Chapter 8 Inheritance

Chapter 8 Inheritance and Polymorphism Oum Saokosal , Head of IT Department National Polytechnic Institute of Cambodia Tel: (855)-12-417214 E-mail: oum_saokosal@yahoo.com

Inheritance Chapter 8 Inheritance and Polymorphism

Inheritance
What is Inheritance?
Why Inheritance?
How to use it?
Superclass & Subclass
Using keyword super
Overriding Methods
The Object class

1. What is Inheritance?

1. What is Inheritance? (1)
OOP has 3 features:
Class Encapsulation
Inheritance
Polymorphism
OOP allows you to derive (create) new objects from existing classes. E.g.
You can create objects from a class:
Circle cir = new Circle();
Word w = new Word(“N P I C”);

But OOP has other mechanisms. One of them is called Inheritance .
Inheritance is a mechanism to make classes inherit properties/methods from an existing class.
Inherit (v) ¬TTYlekrþtMENl¦
Inheritance (n) receiving properties

In fact, every class in Java is always inherited from an existing class, either explicitly or implicitly.
In Java, every class is inherited from java.lang.Object .
To be clear, please look at an example at next slide.

1. What is Inheritance? (4) - Example
Please create a blank class, say, BlankSample
public class BlankSample {
}
Then create a test class, say, TestBlank
public class TestBlank {
public static void main(String[] args){
BlankSample bs = new BlankSample();
System.out.print( bs.toString() );
}
}
The question is why we can call bs.toString()?
If we look at BlankSample, there is toString(). Why?

1. What is Inheritance? (5) - IDE

Where these methods come from?
They are from java.lang.Object . Because every class in Java inherits from java.lang.Object .
To be sure, please look at the API and find out java.lang.Object. Then see its methods.
clone(), equals(Object obj), finalize(), getClass(), hashCode(), notify(),
notifyAll(), toString() and wait()

2. Why Inheritance?

2. Why Inheritance?
Classes often share capabilities
We want to avoid re-coding these capabilities
Reuse of these would be best to
Improve maintainability
Reduce cost
Improve “real world” modeling

2. Why Inheritance? -Benefits
No need to reinvent the wheel.
Allows us to build on existing codes without having to copy it and past it or rewrite it again, etc.
To create the subclass, we need to program only the differences between the superclass and the subclass that inherits from it.
Make class more flexible.

3. How to use it?

3. How to use it? (1)
In Java, to enable a class inherit an existing class, we have to use a keyword “ extends ”. For example, we have Circle class:
public class Circle{
private double radius;
public Circle(){}
public Circle(double radius){
this.radius = radius;
}
public void setRadius (double radius){
this.radius = radius;
}
public double findArea (){
return radius * radius *3.14;
}
}

3. How to use it? (2)
Then we want another class, say, TestCircle , inherits from the Circle class.
public class TestCircle extends Circle{
TestCircle tc1 = new TestCircle ();
tc1. setRadius (5.0);
System.out.println(tc1. findArea ());
}
}
Please note that TestCircle didn’t define setRadius() and getArea() methods but it could use the methods.
The reason is TestCircle inherits from Circle class.

3. How to use it? – Note (1)
Usually inheritance is used to improve features of an existing class.
Please look at the code on page 288, listing 8.1 First Version of the Cylinder class.
The Circle has already the findArea()
So the formula to find Cylinder’s Volume is :
volume = Area * length

public class Cylinder extends Circle {
private double length = 1;
public double getLength(){
return length;
}
public void setLength(double length){
this.length = length;
}
public double findVolume(){
return findArea() * length;
}
}

public class TestCylinder {
Cylinder c1 = new Cylinder();
c1. setRadius (2.5); // from Circle
c1.setLength(5); // from Cylinder
System.out.println(c1.findVolume());
}
}
Please note that the cylinder’s object, c1, could call a method, “ setLength() ”, from Cylinder class and also could call a method, “ setRadius() ”, from Circle class.

4. Superclass & Subclass

4. Superclass & Subclass (1)
The cylinder class inherits features from circle class. Then,
Cylinder is subclass
Circle is superclass
Super inherit Subclass
Circle Cylinder

Quick Check:
C1 <- C2 <- C3 <- C4
What are superclass and subclass?
C1 is the superclass of C2, C3, & C4
C2 are the subclass of C1 and the superclass of C3 & C4
C3 are the subclass of C1 & C2 and the superclass of C4
C4 is the subclass of C1, C2 & C3
It means if we call the final subclass, e.g. C4, then we can use features from C1, C2, C3, and, of course, C4 itself.

4 . Superclass & Subclass (3) – Java API
Please check API Documentation: Javax.swing.JFrame is the subclass of Frame,Window,Container,Component,Object . So if we use JFrame, it means we use features from all of the superclasses.

Sample of using JFrame
import javax.swing.*;
public class TestJFrame extends JFrame {
public static void main( String [] args){
TestJFrame frame = new TestJFrame();
frame .setTitle ( "Hi I am JFrame" );
frame .setSize (400,300);
frame .setVisible ( true );
frame .setDefaultCloseOperation (
JFrame.EXIT_ON_CLOSE);
}
} // Note the underline codes

5. Using keyword super

5. Using keyword super (1)
super is used to call:
Constructors of the superclass
Methods of the superclass

Using keyword super (2)
To call constructors of the superclass
super(); //call no-arg constructor
super(5.0); //call arg constructor
Note
super() :
MUST be written in the 1 st line of subclass constructors
Cannot be written in other methods
Is the only way to call superclass constructor.

Using keyword super (3)
To call methods of the superclass
super.setRadius(5); // setRadius(5);
super.findArea();
super.toString();
Note:
This keyword is not always used to call methods from superclass.
We can call superclass methods by calling directly the methods name. Please look at slide # 14.
However, super is used not to confuse with the name of the overriding methods.

6. Overriding Methods

Overriding Methods (1)
In the real world:
Researchers sometimes never invent or find a new thing. In fact, they just improve an existing thing.
To improve the thing, they just:
Add new features
Modify existing features.

In OOP :
It is true to the both things above. The inheritance helps us to do these. We can:
1. Add new methods to existing class
2. Modify the existing features. It is called Overriding Methods .

Overriding method is a technique to modify a method in the superclass.
Overriding method is a method, defined in subclass, which has the same name and return type to a method in superclass.
For example :
- The Circle has findArea() but Cylinder doesn’t has it. If we call findArea() , it is always the Circle ’s.
- But the cylinder can have findArea() for itself. This implementation is called overriding method.

Please look at the code on page 292, Listing 8.2.

Important Note (1)
1. In the subclass, we can invoke accessible things , e.g. public methods or constructor , from the superclass. E.g.:
- After a class inherits JFrame, then we can call setTitle(), setSize(), setVisible() etc.
In a constructor of subclass, the non-arg constructor of the superclass is ALWAYS invoked. Let see slide “Important Note (2)”.
3. A subclass can NEVER inherit a superclass which has no non-arg constructor . Let see slide “Important Note (3)”.

Important Note (2)
// Circle class
public Circle(){ // non-arg constructor
radius = 5;
}
public double findArea(){
return radius * radius * 3.14;
}
}
//Test Circle class
public class TestCircle extends Circle {
TestCircle tc = new TestCircle();
System .out.println(tc.findArea()); //output: 78.5
}
}

Important Note (3)
// Circle class
//It doesn’t have non-arg constructor Here
public Circle(double radius){
this. radius = radius;
}
public double findArea(){
return radius * radius * 3.14 ;
}
}
//Test Circle class
public class TestCircle extends Circle {
}
}
cannot find symbol symbol: constructor Circle() location: class Circle 1 error

The Object class

The Object class (1)
public boolean equals(Object object)
Indicates whether a object is "equal to" this one. E.g.:
Circle c1 = new Circle();
if(c1.equals(c1)){
}
Note: We have to override it to test our comparison.
public int hashCode()
Returns a hash code value for the object. see “Java Collection Framework.”

The Object class (2)
public String toString()
Return a string that represents the object. e.g.
Circle c1 = new Circle();
c1.toString();
//output: Circle@24efe3
Note: We have to override it to display our wise.

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Chapter 8 Inheritance

by OUM SAOKOSAL on Sep 28, 2009

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