java programming - polymorphism

CHAPTER 8 INHERITANCE AND POLYMORPHISM

Oum Saokosal, Chief of Computer ScienceNational Polytechnic Institute of CambodiaTel: (855)-12-252-752E-mail: oum_saokosal@yahoo.com

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CHAPTER 8Polymorphism

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Polymorphism

1. What is polymorphism?

2. Why polymorphism?

3. Specific uses of polymorphism

4. Notes

5. Remember

6. Casting

7. instanceof Operations

8. Remember

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What is Polymorphism? (1)• OOP has 3 features:

1. Class encapsulation

2. Inheritance

3. Polymorphism

• Polymorphism is a mechanism to allow a single variable to refer to objects from different classes.

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What is Polymorphism? Example (2)

• Assume we have GraduateStudent which inherits from

Student. Here is an example.

public class Student{

public String getClassName(){

return "Student Class";

}

}

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Student

+getClassName():String

UndergraduateStudent

+getClassName():String

GraduateStudent

+getClassName():String

What is Polymorphism? Example (3)

//Continue ...public class GraduateStudent extends Student{

@Override public String getClassName(){

return "GraduateStudent Class";

}

}

public class UndergraduateStudent extends Student{

@Override public String getClassName(){

return “UndergraduateStudent Class";

}

}

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What is Polymorphism? Test 1 (4)

//Test1 1 – Polymorphismpublic class TestPoly1{

public static void main(String[] args){

Student s = new UndergraduateStudent();

System.out.println(s.getClassName());

}

}

Q. What is output?

A. UnderGraduateStudent Class

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That’s polymorphism

What is Polymorphism? Test 2 (5)

//Example 2 – Without Polymorphismpublic class TestNoPoly2 {

public static void main(String[] args){

GraduateStudent gs = new GraduateStudent();

displayUnder(gs);

displayGraduate(new UndergraduateStudent());

}

static void displayUnder(GraduateStudent gs){

System.out.println(gs.getClassName());

}

static void displayGraduate(UndergraduateStudent us){

System.out.println(us.getClassName());

}

}

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What is Polymorphism? Test 2 (6)

//Example 2 – With Polymorphismpublic class TestPoly2 {

public static void main(String[] args){

display(new GraduateStudent());

display(new UndergraduateStudent());

}

//Polymorphic method

public static void display(Student stu){

System.out.println(stu.getClassName());

}

}

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Why Polymorphism? (1)

• Why Polymorphism?1. To choose which method from which class is called at

runtime dynamically.

2. A variable of a parent type can refer to any child and grand child of the parent.

Student stu = new GraduateStudent();

stu.getClassName();

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Student

getClassName():String

UndergraduateStudent

getClassName():String

GraduateStudent

getClassName():String

Parent/Superclass Children/Subclass

Specific Uses of Polymorphism

Methods (1)

Methods• If we don’t use polymorphism, you have to make a new

method for every different parameter data type. Look at slide 8.

• But with polymorphism, you make only one method for every parameter which has the same parent. Look at slide 9.

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Specific Uses of Polymorphism

Array (2)

Array: In Java, every element in an array has the same type, doesn’t it? E.g.

double[] scores = new double[10];

scores[0] = 1.4;scores[1] = “Hi”; //Compile Error

Circle[] circles = new Circle[5];

circles[0] = new Circle(5);

circles[1] = new Circle(20);circles[2] = new Student();//Compile Error

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Specific Uses of Polymorphism

Array (3)

Q. If I do want to store different types of objects in one array, how can I do it?

A. Use polymorphism.

If the objects are inherited from the same parent, we can store them in the same array.

Example:Student[] students = new Student[3];

students[0] = new Student();

students[1] = new GraduateStudent();

students[2] = new UndergraduateStudent();

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Specific usages of Polymorphism

Array (4)public class TestPoly3 { public static void main(String[] args){ Student[] s = new Student[3]; s[0] = new Student(); s[1] = new GraduateStudent(); s[2] = new UndergraduateStudent(); display(s); //Polymorphism }

public static void display(Student[] s){ for(int i = 0; i<s.length; i++){

System.out.println(s[i].getClassName()); } }}

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Note #1 (1)• Look at the codes again.

public class Student{ public String getClassName(){ return "Student Class"; }}public class GraduateStudent extends Student{ @Override public String getClassName(){ return "GraduateStudent Class"; }}public class UndergraduateStudent extends Student{ @Override public String getClassName(){

return “UndergraduateStudent Class"; }}

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GraduateStudent and UndergraduateStudent

override getClassName() method!

Note #1 (2)• Please note that the two subclasses of Student override

getClassName().

Then, we can use polymorphism to refer to getClassName() in the sub class.

public class TestPoly1{

public static void main(String[] args){

Student s;

s = new GraduateStudent();

System.out.println(s.getClassName());

s = new UndergraduateStudent();

System.out.println(s.getClassName());

}

}

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Note #2 (1)

• But now in a subclass GraduateStudent, we add a

new method, say getDegree().public class GraduateStudent extends Student{

public String getClassName(){

return "GraduateStudent Class";

}

public String getDegree(){

return "Msc in Computer Science";

}

}

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Note #2 (2)• And then we want to use polymorphism to call getDegree() in the subclass,public class TestPoly1{

public static void main(String[] args){

Student s = new GraduateStudent();

System.out.println(s.getDegree());

}

}

It will show a compile error because Student class

doesn’t have getDegree().

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Note #3 (1)

• What about if we want add a method, say getID() to

the parent, Student.

public class Student{ public String getClassName(){

return "Student Class";

}

public String getID(){

return “NPIC001";

}

}

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Note #3 (2)

• Then we want to call getID().public class TestPoly1{

public static void main(String[] args){

Student s = new GraduateStudent();

System.out.println(s.getID());

}

}

Q. Is it a compile error?

A. No. As long as getID() is in Student, we can call it without any question.

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The bottom line (1)

1. As long as subclasses inherit from the same parent, you can declare a variable as the parent type and then refer to any subclasses.

Student stu; //stu is parent type

//stu refers to subclasses

stu = new UndergraduateStudent();

stu = new GraduateStudent();

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Student

getClassName():String

UndergraduateStudent

getClassName():String

GraduateStudent

getClassName():String

The bottom line (2)

2. When you call a method which exists both in a parent and subclass, then it will dynamically refer to the method in the subclass. (This is the main reason to use Polymorphism) See slide 7.

3. You cannot call methods that are NOT existed in parent. If you try it, you will get compile error. See slide 18.

4. However, you can call every method in the superclass. See slide 19 & 20.

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Remember (1)• Polymorphism is a mechanism to enable us to choose a

method at runtime dynamically.• Polymorphism always needs Inheritance.• Polymorphism always needs Overridden Methods.• There is no keyword to indicate Polymorphism. • You can recognise it whenever you see a class type that is

different from its constructor. E.g. Student s = new GraduateStudent();

• You can also recognise it, when a class type is Object class. E.g.private void aMethod(Object ob){...}

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Remember (2)• Polymorphism is not as vital as inheritance. That is,

without polymorphism, you still CAN make a Java program; however without inheritance, especially when using Java library code, you CANNOT achieve very much.

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Casting Objects (1)• What is Casting?

Casting is a technique to convert a variable from one primitive type to another one. E.g.:

double d1 = 2.5;

double d2 = 12;

int i1 = 23;

int i2 = 22.5; // Compile Error• How to solve this error?

int i2 = (int) 22.5; // i2 = 22

//Casting from double 22.5 to integer.

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Casting Objects (2)• To understand casting, let’s meet a group of animals

studying polymorphism.

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SR: Smart Rabbit

CB: Curious Ball

CB is a curious student who always asks questions whereas SR is a smart student. She can help to answer CB’s questions.

Casting Objects (3)

CB: Hey Smart Rabbit! Can I ask you some questions?SR: Yes, Of course. What can I do for ya?CB: I have 2 classes, Grape inherits from Fruit.

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Casting Objects (4)

SR: So what?

CB: Grape has four methods. And Fruit has only two methods. Then I want to use polymorphism like this:

Fruit f = new Grape();

So which methods can I call from f?

f._ _ _ ?SR: Then you can call only those methods from Fruit

class like isEatable() and getColor(), and, of course, other 9 methods from Object class.

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Casting Objects (5)

CB: You know, usually when I define a variable with a parent type and refer to a constructor from the same class, I can call every method from it.Grape g = new Grape();

g.getTaste(); // OK

g.getVitamin(); //OK

SR: Yeh, and what is your problem?CB: But now when I use polymorphism, it seems to me

that I couldn’t do what I used to do. Fruit fruit = new Grape();

fruit.getTaste(); // Compile Error

fruit.getVitamin(); // Compile Error

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Casting Objects (6)

CB: So do you have any solution, Rabbit?

SR: Why not? You can use “casting” to cast from Fruit to another Grape variable.

CB: Can you show me?SR: Sure! Here it is:Fruit fruit = new Grape();

Grape grape = (Grape) fruit; // Casting

grape.getTaste(); //OK

grape.getVitamin(); //OK

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Casting Objects (6)

CB: Yeh... but you still create a variable that has Grape as its parent type. It doesn’t make sense to me?

SR: Why not?CB: You know, it is much better if I just create a

variable which has Grape as its type and refer to its constructor since the beginning like this:Grape g = new Grape();

g.getTaste(); // OK

g.getVitamin(); //OK

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Casting Objects (7)

SR: Well, but do you remember polymorphic method?CB: Yes. SR: So can I tell me some about it?CB: polymorphic method is just like other methods but

the type of parameters is always a superclass. For example: public static void display(Object o){

JOptionPane.showMessageDialog(null, o);

}

SR: Yehh! You are good!

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Casting Objects (8)CB: Thank you. But how polymorphic method works with casting?SR: OK! I’ll show you the differences when not using casting and when

using casting.public class TestNoCasting { public static void main(String[] args){ showTaste(new Grape()); } public static void showTaste(Fruit fruit){

JOptionPane.showMessageDialog(null, fruit.getTaste());

} //Error because Fruit has not getTaste()}

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Casting Objects (9)SR: Here is codes when using casting.public class TestCastingObject { public static void main(String[] args){ showTaste(new Grape()); } public static void showTaste(Fruit fruit){ Grape grape = (Grape) fruit; JOptionPane.showMessageDialog(null, grape.getTaste()); //OK }}

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Casting Objects (10)

CB: Wow! Now I got. Casting can help polymorphism more powerful. Both Java and you are very smart.

SR: Not yet dude!CB: What else?SR: What about if I add another class, say Apple

inherits also from Fruit? Let’s see it on the next slide.

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Casting Objects (11)

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Casting Objects (11)SR: Then I pass in Apple object to showTaste() method.public class TestCastingObject { public static void main(String[] args){ showTaste(new Apple()); } public static void showTaste(Fruit fruit){ Grape grape = (Grape) fruit; JOptionPane.showMessageDialog(null, grape.getTaste()); }}//Runtime Error: casting wrong class

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instanceof Operation (1)CB: Wo.. ho.. ho. I didn’t realize it.SR: Don’t worry! Remember Java is number one in the

world.CB: So what is your solution?

SR: We can use instanceof to test whether it is correct object or not before we cast it.

CB: Can you show how?SR: OK, Let’s go to the next slide.

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instanceof Operation (2)public static void main(String[] args) {

showTaste(new Apple());

}

public static void showTaste(Fruit fruit) {

if (fruit instanceof Grape) {

Grape grape = (Grape) fruit; System.out.print(grape.getTaste());

} else if(fruit instanceof Apple) {

Apple apple = (Apple) fruit; System.out.print(apple.getTaste());

}

}

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instanceof Operation (3)

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CB: OK. It seems to me that I know them all, but I want to ask you, Smart Rabbit, whether the 3rd year students in NPIC who are watching us understand it or not.

SR: This is a hard question. I’m not sure either. You’ve better ask them by yourself.

Remember (1)• Casting is a technique to convert a variable from one

class type to another one within an inheritance hierarchy.

That is, if we cast a class to another class in different root, you will get run-time error. E.g:

Fruit fruit = new Grape();Grape grape = (Grape) fruit;// OKApple apple = (Apple) fruit;//Runtime Error

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Remember (2)• A keyword instanceof is an operation to check whether a

class that we want to cast is in the correct hierarchy or not.

• E.g.:Fruit fruit = new Grape();

if(fruit instanceof Grape){

Grape grape = (Grape) fruit;

} else if(fruit instanceof Apple){

Apple apple = (Apple) fruit;

}

fruit.getColor();

fruit.getTaste();

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