Java Programming - Inheritance

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
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Inheritance
Chapter 8 Inheritance and Polymorphism
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Inheritance
1. What is Inheritance?
2. Why Inheritance?
3. How to use it?
4. Superclass & Subclass
5. Using keyword super
6. Overriding Methods
7. The Object class
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1. What is Inheritance? (1)
• OOP has 3 features:
1. Class Encapsulation
2. Inheritance
3. 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”);
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• 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
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• 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.
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1. What is Inheritance? (4) - Example
1. Please create a blank class, say, BlankSample
public class BlankSample {
}
1. 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? 8

1. What is Inheritance? (5) - IDE
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• 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()
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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
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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.
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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){
}
public double findArea(){
return radius * radius *3.14;
}
}
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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.
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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
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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;
}
}
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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.
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4. Superclass & Subclass (1)
• The cylinder class inherits features from
circle class. Then,
– Cylinder is subclass
– Circle is superclass
Super inherit Subclass
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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.
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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.
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• Sample of using JFrame
import javax.swing.*;
public class TestJFrame extends JFrame {
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
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5. Using keyword super (1)
• super is used to call:
1. Constructors of the superclass
2. Methods of the superclass
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Using keyword super (2)
• To call constructors of the superclass
super(); //call no-arg constructor
super(5.0); //call arg constructor
• Note
super():
1. MUST be written in the 1st
line of subclass constructors
2. Cannot be written in other methods
3. Is the only way to call superclass constructor.
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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.
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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:
1. Add new features
2. Modify existing features.
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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.
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• 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.
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• Please look at the code on page 292, Listing 8.2.
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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.
2. 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)”.
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Important Note (2)
//Circle class
public Circle(){ // non-arg constructor
radius = 5;
}
return radius * radius * 3.14;
}
}
//TestCircle class
public class TestCircle extends Circle {
TestCircle tc = new TestCircle();
System.out.println(tc.findArea());//output: 78.5
}
}
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Important Note (3)
//Circle class
//It doesn’t have non-arg constructor Here
public Circle(double radius){
}
return radius * radius * 3.14;
}
}
//TestCircle class
public class TestCircle extends Circle {
}
}
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cannot find symbol
symbol: constructor Circle()
location: class Circle
1 error

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.”
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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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Java Programming - Inheritance

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