Java Programming - Polymorphism

CHAPTER 8 INHERITANCE AND
POLYMORPHISM
Oum Saokosal, Chief of Computer Science
National Polytechnic Institute of Cambodia
Tel: (855)-12-252-752
E-mail: oum_saokosal@yahoo.com
1

Polymorphism
1. What is polymorphism?
2. Why polymorphism?
3. Specific uses of polymorphism
4. Notes
5. Remember
6. Casting
7. instanceof Operations
8. Remember
3

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.
4

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";
}
}
5
Student
+getClassName():String
UndergraduateStudent
GraduateStudent

What is Polymorphism? Example (3)
//Continue ...
public class GraduateStudent extends Student{
@Override public String getClassName(){
return "GraduateStudent Class";
}
}
public class UndergraduateStudent extends Student{
return “UndergraduateStudent Class";
}
}
6

What is Polymorphism? Test 1 (4)
//Test1 1 – Polymorphism
public class TestPoly1{
public static void main(String[] args){
Student s = new UndergraduateStudent();
System.out.println(s.getClassName());
}
}
Q. What is output?
A. UnderGraduateStudent Class
7
That’s polymorphism

//Example 2 – Without Polymorphism
public class TestNoPoly2 {
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());
}
}
8

//Example 2 – With Polymorphism
public class TestPoly2 {
display(new GraduateStudent());
display(new UndergraduateStudent());
}
//Polymorphic method
public static void display(Student stu){
System.out.println(stu.getClassName());
}
}
9

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();
10
Student
getClassName():String
GraduateStudent
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.
11

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
12

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();
13

Specific usages of Polymorphism
Array (4)
public class TestPoly3 {
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());
}
}
}
14

Note #1 (1)
• Look at the codes again.
}
}
}
}
public class UndergraduateStudent extends Student{
return “UndergraduateStudent Class";
}
}
15
GraduateStudent and
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.
Student s;
s = new GraduateStudent();
s = new UndergraduateStudent();
}
}
16

Note #2 (1)
• But now in a subclass GraduateStudent, we add a
new method, say getDegree().
}
public String getDegree(){
return "Msc in Computer Science";
}
}
17

Note #2 (2)
• And then we want to use polymorphism to call
getDegree() in the subclass,
Student s = new GraduateStudent();
System.out.println(s.getDegree());
}
}
It will show a compile error because Student class
doesn’t have getDegree().
18

Note #3 (1)
• What about if we want add a method, say getID() to
the parent, Student.
}
public String getID(){
return “NPIC001";
}
}
19

Note #3 (2)
• Then we want to call getID().
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.
20

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();
21
Student
GraduateStudent

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.
22

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.
• You can also recognise it, when a class type is Object
class. E.g.
private void aMethod(Object ob){...}
23

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.
24

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.
25

Casting Objects (2)
• To understand casting, let’s meet a group of animals
studying polymorphism.
26
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.
27

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.
28

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
29

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:
Grape grape = (Grape) fruit; // Casting
grape.getTaste(); //OK
grape.getVitamin(); //OK
30

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
31

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!
32

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 {
showTaste(new Grape());
}
public static void showTaste(Fruit fruit){
JOptionPane.showMessageDialog(null,
fruit.getTaste());
} //Error because Fruit has not getTaste()
}
33

Casting Objects (9)
SR: Here is codes when using casting.
public class TestCastingObject {
showTaste(new Grape());
}
Grape grape = (Grape) fruit;
grape.getTaste()); //OK
}
}
34

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.
35

Casting Objects (11)
SR: Then I pass in Apple object to showTaste() method.
public class TestCastingObject {
showTaste(new Apple());
}
grape.getTaste());
}
}//Runtime Error: casting wrong class
37

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.
38

public static void main(String[] args) {
showTaste(new Apple());
}
public static void showTaste(Fruit fruit) {
if (fruit instanceof Grape) {
System.out.print(grape.getTaste());
} else if(fruit instanceof Apple) {
Apple apple = (Apple) fruit;
System.out.print(apple.getTaste());
}
}
39

40
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:
Grape grape = (Grape) fruit;// OK
Apple apple = (Apple) fruit;//Runtime Error
41

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.:
if(fruit instanceof Grape){
} else if(fruit instanceof Apple){
Apple apple = (Apple) fruit;
}
fruit.getColor();
fruit.getTaste();
42

Java Programming - Polymorphism

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