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Polymorphism


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Lecture Objectives

To understand the concept ofpolymorphism

To understand the concept ofstatic orearlybinding

To understand the concept ofdynamic orlatebinding


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Polymorphism

Polymorphism comesfrom Greek meaning manyforms.

In Java, polymorphismrefers to the dynamic bindingmechanism that determines whichmethod definitionwill be used when a method name has beenoverridden.

Thus, polymorphismrefers todynamic binding.


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Polymorphism (Contd)

Can treat an object ofa subclass as an object ofitssuperclass A reference variable of a superclass type can point to an

object of its subclass

Person name, nameRef;PartTimeEmployee employee, employeeRef;name = new Person("John", "Blair");employee = new PartTimeEmployee("Susan", "Johnson",

12.50, 45);nameRef= employee;System.out.println("nameRef: " + nameRef);

nameRef: Susan Johnson wages are: $562.5


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Late binding ordynamic binding (run-timebinding): Method to be executed is determined at execution

time, not compile time

Polymorphism: to assign multiple meaningsto the same method name

Implemented using late binding


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The reference variable name ornameRef can pointto anyobject ofthe class Person orthe classPartTimeEmployee

These reference variableshave manyforms, thatis, they are polymorphic reference variables

They can refertoobjectsoftheirown classortoobjectsofthe classesinherited from theirclass


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Can declare a method ofa classfinal using thekeyword final

Ifa method ofa class is declared final, itcannot be overridden with a new definition in aderived class

public final void doSomeThing(){//...

}


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Can also declare a class final using the keywordfinal

Ifa class is declared final, then nootherclass

can be derived from this class

Java does not use late binding formethods thatareprivate, marked final, orstatic


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You cannot automaticallymake reference variableofsubclass type point toobject ofitssuperclass

Suppose that supRef is a reference variable ofasuperclass type and supRef points to an object ofitssubclass:

Can use a cast operator on supRef and make a referencevariable of the subclass point to the object

IfsupRef does not point to a subclass object and you use a

cast operator on supRef to make a reference variable of thesubclass point to the object, then Java will throw aClassCastExceptionindicating that the class cast is notallowed


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Operatorinstanceof: determines whetherareference variable that points to an object isofaparticularclass type

This expression evaluates totrue ifp points to anobject ofthe class BoxShape;otherwise itevaluates tofalse

pinstanceofBoxShape


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Interface variable holdsreference toobject ofa class that implements the interfaceMeasurable x;

Note that the object to whichx refers doesn'thave typeMeasurable; the type ofthe object

issome class that implements theMeasurable interface

Continued

x = new BankAccount(10000);

x = new Coin(0.1, "dime");


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You can call anyofthe interface methods:

Whichmethod is called?

double m = x.getMeasure();


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Dependson the actualobject.

Ifx refers to a bank account, calls

BankAccount.getMeasure() Ifx refers to a coin, callsCoin.getMeasure()

Polymorphism (manyshapes): Behaviorcan

vary depending on the actual type ofan object

Continued


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Called late binding:resolved at runtime

Different fromoverloading;overloading isresolved by the compiler(early binding)


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Dynamic Binding

Different objects can invoke different method definitionsusing the same method name.

The type ofobject being referenced at the time ofthemethod call, not the type ofreference that was declared,determines whichmethod isinvoked.

Forexample, ifthe reference b references a Box object andthe reference t references a Triangle object, b and t invokedifferent definitionsofthe method drawAt() even ofb and tare declared to be oftype Figure.


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Dynamic Binding (Contd)

Considerthe following example:

Figure f;

Box b = new Box(1, 4, 4);f= b;f.drawAt(2);Triangle t = new Triangle(1,2);f= t;f.drawAt(2);


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The method drawAt() isinherited from classFigureand is not overridden.

But, method drawHere() isinvoked within the

definition ofmethod drawAt(), and methoddrawHere() isoverridden.

The type ofobject referred to byfdetermines whichmethod drawHere() isinvoked.

Dynamic Binding (Contd)


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Type Checking and Dynamic Binding

Recall that an object reference to an ancestorclass canreferto an object ofa descendant class.

However, you can invoke only a method in classPerson

with the variable p.

Employee e = new Employee();Person p;p = e;


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However, ifa method isoverridden in the classEmployee, and variablep references an Employeeobject, then the method in classEmployee is used.

The variable determines what methods can be used, butthe type referenced by the object determines whichdefinition ofthe method will be used.

Type Checking and Dynamic Binding (Contd)


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To use a method name in the classEmployee withan object named by the variablep oftype Person,

use a type cast.

Example:

Employee e = (Employee)p;e.setEmployeeNumber(5678);


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However, even a type cast cannot fool Java!

Example:

will use the definition ofthe method drawHere() givenin classBox, not the definition ofdrawHere() given inclassFigure.


Box b = new Box (1, 4, 4);Figure f= (Figure)b;

f. drawHere()


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You are unlikely to assign an object ofa descendanttype to a variable ofa parent type, at least not directly.

But, such an assignment can occurindirectly byproviding an argument ofa descendant type foramethod that has a parameterofan ancestor type.



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Dynamic Binding with the toString()

Method

Recall the method toString() typicallyis used toprepareand return a string, describing an object, foroutput tothe screen.

The name ofthismethod can be omitted, thanks todynamic binding, because one definition ofmethod

println() expects a single argument oftype Object whichit uses toinvoke the method toString() associated withthe object.


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Subtle Difference

Dynamic binding refers to the process carried out by thecomputer.

Polymorphism can be thought ofassomething objectsdo.

Polymorphism, encapsulation, and inheritance, and

considered to be the main featuresofobject-orientedprogramming.


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A Betterequals() Method

Sometimes the method equals() from classObject isoverloaded when it should have been overridden.

Thisoccurs when itsparameteris not oftype Object.

Usually, thisis allright.


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A Betterequals() Method (Contd)

But, ifthe method equals() is called with an object ofclassObject asits argument, the method equals()from classObject will be invoked.

The problemisfixed by changing the formalparameterinthe overriding method so that it is a parameteroftypeObject.


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However, this allows the argument to be any type ofobject,which can produce a run-time error.

But, we can determine ifan object isofthe correct typeusing:

Finally, we should return false when comparing an objectto a null reference.


ObjectinstanceofClass_Name


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The improved equals() method:


public boolean equals(Object otherObject) {

if(otherObject == null)

return false;

else if(!(otherObject instanceof Student))

return false;

else {

Student otherStudent = (Student) otherObject; // Downcast!!

return (this.studentNumbemer == otherStudent.studentNumber));

}

}


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BenefitsofPolymorphism

Polymorphism enablesprogrammers to dealingeneralities and let the execution-timeenvironment handle the specifics.Programmerscan command objects to behave in mannersappropriate to those objects, without knowing thetypesofthe objects (aslong as the objects belongto the same inheritance hierarchy).


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Polymorphismpromotes extensibility: Softwarethat invokespolymorphic behavioris

independent ofthe object types to whichmessages are sent.New object types that canrespond to existing method calls can beincorporated into a system without requiring

modification ofthe base system. Only client codethat instantiates new objectsmust be modified toaccommodate new types.

BenefitsofPolymorphism (Contd)


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1 // PolymorphismTest.java

2 // Assigning superclass and subclass references to superclass and

3 // subclass variables.

4

5 publicclass PolymorphismTest

6 {

7 publicstaticvoidmain( String args[] )

8 {

9 // assign superclass reference to superclass variable

10 CommissionEmployee3 commissionEmployee = new CommissionEmployee3(

11"Sue", "Jones", "222-22-2222", 10000, .06 );

12

13 // assign subclass reference to subclass variable

14 BasePlusCommissionEmployee4 basePlusCommissionEmployee =

15 new BasePlusCommissionEmployee4(

16 "Bob", "Lewis", "333-33-3333", 5000, .04, 300 );

17

18 // invoke toString on superclass object using superclass variable

19 System.out.printf( "%s %s:\n\n%s\n\n",

20 "Call CommissionEmployee3's toString with superclass reference ",

21 "to superclass object", commissionEmployee.toString() );

22

23 // invoke toString on subclass object using subclass variable

24 System.out.printf( "%s %s:\n\n%s\n\n",

25 "Call BasePlusCommissionEmployee4's toString with subclass",

26 "reference to subclass object",

27 basePlusCommissionEmployee.toString() );

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Typical reference assignments

Testing Polymorphism


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29 // invoke toString on subclass object using superclass variable

30 CommissionEmployee3 commissionEmployee2 =31 basePlusCommissionEmployee;

32 System.out.printf( "%s %s:\n\n%s\n",

33 "Call BasePlusCommissionEmployee4's toString with superclass",

34 "reference to subclass object", commissionEmployee2.toString() );

35 } // end main

36 } // end class PolymorphismTest

Call CommissionEmployee3's toString with superclass reference to superclassobject:

commission employee: Sue Jonessocial security number: 222-22-2222gross sales: 10000.00commission rate: 0.06

Call BasePlusCommissionEmployee4's toString with subclass reference tosubclass object:

base-salaried commission employee: Bob Lewis

social security number: 333-33-3333gross sales: 5000.00commission rate: 0.04

base salary: 300.00

Call BasePlusCommissionEmployee4's toString with superclass reference tosubclass object:

base-salaried commission employee: Bob Lewissocial security number: 333-33-3333gross sales: 5000.00commission rate: 0.04

base salary: 300.00

AssignareferencetoabasePlusCommissionEmployee objec

toaCommissionEmployee3 variable

PolymorphicallycallbasePlusCommissionEmployee s

toString method

Testing Polymorphism (Contd)

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