Inheritance - Beginning Java 8 Fundamentals - page 627

Drawing a rectangle...
Drawing a circle...
Name: Rectangle
Area: 8.0
Perimeter: 12.0
Name: Circle
Area: 78.53981633974483
Perimeter: 31.41592653589793

You have finished discussing the main rules of declaring classes and methods abstract . However, there are many
other rules that govern the use of abstract classes and methods in a Java program. Most of those rules (if not all) are
listed below. All rules point to only one basic rule: “Abstract class should be subclassed to be useful and the subclass
should override and provide implementation for the abstract methods.”

abstract even if it does not have an abstract method. • A class may be declared

abstract if it declares or inherits an abstract method. If the class
overrides and provides implementations for all inherited abstract methods and does not
declare any abstract methods, it does not have to be declared abstract . Although it could be
declared abstract .

• A class must be declared

abstract class. However, you can declare a variable of the
abstract class type and call methods using it.

• You cannot create an object of an

abstract class cannot be declared final . Recall that a final class cannot be subclassed,
which conflicts with the requirement of an abstract class that it must be subclassed to be
useful in a true sense.

• An

abstract class should not declare all constructors private . Otherwise, the abstract class
cannot be subclassed. Note that constructors of all ancestor classes (including an abstract
class) are always invoked when an object of a class is created. When you create an object of a
Rectangle class, constructors for the Object class and the Shape class are also invoked. If you
declare all constructors of an abstract class private , you cannot create a subclass for your
abstract class, which makes it the same as declaring an abstract final class.

• An

abstract method cannot be declared static . Note that an abstract method must be
overridden and implemented by a subclass. A static method cannot be overridden. However,
it can be hidden.

• An

abstract method cannot be declared private . Recall that a private method is not
inherited and hence it cannot be overridden. The requirement for an abstract method is that
a subclass must be able to override and provide implementation for it.
An abstract method cannot be declared native , strictfp , or synchronized . These keywords refer to
implementation details of a method. The native keyword denotes that a method is implemented in native code
as opposed to Java code. The strictfp keyword denotes that the code inside a method uses FP-strict rules for
floating-point computations. Please refer to http://en.wikipedia.org/wiki/Strictfp for more details on the
strictfp keyword and its usage. The synchronized keyword denotes that the object on which the method is invoked
must be locked by the thread before it can execute method's code. Since an abstract method does not have an
implementation, the keywords that imply an implementation cannot be used for an abstract method.

• An