Creating Your Own Classes Sxi

8/11/2019 Creating Your Own Classes Sxi

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Introduction to Programming 1 1

10 Creating your ownClasses


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Topics Defining your own classes Declaring Fields

Instance Variables Class Variables or Static Variables

Declaring Methods Multiple return statements

The this reference Overloading Methods Declaring Constructors The this ! Constructor Call


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Topics "ac#ages

Importing "ac#ages Creating your own pac#ages

$ccess Modifiers default% public% private% protected!


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Defining your own classes Sample Class to create&

contains information of a Student and operations needed for acertain student record

To create class&'( Thin# on where you will be using your class and how your class will be used()( Thin# of an appropriate name for the class*( +ist all the information or properties that you want your class to contain,( +ist down the methods that you will be using for your class(


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Defining your own classes -ow% to define our class we write%

where%public . means that our class is accessible to other classes outside the

pac#ageclass . this is the #eyword used to create a class in /avaStudent0ecord . a uni1ue identifier that describes our class

public class StudentRecord {

//we'll add more code here later

}


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Defining your own classes:

Coding Guidelines'( Thin# of an appropriate name for your class( Don2t 3ust callyour class 456 or any random names you can thin# of(

)( Class names should start with a C$"IT$+ letter(

*( The filename of your class should have the S$M7 -$M7 asyour class name(


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Declaring Fields +et us now write down a list of information that a student

record can contain( For each information% also list what datatypes would be appropriate to use(

name . Stringaddress . Stringage . intmath grade . doubleenglish grade . doublescience grade . doubleaverage grade . double


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Declaring Fields:Instance Variables

-ow that we have a list of all the properties we want to addto our class% let us now add them to our code(

where%private . means that the variables are only accessible

within the class( Other ob3ects cannot access these variables directly( 8e will cover more

about accessibility later(

public class StudentRecord { private String name; private String address;

private int age; private double mathGrade; private double englishGrade; private double scienceGrade; private double average; //we'll add more code here later

}


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Declaring Fields:

Coding Guidelines'( Declare all your instance variables on the top of the classdeclaration(

)( Declare one variable for each line(

*( Instance variables% li#e any other variables should start witha SM$++ letter(

,( 9se an appropriate data type for each variable you declare(

:( Declare instance variables as private so that only class

methods can access them directly(


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Declaring Fields:

Static Variables $side from instance variables% we can also declare classvariables or variables that belong to the class as a whole(

The value of these variables are the same for all the ob3ects

of the same class( To declare a static variable%

public class StudentRecord {//instance variables we have declaredprivate static int studentCount;//we'll add more code here later

}

we use the #eyword static to indicate that a variable is astatic variable(


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Declaring Fields So far% our whole code now loo#s li#e this(

public class StudentRecord { private String name; private String address; private int age; private double mathGrade; private double englishGrade; private double scienceGrade; private double average; private static int studentCount; //we'll add more code here later

}


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Declaring Met ods +et2s now list down the set of functionalities that we want our

class to have(

8e want to access information about the student li#e whatthe student2s name is% what2s his;her address and otherinformation(


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Declaring Met ods:

!ccessor " Mutator Met ods $ccessor Methods Methods used to read values from our class variables(

Mutator Methods Methods used to write or change values of our class variables(

-OT7& 5ou can restrict access to a class variable by not

creating an accessor method for it(


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Declaring Met ods:

!ccessor " Mutator Met ods To summari=e% we have the list of accessor and mutatormethods and what it can do%

name . readwrite address . read% writeenglish grade . read% writemath grade . read% writescience grade . read% writeaverage grade . compute;read

studentCount . read


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Declaring Met ods:

!ccessor " Mutator Met ods $n accessor method usually starts with aget>-ameOfInstanceVariable?

$ mutator method usually starts with a

set>-ameOfInstanceVariable?


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Declaring Met ods:

!ccessor " Mutator Met ods So% given the previous list% we can have the following namesfor our accessor and mutator methods%

name . get-ame% set-amewrite address . get$ddress% set$ddressenglish grade . get7nglish@rade% set7nglish@rademath grade . getMath@rade% setMath@radescience grade . getScience@rade% setScience@radeaverage grade . get$verage@rade

studentCount . getStudentCount


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Declaring Met ods:

!ccessor " Mutator Met ods One sample implementation of a get ! method%

where%

public . means that the method can be called from ob3ects outside the classString . is the return type of the method( This means that the method shouldreturn a value of type Stringget-ame . the name of the method

! . this means that our method does not have any parameters

public class StudentRecord {private String name;

::

public String getName(){ return name;

}}


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Declaring Met ods:

!ccessor " Mutator Met ods The statement%return name;

in our program signifies that it will return the value of theinstance variable name to the calling method(

-OT7& The return type of the method should have the samedata type as the data in the return statement(


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Declaring Met ods:

Multiple return state#ents 5ou can have multiple return statements for a method aslong as they are not on the same bloc#(

5ou can also use constants to return values instead ofvariables(


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Declaring Met ods:

Multiple return state#ents 7Aample&public String getNumberInWords( int num ){

String defaultNum = "zero";

if( num == 1 ){return "one"; //return a constant}else if( num == 2){

return "two"; //return a constant}

//return a variablereturn defaultNum;

}


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Declaring Met ods:

!ccessor Met ods One sample implementation of a set ! method%

where%public . means that the method can be called from ob3ects outside the classvoid . means that the method does not return any valueset-ame . the name of the method

String temp! . parameter that will be used inside our method

public class StudentRecord {private String name;

:

:public void setName( String temp ){

name = temp;}

}


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Declaring Met ods:

!ccessor Met ods The statement%name = temp;

assigns the value of temp to name and thus changes thedata inside the instance variable name(

-OT7& Set methods don2t return values(


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Declaring Met ods:

Coding Guidelines'( Method names should start with a SM$++ letter(

)( Method names should be verbs

*( $lways provide documentation before the declaration of themethod( 5ou can use 3avadocs style for this(

D l i M d


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Declaring Met ods:!ccessor Met ods

For the static variable studentCount% we can create a staticmethod to access its value(

where%public . means that the method can be called from ob3ects outside the class

static . means that the method is static and should be called by typing% >Class-ame?(>method-ame?(int . is the return type of the method( This means that the method should return a value of type intgetStudentCount .the name of the method

! . this means that our method does not have any parameters

public class StudentRecord { :

private static int studentCount;:

:public static int getStudentCount(){

return studentCount;}

}


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Code for Student$ecord

Class1 public class StudentRecord2 {3 private String name;4 private String address;5 private int age;6 private double mathGrade;

7 private double englishGrade;8 private double scienceGrade;9 private double average;10 private static int studentCount;1112 /**13 * Returns the name of the student14 */15 public String getName(){16 return name;17 }


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Class18 /**19 * Changes the name of the student20 */21 public void setName( String temp ){22 name = temp;23 }

2425 // other code here ....2627 /**28 * Computes the average of the english, math and

science

29 * grades30 */31 public double getAverage(){32 double result = 0;33 result = ( mathGrade+englishGrade+scienceGrade )/3;34 return result;

35 }


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class36 /**37 * returns the number of instances of StudentRecords38 */39 public static int getStudentCount(){40 return studentCount;41 }42 }


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T e %t is& reference The this reference is used to access the instance variables

shadowed by the parameters(

To understand this better% let2s ta#e for eAample the set$gemethod( Suppose we have the following declaration forset$ge(

The parameter name in this declaration is age % which hasthe same name as the instance variable age ( Since theparameter age is the closest declaration to the method% thevalue of the parameter age will be used(

public void setAge( int age ){age = age; //WRONG!!!

}


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T e %t is& reference In order to correct this mista#e% we use the Bthis reference(

To use the this reference% we type%

this.[nameOfTheInstanceVariable]

So for example, we can now rewrite our code to,public void setAge( int age ){

this.age = age;}


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T e %t is& reference -OT7& 5ou can only use the this reference for instance

variables and -OT static or class variables (


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'(erloading Met ods Method overloading

allows a method with the same name but different parameters% tohave different implementations and return values of different types(

0ather than invent new names all the time% method overloading canbe used when the same operation has different implementations(

( l d d


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'(erloading Met ods:Sa#ple )rogra#1 /**

2 * overloaded method print() - one parameter3 */4 public void print( String temp )5 {6 System.out.println("Name:" + name);7 System.out.println("Address:" + address);8 System.out.println("Age:" + age);9 }1011 /**12 * overloaded method print() - three parameters13 */

14 public void print(double eGrade,double mGrade,double sGrade)15 {16 System.out.println("Name:" + name);17 System.out.println("Math Grade:" + mGrade);18 System.out.println("English Grade:" + eGrade);19 System.out.println("Science Grade:" + sGrade);

20 }


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'(erloading Met ods:

Sa#ple )rogra# -ow calling those methods in main !1 public static void main( String[] args ) {2 StudentRecord annaRecord = new StudentRecord();3 annaRecord.setName("Anna");4 annaRecord.setAddress("Philippines");5 annaRecord.setAge(15);6 annaRecord.setMathGrade(80);7 annaRecord.setEnglishGrade(95.5);8 annaRecord.setScienceGrade(100);9 //overloaded methods10 annaRecord.print( annaRecord.getName() );11 annaRecord.print( annaRecord.getEnglishGrade(),12 annaRecord.getMathGrade(),13 annaRecord.getScienceGrade());14 }


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'(erloading Met ods:

Sa#ple )rogra# 'utput For print call in line ' % the output is%

For print call in line ''% the output is%

Name:AnnaAddress:PhilippinesAge:15

Name:AnnaMath Grade:80.0English Grade:95.5Science Grade:100.0


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'(erloading Met ods $lways remember that overloaded methods have the

following properties& the same name different parameters return types can be different or the same


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Declaring Constructors Constructors are important in instantiating an ob3ect( It is a

method where all the initiali=ations are placed(

"roperties of a constructor&'( Constructors have the same name as the class

)( $ constructor is 3ust li#e an ordinary method% however only thefollowing information can be placed in the header of the constructor%scope or accessibility identifier li#e public(((!% constructor2s nameand parameters if it has any(

*( Constructors does not have any return value

,( 5ou cannot call a constructor directly % it can only be called by usingthe new operator during class instantiation(


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Declaring Constructors:

T e default constructor 7very class has a default constructor( The default constructor is the constructor without any

parameters(

If the class does not specify any constructors% then animplicit default constructor is created(

7Aample&public StudentRecord() { //some code here

}


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Declaring Constructors:

Constructor o(erloading Since constructors are 3ust li#e methods% they can also beoverloaded(

7Aample&1 public StudentRecord(){2 //some initialization code here3 }45 public StudentRecord(String temp){6 this.name = temp;

7 }89 public StudentRecord(String name, String address){10 this.name = name;11 this.address = address;12 }


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Declaring Constructors:Constructor o(erloading

To use these constructors% we have&1 public static void main( String[] args )2 {3 //create three objects for Student record4 StudentRecord annaRecord=new StudentRecord("Anna");5 StudentRecord beahRecord=new StudentRecord("Beah",6 "Philippines");7 StudentRecord crisRecord=new StudentRecord ();8 }


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Declaring Constructors:T e %t is*+& constructor call

Constructor calls can be chained% meaning% you can callanother constructor from inside another constructor(

8e use the this ! call for this(

There are a few things to remember when using the thisconstructor call&

'( 8hen using the this constructor call% IT M9ST OCC90 $S T


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Declaring Constructors:T e %t is& constructor call

1 public StudentRecord(){2 this("some string");3 }45 public StudentRecord(String temp){6 this.name = temp;7 }89 public static void main( String[] args ){10 StudentRecord annaRecord = new StudentRecord();11 }


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)ac,ages "ac#ages are /ava s means of grouping related classes and

interfaces together in a single unit interfaces will bediscussed later!(

This powerful feature provides for a convenient mechanismfor managing a large group of classes and interfaces whileavoiding potential naming conflicts(

)


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)ac,ages:I#porting )ac,ages

To be able to use classes outside of the pac#age you arecurrently wor#ing in% you need to import the pac#age ofthose classes(

Ey default% all your /ava programs import the 3ava(lang(Gpac#age% that is why you can use classes li#e String andIntegers inside the program even though you haven2timported any pac#ages(

The syntaA for importing pac#ages is as follows%

import nameOfPackage;

)


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)ac,ages:I#porting )ac,ages

7Aample&

The first statement imports the specific class Color while theother imports all of the classes in the 3ava(awt pac#age(

import java.awt.Color;import java.awt.*;

)


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)ac,ages:Creating your own pac,ages

Suppose we want to create a pac#age where we will placeour Student0ecord class% together with other relatedclasses( 8e will call our pac#age% SchoolClasses(

Steps&'( Create a folder named SchoolClasses(

)( Copy all the classes that you want to belong to this pac#age insidethis folder(

*( $dd the following code at the top of the class file(package SchoolClasses;

public class StudentRecord{. . . .

)


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)ac,ages:Creating your own pac,ages

"ac#ages can also be nested(

In this case% the /ava interpreter eApects the directorystructure containing the eAecutable classes to match thepac#age hierarchy(


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!ccess Modifiers 8hen creating our classes and defining the properties and

methods in our class% we want to implement some #ind ofrestriction to access these data(

In /ava% we have what we call access modifiers in order toimplement this(

Four different types of member access modifiers in /ava& public private protected default(

!ccess Modifiers:


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!ccess Modifiers:default access

default access $lso called pac#age accessibility This specifies that only classes in the same pac#age can have

access to the class2 variables and methods(

There are no actual #eyword for the default modifierH it is applied inthe absence of an access modifier(

7Aample&public class StudentRecord {

//default access to instance variableint name;//default access to methodString getName(){

return name;}

}

!ccess Modifiers:


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!ccess Modifiers:public access

This specifies that class members are accessible to anyone%both inside and outside the class(

$ny ob3ect that interacts with the class can have access tothe public members of the class(

7Aample&

public class StudentRecord {//public access to instance variablepublic int name;

//public access to methodpublic String getName(){

return name;}

}

!ccess Modifiers:


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!ccess Modifiers:protected access

This specifies that the class members are accessible only tomethods in that class and the subclasses of the class(

7Aample&public class StudentRecord {

//protected access to instance variableprotected int name;

//protected access to methodprotected String getName(){

return name;}}


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!ccess Modifiers:


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!ccess Modifiers:Coding Guidelines

The instance variables of a class should normally bedeclared private% and the class will 3ust provide accessormethods which are declared as public! to these variables(


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Su##ary Defining your own classes Declaring Fields Instance Variables% Class Variables or

Static Variables! Declaring Methods 0eturning values and Multiple return statements The this reference Overloading Methods Declaring Constructors The this ! Constructor Call "ac#ages $ccess Modifiers default% public% private% protected!

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Creating Your Own Classes Sxi