If at execution time the reference of a subclass object has been assigned to a variable of one of its direct or indirect superclasses, it is acceptable to cast the reference stored in that superclass variable back to a reference of the subclass type. For example, line 46 implicitly invokes method toString of the object to which currentEmployee refers. Note that abstract class names e. Also, in the loop that processes all the shapes in the array, determine whether each shape is a TwoDimensionalShape or a ThreeDimensionalShape. Similarly, constants should be declared without key- words public, static and final, because they, too, are redundant. You can create an interface that describes the desired functionality, then im- plement this interface in any classes that require that functionality. Java does not allow subclasses to inherit from more than one superclass, but it does allow a class to inherit from a superclass and implement more than one interface.
However, these operators cannot be used to compare the contents of objects. We now modify our implementation to incorporate inheritance, using class Withdrawal as an example. Implementing an interface is like signing a contract with the compiler that states, “I will declare all the methods specified by the interface or I will declare my class abstract. Be sure to include private reference-type attributes based on class Transaction’s associations. Finally, the application should output the payment amount for each object. As in the class diagram of Fig.
Classes MyOval and MyRectangle payfoll provide a no-argument constructor and a constructor with argu- ments for the coordinates, color and determining whether the shape is filled.
Many scientists blame these greenhouse gases for the phenomenon called global warming. Line 38 assigns the reference to a CommissionEmployee object to employees[ 2 ].
You can create an interface that describes the desired functionality, then implement this interface in any classes that require that functionality. As indicated by the comments in linesclass Employee of Figure To demonstrate creating and using interfaces, we modify our payroll appli- cation to create a general accounts payable application that can calculate payments due for company employees and invoice amounts to be billed for purchased goods.
As another example, suppose we design a video game that manipulates objects of classes Martian, Venusian, Plutonian, SpaceShip and LaserBeam. Line 37 assigns the reference to an HourlyEmployee object to employees. The device-driver methods are declared as abstract methods in the abstract superclass.
Once a class implements an interface, all objects of that czse have an is-a relationship with the interface type, and all objects of the class are guaranteed to provide the functionality described by the interface. We maintain an array of Employee variables, each of which holds a reference to an Employee object of course, there cannot be Employee objects because Employee is an abstract class—because of inheritance, however, all objects of all subclasses of Employee may nevertheless be thought caxe as Employee objects.
Java™ How To Program (Early Objects), Tenth Edition by Harvey Deitel, Paul Deitel
The following sections implement the Employee class hierarchy. Finally, the program polymorphically determines and outputs the type of each object in the Employee array. Com- plete the following steps to create the new application: The drawLine method of the Graphics class will connect the two points supplied with a line.
An interface declaration stucy with the keyword interface and contains only constants and abstract methods. A class diagram models a realization as a dashed arrow with a hollow arrowhead pointing from the implementing class to the interface.
As you’ll soon see, when we invoke each employee’s earnings method off a superclass Employee reference, the correct earnings calculation is performed due to Java’s polymorphic capabilities.
The same message in this case, move sent to a variety of objects has “many forms” of results—hence the term polymorphism.
We use abstract class Employee to represent the general concept of polymorphixm employee. Method toString calls superclass method toString line 52 to obtain the Employee -specific information i. A superclass reference can be used to invoke only methods of the superclass and the superclass can invoke overridden versions of payrolp in the subclass.
We explore GUI programming in Chapters 11 and Such classes provide implementations of every method they declare some of the implementations can be inherited.
(PDF) Chapter 10 Object Oriented Programming | Yucheng Liao –
However, these operators cannot be used to compare the contents of objects. In fact, a class can implement as many interfaces as it needs, in addition to extending one other class.
Normally, such validation should be provided. Since abstract methods are meant to be overridden so that they can process objects based on their types, it would not make sense to declare a static method as abstract.
Chapter Object-Oriented Programming: Polymorphism – CodeProject
Creating the Array of Employees Line 33 declares employees and assigns it an array cass four Employee variables. Interface Payable contains public abstract method getPaymentAmount line 6. Line 70 invokes the getClass method on the object to get its runtime class i.
If the instanceof expression in line 49 is true, lines 53—60 perform the special pro- cessing required for the BasePlusCommissionEmployee object.
Imagine that each of these classes extends superclass Animal, which contains a method move and maintains an animal’s current location as x-y coordinates.
Creating and Using Interfaces invokes method toString via a Payable interface reference, even though toString is not declared in interface Payable—all references including those of interface types refer to objects that extend Object and therefore have a toString method.
Classes Withdrawal and Deposit require this additional attribute to store the amount of money that the user wishes to withdraw or deposit. The compiler prevents this assignment because a Com- missionEmployee is not a BasePlusCommissionEmployee—the is-a relationship applies only between the subclass and its superclasses, not vice versa.