Oop Concepts & Abstract Class ,Inheritance

8/6/2019 Oop Concepts & Abstract Class ,Inheritance

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Day2

Java Essentials


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Primitive Data Types in Java

Java Programming language is strongly typed All variables must be first declared before they can be used

Byte: 8 bit signed integer. Values minimum (-128) tomax 127(inclusive)

Short: 16 bit signed integer. Values min- (-32,768)to 32767(inclusive)Int: 32 bit signed integer. Values minimum- (-2,147,483,648 ) to (2,147,483,647 )Long:64 bit signed integer

Float: 32 bit(3.4e-038 to 3.4e+038)Double:64 bitChar:16 bit Unicode character Boolean can only take true or false


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Scope

Local Scope: Variable will not be available outside itsscopepublic static void main(String args[])

{ int x;if(x==10){ int y = 20;

System.out.println(x is + x+y is + y);

}System.out.println(y is +y);

//error }


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Scope Error

public static void main(String args[]){int bar =1;{

int bar =2;

}

}


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Type Casting

Automatic Type Conversions Two types are compatible Destination type is larger than source type This type of conversion is called widening

conversion

Casting Incompatible Typesint a;byte b;b = (byte) a;


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Type Casting

Truncation Conversionint i;double d = 1.23;i = (integer)d;

Automatic Type Promotion

byte b = 5 0;b = b*2;


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Classes and objects

A class creates a new data type that can beused to create objects. Act as a blueprintA class is a logical constructObject is nothing but an instance of the classAn object has physical reality i.e. it occupiesspace in memory


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Declaring Objects

Class-var = new classname();Classname followed by parentheses specifiesthe constructor for the classA constructor defines what occurs when anobject of a class is created


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Introducing methods

Although it is perfectly fine to create a class that onlycontains data, rarely happensMost of the time methods to access the instancevariables defined by classWhat if another part of the program want volume?

Type name (parameter-list){body of method

}Type specifies the type of data returned. Can be any valid

type.return value;


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Method that takes parameter

Adding a Method That Takes Parametersint square(){Return 10*10

}

Int square(int i){return i*I;}

Int x = square( 5 );


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Parameter is a variable defined by a methodthat receives a value when method is called

An argument is a value that is passed to amethod when it is invoked


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Can use a parameterized method to improve Box classIn previous example

mybox1.width=10; etc

Code is clumsy and error proneNever a good idea to access variables directlyVoid setDim(double w, double h, double d){Width = w; height = h; depth = d;

mybox1.setDim(10, 20, 5 );Vol=mybox1.volume();


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Constructors

It can be tedious to initialize all of the variablesin a class each time an instance is createdEven setDim is not rightA constructor initializes an object immediatelyupon creationIt has the same name as the class in which it

resides and is syntactically similar to methodOnce defined, constructor is automaticallycalled immediately after the object is created,before new operator completes


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Constructors

Constructors do not have any return type, noteven voidImplicit return type of a class constructor isthe class type itself Creating an instance will have fully initializedusable object immediately

Rework Box example so that dimensions areautomatically initializedReplace setDim with a constructor


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Constructors

Box2(){System.out.println("Constructing Box");width = 10;height = 10;depth = 10;}


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Parameterized Constructors

In the previous example it is not very useful

So add parameters to constructor

Box(double w, double h, double d){Width=w; height=h;

}


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The this keyword

Sometimes the method will need to refer to theobject that invoked itTo allow this java, java defines the thiskeywordthis can be used inside any method to refer to the current objectIs always a reference to the object on whichmethod was invoked


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Instance Variable Hiding

Illegal in java to declare to declare two localvariables with same name inside the same or enclosing space

Box(double width, double height, doubledepth){this.width=width;this.height=height;this.depth=depth}


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Overloading methods

In java it is possible to define two or moremethods within the same class that share thesame name as long as different parameter

declarationThis is called method overloadingOne of the ways java implementspolymorphism


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Overloading methods

One way java implements polymorphism i.e. oneinterface, multiple methodsIn languages that do not support method overloading,each method has unique nameIn C, abs() absolute value of integer labs() absolute value of a long integer fabs() absolute value of a floating point value

In java standard, abs method is overloaded to handleall numeric typesLeft to the compiler to chose right version


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Argument Passing

Call-by-value: Copies value of an argument into formalparameters. Therefore changes made to parameter of the subroutine have no effects on the argument usedto call.

Call-by-reference: A reference to argument, not valueis passed. Changes made to parameter will changeargument

When a simple type is passed , it is done by use of callby value. Objects are passed by use of call-by-reference


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Inheritance

Very important component of object-orientedprogramming as allows hierarchicalclassifications

Each class can use things that are unique to itAnimal (Age, sex, weight) Mammal (Gestationperiod, etc) Canine (Hunting Skills, tail length)

Dogs (trained, indoor/outdoor) Labrador


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Inheritance

Enables you to create more specific classesEnables software reuseParent class is called superclassChild class is called subclass


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Inheritance Example

Class A{ int i; int j;

showij(){System.out.println(I and j:+ i+ + j);}

}


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Inheritance Example

Class B extends A{int k;void showk();

{ System.out.println(k);}

void sum(){

system.out.println(i+j+k);}

}


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Inheritance Example

Class SimpleInheritance{Public static void main(String args[]}A superObj = new A();B subObj = new B();

superObj.i = 10;superObj.j = 20;

superObj.showij();subObj.i = 7; subObj.i = 8; subObj.k = 9 ;subObj.showij(); subObj.showk();


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Inheritance

We can use a super class directlyUnlike C, Java does not support theinheritance of multiple classes into a single

sub classAlthough a subclass includes all of themembers of its super class, cannot accessprivate members


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Cannot Access Private Data

Class A{ int i;

private int j;setij(int x, int y){i=x; j=y;

}

}


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Error Accessing Private Data

Class B extends A{ int total;

void sum(){ total = i+j}

}


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Super class variable can Refrencesub class variableWhen a reference to a subclass object is

assigned to a superclass reference variable,only access to those parts which are defined in

superclass

Boxweight weightbox = new Boxweight(3, 5 ,7, 9 );Box plainbox = new Box();.plainbox = weightbox;


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Super class variable can Refrencesub class variable{ Car x;

Ford escort;x = escort;

// but cannot access ford specific features cozthe parent class does not know about them


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Using super

Classes derived from box were notimplemented as efficiently and robustly.Duplicate in superclass, inefficient

Also subclass must be granted access tothese membersBut usually superclass keeps details of itsimplementations to itself i.e. encapsulationJava provides a solution to this with keywordsuper


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Using super

A subclass can call a constructor methoddefined by its superclass by use of followingform

super(parameter-list);Parameter list specifies any parameters needed

by the constructor in superclassSuper() must always be the first statement inside

subclass constructor


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Using Super

Class BoxWeight extends Box{double weight;BoxWeight(double w, double h, double d,double m);super(w,h,d);weight = m;

}


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Second use of super

super.member


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Using Super Class A{

int i;}Class B extends A{

int i;B(int a, int b){ super.i = a; i=b;}

void show(){System.out.println(I in superclass + super.i);System.out.println( I in subclass: + i);}

Class useSuper{public static void main(String args[])B obj1 = new B(1, 2);

obj1.show();}


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When Constructors are Called

Class A{A(){ System.out.println( inside As const);}

Class B extends A{B(){ System.out.println( inside Bs const);}

}

Class C extends B{C(){ System.out.println( inside Cs const);}

}


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Class callConst{ C obj = new C();}

OutputInside As Constructor

Inside Bs Constructor Inside Cs Constructor


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Method Overriding

When method in a subclass has same nameand type signature as method in its superclassthen method in subclass is said to override

method in superclass


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Dynamic Method Despatch

Class A{callme(){ System.out.println( inside As callme);}

Class B extends A{callme(){ System.out.println( inside Bs call me);}

}Class C extends B{

callme(){ System.out.println( inside Cs callme);}

}


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Class Dispatch{ public static void main(String args[])

A a = new A();

B b = new B();C c = new C();A r ; r = a;r.callme(); r = b;

r.callme(); r =c;r.callme();

}


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Abstract Classes

Super class that declares the structure of agiven abstraction without providing completeimplementation

Super class that defines a generalized formand leave specific implementation tosubclassesAll methods which needs to be declared aredeclared as abstractAny class which contains abstract methods isdefined as abstract class


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Abstract Classes

abstract class A{ abstract void callme();

void callmetoo()

{ System.out.println(This is a concrete method);}class B extends A{B b = new B();

void callme(){ System.out.println(Bs implementation of callme);}

}


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Using final

Keyword final has three usesCreate equivalent of a named constantWhen used with method name, it prevents themethod from being overridenWhen used with a class name it prevents theclass from being inherited.


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Passing Objects as Parameters

Both Common and right to pass objects toclassesAlready defined classes as well as custom

classes can be sent as parameters


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Argument Passing

Two Ways through which a computer Language can pass Arguments

Call-by-value Method Copies the value of an

argument into formal parameters of subroutine Call-by-reference Reference to an argument (not

values of the arguments) is passed to theparameter. Reference used to access actualarguments


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The Object Class

Object Clone()boolean equals(Object obj)Void finalize()Class getClass()String toString()Void notify

Void notifyAllVoid wait()


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Lab3

Oop Concepts & Abstract Class ,Inheritance

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