chapter 8 inheritance

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Chapter 8 Inheritance and Polymorphism

Oum Saokosal, Head of IT DepartmentNational Polytechnic Institute of Cambodia

Tel: (855)-12-417214E-mail: [email protected]

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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 super6. Overriding Methods

7. The Object class

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1. What is Inheritance?

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1. What is Inheritance? (1)

• OOP has 3 features:1. Class Encapsulation 2. Inheritance3. 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, BlankSamplepublic class BlankSample {

}

2. Then create a test class, say, TestBlankpublic 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?

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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?

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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?

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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){ this.radius = 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{ public static void main(String[] args){

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

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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

Circle Cylinder

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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 JFrameimport 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

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5. Using keyword super (1)

• super is used to call:

1. Constructors of the superclass2. 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 methods3.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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6. 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 features2. 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 classpublic class Circle{

private double radius;

public Circle(){ // non-arg constructor

radius = 5; }

public double findArea(){

return radius * radius * 3.14;

}

}

//TestCircle classpublic class TestCircle extends Circle {


TestCircle tc = new TestCircle();

System.out.println(tc.findArea());//output: 78.5

}

}

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Important Note (3)//Circle classpublic class Circle{ private double radius;

//It doesn’t have non-arg constructor Here public Circle(double radius){

this.radius = radius; } public double findArea(){ return radius * radius * 3.14; }}

//TestCircle classpublic class TestCircle extends Circle {

public static void main(String[] args){}

}

cannot find symbolsymbol: constructor Circle()location: class Circle

1 error

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The Object class

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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@24efe3Note: We have to override it to display our wise.

chapter 8 inheritance

Technology

public class circle

circle class

public class testcylinder

test class

blank class

inheritance chapter

public circledouble

public doublefindarea