understanding polymorphism


public class Seed{ public void grow() {

System.out.println( “Seed grow”); }}

public class Test{ public Test() { Seed x = new Apple(); x.grow(); } public static void main( String[] args ) { Test t = new Test(); }}

What is the output of program

Test?public class Apple extends Seed{ public void grow() { System.out.println( “Apple grow” ); }}


In Java the simplest category of variables are the primitives.

OOP Basics

int x;

x = 0;

The primitive variables are notnot objects.

Only one slot in memory is used to hold this variable.

1


x 0

• When we declare a primitive variable, it will occupy a single slot in memory. That slot will contain the value.

int x;x = 0;

1 2 3 4 5 6

7 8 1211109

2


Now, we want to build an object. I have declared “t” to be a reference to an object of type Employee.

At this point, the reference “t” does not point to any object.

Soon, it will point to an object of type Employee, but now the object doesn’t exist.

OOP Basics

Employee t;

3


Employee t = new Employee();

When this statement executes, the new keyword executes the default Constructor for Employee, which actually creates an object in memory and writes the location of that object in t.

Now t points to the new Employee object.

OOP Vocabulary

4


• It’s critical to understand the difference between a reference and the object to which it refers.

Employee t;

t = new Employee();

OOP Basics

5


t

• When we declare a reference and instantiate the class,the location of the object is written in the reference.The reference points to the object.


6


• Now, let’s explore this object called employee.

OOP Basics

public class Employee{

}

7


• I have added two instance variables: name and pay.

OOP Basics

public class Employee{ private String name; private double pay;

}

8


• I have added a default constructor.

OOP Basics


public Employee() { name = “”; pay = 0.0; }}

9


t

• This is what we have done. The object has instance variables, and methods we will use to operate on those instance variables.

• If I want to make a concrete instance of this class, I need another class--a driver with a main method--to instantiate this class.


name=“” pay=0.0

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• I have added getters and setters for the name.This is all pretty trivial so far.• The purpose for examining this class is to better understand its reference.

OOP Basics


public Employee() { name = “”; pay = 0.0; } public String getName() { return name; } public void setName( String n ) { name = n; }}

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t Employee

Employee()

String getName()

void setName( String)

Up close and personal with this reference. What do we know from this reference? We know:• It can point to some object. • It expects that object to be of type Employee.• It knows that the object has these methods.

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public class Employee{ private double pay; private double rate; private int hours; public Employee() { pay = 0.0; rate = 0.0; hours = 0; } public double getPay() { return pay; } public double calcPay() { pay = rate * hours; }}

• Here I have made a first stab at making this class do some work. I have given it the ‘calcPay()’ method.

Unfortunately, this calcPay() method is only useful for an hourly employee.

OOP Basics

13


public abstract class Employee{ private double pay; public Employee() { pay = 0.0; } public double getPay() { return pay; } public abstract double calcPay();

}

• Now, I’ve changed the ‘calcPay()’ method. I’ve made it abstract.Also, the class itself becomes abstract.

OOP Basics

14


public class HourlyEmployee extends Employee{ private double rate; private int hours;

public HourlyEmployee( double r, int h ) {

rate = r; hours = h; } public double calcPay() {

return rate * hours; }

}

• What is the sequence of actions as this class is instantiated?

OOP Basics

SuperclassSubclass

//Implicit call to Superclass constructor

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public class Employee{ public Employee() { System.out.println( “Employee constructor”); } }

public class HourlyEmployee extends Employee{ public HourlyEmployee() { System.out.println( “HourlyEmployee constructor”); }}

public class TestEmployee{ public TestEmployee() {

HourlyEmployee he = new HourlyEmployee(); } public static void main( String[] args ) {

TestEmployee te = new TestEmployee(); }}

public class Object{ public Object() { }}

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Superclasses and Subclasses• “is a”

• Before the subclass HourlyEmployee object could be instantiated, we first had to instantiate the superclass Employee object.

• A subclass is an instance of the superclass. It contains everything the superclass contains.

• Because a subclass contains everything its superclass contains, we can say: “A subclass is an instance of its superclass.”

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• Whenever you extended another class, (a Superclass) you have created an instance of every class above your classevery class above your class in the class hierarchy.

• When the Constructor for HourlyEmployee fires, it silently also fires the Constructors for Employee and every other class above it in the hierarchy.

Superclasses and Subclassespublic class HourlyEmployee extends Employee{

...}

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Superclasses and Subclasses

• Every method, every data variable present in the Superclass is also present in the Subclass.

• Make sure you understand this:

If my class has everything above it—then I can say it isis an example of the classes above it.

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The super

Reference

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• Already, we know that the Superclass-Subclass interaction has a big effect on the Constructor.

• The Superclass Constructors are either implicitly or explicitly called first in the Constructor of a Subclass.

The super Reference

super( whatever the Superclass needs)

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The super Reference


• Using this super reference, you can call the call the ConstructorConstructor of your Subclass’ Direct Superclass.

• If you use this super reference, then it must be the very first statement in the Subclass’ Constructor.

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public class HourlyEmployee extends Employee{ double rate; int hours;

public HourlyEmployee() {

// implicit (hidden) call to Superclass Constructor.rate = 0.0;hours = 0;

}

public HourlyEmployee( double r, int h ) {

// implicit (hidden) call to Superclass Constructor.rate = r;hours = h;

}

public double calc_pay() { return hours * rate; } }

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public class HourlyEmployee extends Employee{ double rate; int hours;

public HourlyEmployee() {

super( “”, “” ); // Explicit call to Superclass Constructorrate = 0.0;hours = 0;

}

public HourlyEmployee( String n, String s, double r, int h ) {

super( n, s ); // Explicit call to Superclass Constructorrate = r;hours = h;

}

public double calc_pay() { return hours * rate; } }

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• There is a similar statement that is used to call your Subclass’ constructor within your Subclass.

• Don’t get the two confused.

The super Reference


this( whatever the Subclass needs)

25


Subclass can be Treated

as an Object of the

Superclass

26



• A Subclass contains more than its Superclass contains.

• Because of that, we say inside of a Subclass is a complete copy of its Superclass.

27



• On previous slides, we said a Subclass is an object of the Superclass type.

• Now, we take that one step further.

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• A Subclass contains everything ( and more) required to make a complete example of the Superclass...

• Thus, we say an object of a Subclass can be treatedtreated as an object of its Superclass.

• In other words, in certain situations, we can ignore the difference between the Superclass and Subclass objects.

29



“An object of a Subclass can be treated as an object of its Superclass.”

• What would stop this from being true?

• Is there any part of the Superclass that is missing from the Subclass?

Nope—it’s all there.

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“An object of a Subclass can be treated as an object of its Superclass.”

• Certainly if we are asking a Subclass to fill in for its parentfill in for its parent, we throw away the extra stuff that the Subclass added, but—because a Subclass “is an” example of its Superclass—we can “treat the Subclass as an example of the Superclass.”

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• And, if we only want to call the methods

from the Superclass

and access the data variables that come

from the Superclass—what’s the difference?

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• Doing this might be interesting.

• As long as we treat the reference like it only refers to the Superclass, we can handle a Subclass object from a Superclass “reference.”

• As long as we only call the methods that exist in the Superclass, this will work fine.

33


public class Employee{ String name; String SSN;

public double calc_pay() { return 0.0; }}

public class HourlyEmployeeextends Employee

{ double hourly_rate; int hours;

public double calc_pay() { return hours * hourly_rate; } }

public class SalariedEmployeeextends Employee

{ double monthlySalary;

public double calc_pay() { return monthlySalary; } }

34


public class TestEmp{

public static void main( String args[] ){ double money = 0.0; HourlyEmployee hour; hour = new HourlyEmployee();

money = hour.calc_pay(); System.out.println( “Money=” + money );}

}

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public static void main( String args[] ){ double money = 0.0; SalariedEmployee salr; salr = new SalariedEmployee();

money = salr.calc_pay(); System.out.println( “Money=” + money );}

}

36



public static void main( String args[] ){ double money = 0.0; Employee empl; empl = new SalariedEmployee();

money = empl.calc_pay(); System.out.println( “Money=” + money );}

}

• As long as we only call the methods that exist in the Superclass, this will work fine.

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public static void main( String args[] ){ double money = 0.0; Employee empl; empl = new SalariedEmployee();


}

• So, why in the world might we want to do this?

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public class RetiredEmployeeextends Employee

{ double monthlyPension; public double calc_pay() { return monthlyPension; } } • Imagine if—after we built our system—we

decided to add a class, for Retired Employees.

• If we had built our program on a Superclass reference, then we wouldn’t have to rebuild anything.• The runtime environment sees what kind of object we have instantiated, and calls the right method override !

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public static void main( String args[] ){ double money = 0.0; Employee empl; empl = new RetiredEmployee();


}

• Caution, this works only when you’re calling a method that exists in your Superclass, not one that exists only in the Subclass.(And so now you see the advantage of identifying a bunch of empty methods in your Superclass.)


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• And (drumroll…) the whole process is called...


42


• More on Polymorphism

• We can create a Superclass reference that is an array.

• Then, when we instantiate the array, we can attach all different kinds of Subclass objects to the Superclass array reference.


43


• As long as we’re only calling methods that exist in the Superclass, we can work our way through the array and it will call all the correct overridden versions of each individual Subclass object.


44


• The Superclass reference only knows about the methods that exist in the Superclass.

• The Superclass only tries to call the methods it knows about.

• That’s perfectly fine, because all the methods in the Superclass are available in its Subclass.


45


• Therefore the Superclass isn’t aware that the object it references can do a whole lot more than the Superclass thinks it can.


46

Superclasses and Subclasses• So, “We can create a Superclass reference array that actually points to a Subclass object.”

• As long as we treat those Subclass objects as if they were Superclass objects, (meaning we only call the methods in the Superclass) we have no problems.


47

Superclasses and Subclasses• The Subclass knows how to do everything its parent Superclass can do.

• The kids can do everything the parent can.


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• However, if we try to do it the other way around—treating the Superclass as if it were one of its children—then we can have...


problemsproblems• Why? The Subclass can do many things the Superclass cannot.

• The kids can do many things the parent cannot.


49


• If we called the Superclass with the Subclass’s reference, then you might expect the Subclass reference can do all the things the kids can—and you’d be wrong!


50


• If you want to go the other way, attach a Superclass object to a Subclass reference, you have to do an explicit cast, as a way of informing the compiler that you really want to do this dumb thing.


51


• “A Superclass object is notnot a Subclass object.”

An HourlyEmployee is an Employee(Subclass is a Superclass object)

but…

An Employee is NOT an HourlyEmployee (Superclass is NOT a Subclass object)


52


• If you want to actually call some of the methods that don’t exist in the Superclass, then you have to first cast the object back to a reference of its own kind.


53

A Case Study of Inheriting a Class:

The Rosetta Stone of Inheritance


54

• We have a class “Point” and its subclass “Circle”.

A Case Study of Inheriting a Class

public class Point{}

public class Circle extends Point{}

55



public class Test{ public Test() {

Point p = new Point(); Circle c = new Circle(); }

public static void main( String[] args ) { }}

56

Point p;

p = new Point();

p

Circle c;

c = new Circle();

c

57



public class Test{ public Test() {

Point p = new Circle();Circle c = p;

}

public static void main( String[] args ) { }}

Now, we have a Superclass

reference, pointing to a subclass

object.Is this legal?

Is this useful?

58

p = c;

p

c

What if we try to call only the methods that both share?

Will this “p” reference work correctly?

This will work because any methodsone would expect to find in a Point reference (p) already exist in the Circle object.

59

c = p;

p

c

No ! A Circle reference would expect many methods that you won’t find in a Point object.

Will this work for “c”?

For this to even remotely work, you would need to do a “downcast” ?


60


• We’re attempting to do this error by casting a Subclass reference to a Superclass object.

circleRef = (Circle) pointRef;

• It’s useful to use the operator instanceof

p instanceof Circle

• This operator will tell us if p is a reference to an object of type Circle.


61


• Attempting to have a Superclass reference point to a Subclass object would lead to an error known as a ClassCastException.


62

Implicit Subclass-object

toSuperclass-object

Conversion


63

• Joe was feeling entrepreneurial, and so he decided to buy a piece of land in Arkansas. • Joe paid $50 for what the deed described as a wooden shack in the swamp.

• But, to his great surprise, when he got there, Joe discovered he hadn’t bought a shack in a swamp, but rather, the Taj Mahal on a beautiful lake. Joe was ecstatic.

• Joe got more than he was promised.


• Mary was feeling entrepreneurial, and so she decided to buy a huge Mansion on an estate in Arkansas. • Mary paid $5 Million for what the deed described as a palatial estate on 40 acres.

• But, to her great surprise, found she hadn’t bought a magnificent Mansion and estate, but rather, a Doublewide trailer next to a dump. Mary was horrified.

• Mary got less than was promised. 64


65

• Joe’s situation is akin to Referring to a Subclass object with a Superclass reference.

You get more than you expected. It’s a pleasant surprise.

SuperclassRef = new SubclassObject


66

• Mary’s situation is akin to Referring to a Superclass object with a Subclass reference .

You get less than you expected. It’s a tragic mistake.

SubclassRef = new SuperclassObject


67

Implicit Subclass-object to Superclass-object Conversion

• There are four possible conversions that can occur:

1.) Refer to a Superclass object with a Superclass reference.

This is routine


68


2.) Refer to a Subclass object with a Subclass reference.

This is also routine


69


3.) Refer to a Subclass object with a Superclass reference

This is safesafe because the Subclass object isis an object of its Superclass.

Such code can only refer to Superclass methods.


70

Why might want to do that?

Suppose many classes inherit from one Superclass. • You can make an array of Superclass

references. • Then, you can treat them all the same,

as if they really were all Superclassobjects.

• As long as you call methods that exist inthe Superclass (but were overriddenby each of the various Subclasses), then the runtime system calls the correct overridden method for each one!


71

This process is known as...


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4.) Refer to a Superclass object with aSubclass reference .

This is a syntax error! It doesn’t make sense. It is only remotely possible ifif the Subclass is explicitly cast into a Superclass reference, which is a way of telling the compiler you are doing this damn-fool thing with your eyes open.


73


So, why is that a problem?

Here’s the problem:A Subclass contains more and does more than its

Superclass parent.

If you use a Subclass reference to point to a Superclass object, you are implying that the object has more data variables available than it really does—and if someone made that assumption—they would be sorely mistaken.


74

The Right Way to Use

Polymorphism


75

class Employee{

private String ssn;

public Employee(){

ssn = "";}

public Employee( String soc_num ){

ssn = soc_num;}

public String getSSN(){

return ssn;}

public double calcPay(){

return 0;}

}

Employee is our Superclass. As we see, it contains the instance

variable String ssn, a Constructor, the Accessor method getSSN() and the

emptyempty method calcPay(), which we will need to override in

all the Subclasses because they will all need to calculate pay

differently.


class HourlyEmployee extends Employee{

double hourlyRate;int hoursWorked;

public HourlyEmployee(){

hourlyRate = 0.0;hoursWorked = 0;

}

public void setHourlyRate( double rate ){

hourlyRate = rate;}

public void setHoursWorked( int hours ){

hoursWorked = hours;}

public double calcPay(){

return hourlyRate * hoursWorked;}

} 76

Instance variables specific to the Subclass

Default Constructor

The overridden method inherited from the Superclass.


77

public class DemoPolymorphism{

double pay = 0;public DemoPolymorphism(){

Employee supEmp = new HourlyEmployee();HourlyEmployee subHrEmp = new HourlyEmployee();

pay = supEmp.calcPay();System.out.println( "Superclass pay = " + pay );

subHrEmp.setHourlyRate( 5.65 );subHrEmp.setHoursWorked( 20 );pay = subHrEmp.calc_Pay();System.out.println( "Subclass pay = " + pay );

supEmp = subHrEmp;pay = supEmp.calcPay();System.out.println( "Superclass pay = " + pay );

}public static void main( String[] args ){

DemoPolymorphism demo = new DemoPolymorphism();System.exit( 0 );

}}

We declare a reference to an object of type Employee called supEmp. However, we instantiate an HourlyEmployee object and assign the reference to the Superclass Employee. This works because—after all—HourlyEmployee isis an Employee.


78

public class DemoPolymorphism{

double pay = 0;public DemoPolymorphism(){

Employee supEmp = new HourlyEmployee();HourlyEmployee subHrEmp = new HourlyEmployee();

pay = supEmp.calcPay();System.out.println( "Superclass pay = " + pay );

subHrEmp.setHourlyRate( 5.65 );subHrEmp.setHoursWorked( 20 );pay = subHrEmp.calc_Pay();System.out.println( "Subclass pay = " + pay );

supEmp = subHrEmp;pay = supEmp.calcPay();System.out.println( "Superclass pay = " + pay );

}public static void main( String[] args ){

DemoPolymorphism demo = new DemoPolymorphism();System.exit( 0 );

}}


79

Superclass pay = 0.0Subclass pay = 113.0Superclass pay = 113.0Press any key to continue . . .


80

The Wrong Way to Use Polymorphism—a Syntax

Error—(Without An Explicit Cast)


81

public class CastSubTypeReftoSuperTypeObject{

double pay = 0;public CastSubTypeReftoSuperTypeObject(){ // Syntax Error ! ! ! HourlyEmployee subHrEmp = new Employee(); // Error!

// The error is not yet revealed. pay = subHrEmp.calcPay(); System.out.println( "Subclass pay = " + pay );

// Error because the object doesn’t have these // methods available.

subHrEmp.setHourlyRate( 5.67 ); subHrEmp.setHoursWorked( 20 ); pay = subHrEmp.calcPay(); System.out.println( "Subclass pay = " + pay );}public static void main( String[] args ){ CastSubTypeReftoSuperTypeObject demo; demo = new CastSubTypeReftoSuperTypeObject();

System.exit( 0 );}

}

In the program, we know that subHrEmp was declared as a type HourlyEmployee. But instead of getting a Mansion (HourlyEmployee), the reference was assigned to a Doublewide Trailer (Employee). The object doesn’t have these methods and these message calls won’t work!


82

C:\CastSubTypeReftoSuperTypeObject.java:59: Incompatible type for declaration. Explicit cast needed to convert Employee to HourlyEmployee.

HourlyEmployee subHrEmp = new Employee(); // Error! ^

1 error

Tool completed with exit code 1

At this point, the compiler is extremely unhappy.So, let’s give it what it is specifically asking for—an explicit cast—and see what happens then.


public class CastSubTypeReftoSuperTypeObject{

double pay = 0;public CastSubTypeReftoSuperTypeObject(){ // Now we’ve Band-Aided the Error with An

// Explicit cast HourlyEmployee subHrEmp; subHrEmp = (HourlyEmployee) new Employee();

// The error is not yet revealed. pay = subHrEmp.calcPay(); System.out.println( "Subclass pay = " + pay );

// Error because the object doesn’t have these // methods available.

subHrEmp.setHourlyRate( 5.67 ); subHrEmp.setHoursWorked( 20 ); pay = subHrEmp.calcPay(); System.out.println( "Subclass pay = " + pay );}public static void main( String[] args ){ CastSubTypeReftoSuperTypeObject demo; demo = new CastSubTypeReftoSuperTypeObject(); System.exit( 0 );}

}83


84

Exception in thread "main" java.lang.ClassCastException: Employee at CastSubTypeReftoSuperTypeObject.<init>(CastSubTypeReftoSuperTypeObject.java:60) at CastSubTypeReftoSuperTypeObject.main(CastSubTypeReftoSuperTypeObject.java:75)

Press any key to continue . . .

At this stage, the compiler is incensed—it trusted us when we asked for that Explicit cast, and look what happened.

understanding polymorphism

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