java programming - inheritance

Chapter 8 Inheritance and Polymorphism

Oum Saokosal, Chief of Computer ScienceNational Polytechnic Institute of Cambodia

Tel: (855)-12-252-752E-mail: oum_saokosal@yahoo.com

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InheritanceChapter 8 Inheritance and Polymorphism

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Inheritance1. 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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1. What is Inheritance? (2)

• 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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1. What is Inheritance? (3)

• 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? 8

1. What is Inheritance? (5) - IDE

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1. What is Inheritance? (6)

• 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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3. How to use it? – Note (2)

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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3. How to use it? – Note (3)

public class TestCylinder {

public static void main(String[] args){

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

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Circle Cylinder

4. Superclass & Subclass (2)

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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4. Superclass & Subclass (4)

• Sample of using JFrameimport 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

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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 superclasssuper(); //call no-arg constructor

super(5.0); //call arg constructor

• Note

super():1. MUST be written in the 1st line of subclass constructors2. 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 superclasssuper.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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Overriding Methods (2)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 class2. Modify the existing features. It is called Overriding Methods.

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Overriding Methods (3)• 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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Overriding Methods (3)• 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 {

public static void main(String[] args){

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){

}

}36

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

1 error

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@24efe3

Note: We have to override it to display our wise.

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