Concrete classes provide the specifics that make it reasonable to instantiate objects. In the previous example, we avoided several compilation errors by downcasting an Employee variable to a BasePlusCommissionEmployee variable in lines 53— The Employee constructor does not validate its parameters in this example; normally, such validation should be provided. The condition at line 49 ensures that this is the case. We introduced abstract classes, which allow you to provide an appropriate superclass from which other classes can inherit. Concrete class SalariedEmployee extends Employee and inherits its superclass’s realization relationship with interface Payable. Lines 22—30 output the string representation and earnings of each of these objects.
An abstract class can include methods with implementations and abstract meth- ods. Karen Price social security number: Recall that when a class imple- ments an interface, it makes a contract with the compiler stating either that the class will implement each of the methods in the interface or that the class will be declared abstract. Lines 44—65 iterate through array employees and invoke methods toString and earnings with Employee control variable currentEmployee. However, the write call itself really is no different from the write to any other device in the system: Line 70 invokes getClass on the current object to get its runtime class. We then modify our class diagram to incorporate the new inheritance relationships.
You make a class abstract by declaring it with keyword abstract. Otherwise, these subclasses, too, will be abstract. Line 36 as- signs the reference to a SalariedEmployee object to employees.
(PDF) Chapter 10 Object Oriented Programming | Yucheng Liao –
Consider the following example of polymorphism. The program should generate random shapes and store them in an array of type MyShape.
The method calculates the total payment required to pay the invoice. A program can create an array of superclass variables that refer to objects of many subclass types. Making the class final also prevents programmers from creating subclasses that might bypass security restrictions. The company wants to write an application that performs its payroll calculations polymorphically. Classes BalanceInquiry, Withdrawal and Deposit share integer attribute accountNumber, so we factor out this common attribute and place it in superclass Transaction.
Invoice implements Payable, so an Invoice object also is a Pay- able object, and we can assign the reference of an Invoice object to studu Payable variable. Lines iterate through array employees and invoke methods toString and earnings with Employee control variable currentEmployee. We use superclass references to manipulate both su- perclass objects and subclass objects polymorphically.
Classes that can be used to instantiate objects are called concrete classes. The concept of polymorphism. We now continue our study of object-oriented programming by explaining and demonstrating polymorphism with inheritance hierarchies.
Many scientists blame these greenhouse gases for the phenomenon called global warming. Since MyShape represents any shape in general, you cannot stury a draw method without knowing exactly what shape it is. The Java compiler does allow the assignment of a superclass reference to a subclass variable if we explicitly cast the superclass reference to the subclass type—a technique we discuss polymorhism detail in Section In addition, method toString calls superclass method toString line 56 to obtain the Employee-specific information i.
To apply inheritance, we first look for commonality among classes in the system.
In the Java API, the vast majority of classes are not declared final. This class cannot be extended, so programs that use Strings can rely on the functionality of String objects as specified in the Java API.
Java™ How To Program (Early Objects), Tenth Edition by Harvey Deitel, Paul Deitel
At execution time, if the object to which the reference refers is not a sub- class object, an exception will occur. This method is declared abstract in superclass Shape, so each concrete subclass must implement method draw in a manner specific to that shape.
Only client code that instantiates new objects must be modified to accommodate new types. Now that you have seen a complete application that processes diverse subclass objects polymorphically, we summarize what you can and cannot do with superclass and subclass objects and variables. Using an Interface To use an interface, a concrete class must specify that it implements the interface and must declare each method in the interface with the signature specified in the interface declara- tion.
Chapter 10: Object-Oriented Programming: Polymorphism
Self-Review Exercises Section Then, all the objects of your class have the is-a relationship with Serializable. Class Invoice also contains a constructor linesget and set methods lines that manipulate the class’s instance variables and a toString method lines that returns a string representation of an Invoice object.
Of course, there cannot be Employee objects, because Employee is an abstract class. We create an enhanced employee hierarchy to solve the following problem: The power and flexibility of interfaces is used frequently throughout the Java API. These assignments are natural—for example, a CommissionEmployee3 variable’s primary purpose is to hold a reference to a CommissionEmployee3 object.