Note, however, that this example does little to illustrate the power of polymorphic behavior because only one class extends the class named Base and only one version of the abstract method named test exists. Thus, the system is not required to select among two or more overridden versionsof the method named test .)
The important code
The following code fragment shows the instantiation of an object of the class named A and the assignment of that object's reference to a reference variable of type Base . Then the fragment calls the method named test on the reference variable.
Base myVar = new A();
myVar.test(); This causes the overridden version of the method named test , shown in the following fragment, to be called, which causes the letter A to be displayed on the standard output device.
public void test(){
System.out.print("A ");}//end test() A. Compiler Error
Classes can be final or abstract, but not both
A class in Java may be declared final . A class may also be declared abstract . A class cannot be declared both final and abstract .
Behavior of final and abstract classes
A class that is declared final cannot be extended. A class that is declared abstract cannot be instantiated. Therefore, it must be extended to be useful.
An abstract class is normally intended to be extended.
Methods can be final or abstract, but not both
A method in Java may be declared final . A method may also be declared abstract . However, a method cannot be declared both final and abstract .
Behavior of final and abstract methods
A method that is declared final cannot be overridden. A method that is declared abstract must be overridden to be useful.
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