Java OOP Programming language (Part 5) - Inheritance

Java Programming –
Inheritance
Oum Saokosal
Master’s Degree in information systems,Jeonju
University,South Korea
012 252 752 / 070 252 752
oumsaokosal@gmail.com

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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
3

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”);
5

•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.
6

• 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.
7

1. What is Inheritance? (4) - Example
1. Please createa blankclass,say, BlankSample
public class BlankSample {
}
2. 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 iswhy we can call bs.toString()?
If we look at BlankSample,there istoString(). Why? 8

1.What is Inheritance? (5) - IDE
9

• 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()
10

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
12

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 differencesbetween the superclass
and the subclass that inherits from it.
• Make class more flexible.
13

3. How to use it? (1)
• In Java, to enable a class inherit an existing class, we have to usea 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;
}
}
15

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.
16

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 alreadythe findArea()
• So the formulato findCylinder’sVolumeis :
volume =Area * length
17

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;
}
} 18

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.
19

4. Superclass & Subclass (1)
•The cylinder class inherits features
from circle class.Then,
• Cylinder is subclass
• Circle is superclass
Super inherit Subclass
21
Circle Cylinder

Quick Check:
C1 <- C2 <- C3 <- C4
What are superclass and subclass?
- C1 is thesuperclass of C2,C3, &C4
- C2 are the subclass ofC1 and thesuperclass ofC3 &C4
- C3 are the subclassof C1 &C2 andthe superclass ofC4
- C4 is the subclass ofC1,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.
22

4. Superclass & Subclass (3) – Java API
• Please checkAPIDocumentation:
Javax.swing.JFrame is the subclassof
Frame,Window,Container,Component,Object.
So if we use JFrame, it meanswe use features from all of
the superclasses.
23

• 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 24

5. Using keyword super (1)
super is used to call:
1. Constructors of the superclass
2.Methods of the superclass
26

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 callsuperclassconstructor.
27

Using keyword super (3)
• To call methods of the superclass
super.setRadius(5); // setRadius(5);
super.findArea();
super.toString();
Note:
• This keyword isnot alwaysused to call methodsfrom superclass.
• We can call superclassmethodsby calling directly the methods
name.Please look at slide # 14.
• However, super isused not to confuse with the name of the
overriding methods.
28

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.
30

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.
31

• 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 callfindArea(), it is always
the Circle’s.
- But the cylindercan have findArea() for itself.
This implementationis calledoverridingmethod.
32

•Please look at the code on page 292,
Listing 8.2.
33

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 classinheritsJFrame, then we can call
setTitle(), setSize(), setVisible() etc.
2.In a constructor of subclass, the non-arg
constructor of the superclass is ALWAYS
invoked.
3.A subclass can NEVER inherit a superclass
which has no non-arg constructor. Let see slide
“Important Note (3)”.
34

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
}
} 35

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 {
}
}
36
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.”
38

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.
39

Java OOP Programming language (Part 5) - Inheritance

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