7. JAVA InheritanceAndInterfaces

7/30/2019 7. JAVA InheritanceAndInterfaces

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Inheritance and Interfaces

One of the most important concepts of OOP


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Module Introduction

Inheritance

Constructor Inheritance

Overriding Methods

Overloading of methods

abstract class

Using final keyword

Interfaces

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#1 - Inheritance

Explain the concept of inheritance

State the purpose of method overriding

State the use of a super keyword.

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What is Inheritance?

The process, whereby characteristics and behavior are transmittedfrom a parent to a child entity, is called inheritance.

Create a new class from the existing class.reuse the fields and methods

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Basic concepts of Inheritance

SuperclassThe class from which the subclass is derived is called a

Base class or parent class.

Subclass

Class that is derived from another classDerived class, extended class, or child class

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Inheritance basic concepts cont

Use extends keyword to create a subclass.

A class can be directly derived from only one class (singleinheritance)

If a class does not have any superclass, then it is implicitly derivedfrom Object class.Object class is parent of all Java classes

A subclass can inherit all the protected members of itssuperclass.

Constructors are not inherited by subclassesThe constructor of the superclass can be invoked from the subclass.

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Example of Inheritance

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super keyword

Use to access superclasss members and constructors fromsubclass

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#2 - Constructor Inheritance

Describe Constructor Inheritance

Constructor Chaining

Rules for Constructors

Explicitly invoke the base class constructor

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In Java, cannot inherit constructors like inherit methods.

The instance of the derived class will always first invoke theconstructor of the base class followed by the constructor of the

derived class.

Can explicitly invoke the base class constructor by using thesuper keyword in the derived class constructor declaration

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Parent

F1

F2

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The instance of the derived class will always first invoke theconstructor of the base class followed by the constructor of thederived class

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Rules for Constructors (MUST be clear and rem em ber)

A default constructor will be automatically generated by thecompiler if no constructor is specified within the class.

The default constructor is ALWAYS a no-arg constructor.

If there is a constructor defined in the class, the defaultconstructor is no longer used.

If you dont explicitly call a base class constructor in a derivedclass constructor, the compiler attempts to silently insert a callto the base classs default constructor before executing thecode in the derived class constructor

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Compiler-Generated Constructor Code

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Can yo u see any b ug ?

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How to fixed?

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Explicitly invoke the base class constructor

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#3 - Overriding Methods

Method Signatures

Define method overriding.

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Method Signatures

Method has a signature comprises of:The number of parametersThe data types of parametersThe order in which the parameters are written.

The return type of a method is not a part of its signature

The signature of the method is written in parentheses next tothe method name.

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Overriding Methods

Subclass define new method with the same signature as thesuperclass method.

Overridden method cannot have a weaker access specifier thanthe access specifier of the method it overrides.

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Example of Overriding Methods

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super keyword

Use to access superclasss members and constructors fromsubclass

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Example of Using super keyword

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#4 Overloading Methods

Describe method overloading

State and explain the this keyword.

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Overloading Methods

Declaring more than one method with the same method namebut different signatures

Constructor overloading

Allows multiple ways of creating instances

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Example of Overloading Methods

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"this " keyword

Refer to the current object of the class.

Used to resolve conflicts

between variables having samenames and to pass the currentobject as a parameter

Cannot use the this keyword

with static variables andmethods,

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#5 - Abstract Classes

Define abstract classes

Describe how to implement abstract classes

Define abstract methods

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abstract Classes

To serve as a framework that provides certain behavior for other classes.

Contain zero or more abstract methods

Cannot be instantiated.

Can be inherited.

The subclass must implement abstract methods declared in baseclass

Otherwise it must be declared as abstract.

Declare an abstract class by using the keyword abstract precede class keyword

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abstract Methods

Method has only declaration and no implementationIs prefixed with the abstract keyword. The declaration does not contain any braces and is terminated by asemicolon.

An abstract method is only a contract that the subclass will provideits implementation

If a class includes abstract methods, the class itself must bedeclared abstract

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Example of abstract Classes and abstract Methods

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#6 - Final Variables, Methods, Classes

Final Variables

Final Methods

Final Classes

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Final Variables

final keyword is used with variables to indicate that they areconstant identifiers.

Constant variables are assigned a value at the time of

declaration and will not change anytime later.

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Final Methods

To prevent a method from being overridden or hidden in a Javasubclass.

If a change in implementation of method effects the consistentstate of the object

Method that are declared private or part of the final class areimplicitly final.

The final method cannot declared as abstract.

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Final Classes

A class that cannot be subclassed.

May or may not have final methods.

Final classes can be instantiated

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#7 - Interfaces

Introduction to Interfaces

Implementing Multiple Interfaces

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What is an Interface?

An interface is defined as a reference type

Has only final variables, abstract methods signatures.Interface Methods Do Not Contain Method Bodies

Cannot be instantiated.

Can only be inherited by classes or other interfaces.

A class that implements an interface is requi red to provide imp lementat ions for a l l the method s of the interface or elseshould be declared abstract.

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Example of Interface

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Implementing Multiple Interfaces

An interface can extend one or more interfaces

Multiple interfaces can be implemented in a single class.

This implementation provides the functionality of multipleinheritance.

Implement multiple interfaces by placing commas between theinterface names when implementing them in a class.

A class must implement all inherited interface methods

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Example of Implementing Multiple Interfaces

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Thats about all for today!

Inheritance


Overriding Methods

Overloading of methods

abstract class

Using final keyword

Interfaces

Thank you al l for yo ur a t tent ion and pat ient !

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7. JAVA InheritanceAndInterfaces