Java Technology: Classes And Inheritance

Java Technology Classes and Inheritance Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

Session Plan 

The basic constructs in Java 

To introduce Classes, Objects and Methods



To introduce Inheritance



Implementing Inheritance in Java



To introduce Polymorphism



Structuring program by types of polymorphism

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Classes & Objects in Java 

Class 

A class defines a real world or abstract entity.



A class defines both the behavior and attributes of a group of objects with similar characteristics



Object 

An object is an instance or variable of a class. It is said to belong to the class.



An object can be distinguished from other objects by its attributes.

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Example : Objects and Classes object

class class Student char name int rollNo setName()

Jodie

Daria

Jane

Brittany

R001

R002

R003

R004

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setRollNo() calcMarks()

Constituents of a Class public class myClass Data Members { (State) private int a; myClass() { Constructor a=0; } public static void main(String args[]) { System.out.println(“Hello”); } } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Startup Method

Creating Objects in Java 

The New Keyword



E.g. :

Default Constructor

myClass myClassObject= new myClass(); Or myClass myNextClassObject= new myClass(5); User Defined Constructor Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Access Specifiers 

Access Control modifiers



Four Access Control Modifiers in Java 

public



private



protected



package [default access specifier]

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Access Specifiers 

Access specifiers could be used on classes in Java



All classes belong to packages in Java



“public” types are the only ones accessible outside the package



private and protected specifier are invalid for classes

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Access Specifiers Private

Default Specifier

Protected

Public

Same Class

Yes

Yes

Yes

Yes

Same Package Sub Class

No

Yes

Yes

Yes

Same Package NonSub Class

No

Yes

Yes

Yes

Different Package Sub Class

No

No

Yes

Yes

Different Package Non-Sub Class

No

No

No

Yes

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Modifiers in Java 

Access Specifiers – Access Control modifiers



static



final



abstract



synchronized

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Static 

“static” -> the members can be used before the object is created. Can be accessed without reference to any object



For variable: 



Global Variables (for all objects)

For methods: 

Can call only static methods



Can refer to only static variables

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Final 

“final” modifier has a meaning based on its usage



For variable: 

Primitives: read-only



Objects: reference is read-only



use all upper case letters



For methods: no overriding



For classes: no inheritance

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Abstract 

“abstract” modifier is used to defer an operation



Cannot be used for variables



For methods: no implementation



For classes: no instantiation

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Abstract – Rules to follow 

The following cannot be marked with “abstract” modifier 

Constructors



Static methods



Private methods



Methods marked with “final” modifier

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Methods in Java 

Methods are written within a class



Override a method



Overload a method



Parameter passing 

By Value



By reference

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Nested Classes & Inner Classes 

A class written within another class is known as nested class



The scope of the nested class is bounded by the scope of its enclosing class.



A nested class has access to the members, including private members, of the class in which it is nested.



The enclosing class does not have access to the members of the nested class Overload a method

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Inheritance in Java 

Inheritance in Java is achieved using

Parent

‘extends’ keyword. Multiple Inheritance is not allowed. E.g. : Class parent {………}

Child

Class child extends parent

Inheritance in Java (Single Inheritance)

{………} Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Inheritance in Java GrandParen t

Account

Parent

Child Multilevel Inheritance Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

Savings A/C

Current A/C

Hierarchical Inheritance

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Inheritance in Java Paternal

Matern al

Student

Test Child

Final Result

Multiple Inheritance (Not Allowed) Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

Sports

Hybrid Inheritance (Not Allowed) 19

Inheritance in Java GrandParen t

Paternal

Maternal

Parent Child

Child

Not Allowed in Java

Allowed in Java Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Inheritance in Java Following are the code for the diagram in the slide shown before : Class Parent extends GrandParent {

Class Child extends Paternal, Maternal {

}

}

Class Child extends Parent { }

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The code written above is not acceptable by Java 21

The super keyword 

Super is a java keyword that allows a method to access/refer the hidden variables and overriden methods of the super class.

Example:





Super.aNumber,



Super.getNumber()

The method super() is used to invoke the constructor of super class.

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Abstract class: 

The classes without complete implementation.



The keyword used is abstract



Abstract classes cannot be instantiated.



Abstract classes contain zero or more abstract methods, which are later implemented by concrete classes.

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Abstract class Example <> Vehicle Abstract drive()

Car Drive(){}

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Bus Drive(){}

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Polymorphism 

Polymorphism means, behaving differently at different time depending on the situation



Types 

Method Overloading (Compile time/static Polymorphism)



Method Overriding (Run time/dynamic polymorphism)

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Method Overloading 

Java allows to have more than one method in the class to have same name, but different signature.



Signature includes method name and parameter types.



Example: 

int add (int a, int b);



float add (float a, float b);

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Constructor Overloading public class myMath { private int x; myMath(){ x=0; } myMath(int i) { x=i; } int add ( int a, int b){ return (a+b); } }

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Data Members (State)

Constructor

One more Constructor (Overloading)

Method (Behavior)

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Method Overriding 

A method defined in the base class can be overriden in the derived class.



This will change the behaviour in the derived class.



The signature should be same in both classes.

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Rules of overriding 

A subclass CANNOT override methods that are declared final



A subclass MUST override methods that are declared abstract

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Overloading vs Overriding 

Overloading means the method name is same but the signature is different. Applicable only within a class.



Overriding means the name and signature are same but behaviour may be different. Applicable in class hierarchies.

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Summary 

Constituents of a Class, Creating Objects in Java



Modifiers in Java (Access Control Modifiers+ Others)



Inheritance and its types in Java



Abstract Class



Polymorphism in Java

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