Lecture 2 classes i

1. Introduction to Computer Science II COSC 1320/6305 Lecture2:Defining Classes I (Chapter 4)

Introduce toclassesandobjectsinJava .

Understand how some of the OO concepts learnt so far are supported in Java.

Understand important features in Javaclasses .

Java is a true OO language and therefore the underlying structure of all Java programs isclasses .

Anything we wish to represent in Java must be encapsulated in aclassthat defines the state and behaviour of the basic program components known asobjects .

Classescreateobjectsandobjectsusemethodsto communicate between them. They provide a convenient method forpackaging a group of logically related data itemsandfunctions that work on them .

Aclassessentiallyserves as a templatefor anobjectand behaves like a basic data type int. It is therefore important to understand how thefieldsandmethodsare defined in a class and how they are used to build a Java program that incorporates the basic OO concepts such asencapsulation ,inheritance , andpolymorphism .

Aclass is a collection offields(data) andmethods (procedure or function) that operate on that data.

Aclass is a collection offields (data) andmethods (procedure or function) that operate on that data.

The basic syntax for aclassdefinition:

Bare bone class no fields ,no methods

class ClassName[ extendsSuperClassName ]

[ fields declaration ]

[ methods declaration ]

Addfields

Thefields(data) are also called theinstancevariables .

Aclasswith only data fields has nolife .Objectscreated by such a classcannot respond to any messages .

Methodsare declared inside the body of the class but immediately after the declaration ofdata fields .

The general form of amethod declarationis:

Declare theCircle class , have created a new data type Data Abstraction

Can define variables ( objects ) of that type:

CircleaCircle ;

CirclebCircle ;

aCircle ,bCirclesimply refers to a Circleobject ,not an object itself .

CircleaCircle ;

CirclebCircle ;

Objectsare created dynamically using thenewkeyword.

aCircleandbCirclerefer toCircle objects

Theobjectdoesnot have a referenceand cannot be used in future.

Theobjectbecomes a candidate for automaticgarbage collection .

Java automatically collects garbage periodicallyand releases the memory used to be used inthe future.

Similar toCsyntax for accessing data defined in a structure.

UsingObjectMethods :

// Circle.java:Contains both Circle class and its user class

//Add Circle class code here

classMyMain

public staticvoid main(String args[])

CircleaCircle ;// creating reference

aCircle= newCircle() ;// creating object

aCircle . x = 10;// assigning value to data field

aCircle . y = 20;

aCircle . r = 5;

double area =aCircle . area() ; // invoking method

double circumf =aCircle . circumference ();

System.out.println("Radius="+ aCircle . r+" Area="+area);

System.out.println("Radius="+ aCircle .r+" Circumference ="+circumf);

Classes,objects , andmethodsare the basic components used in Java programming.

We have discussed:

How to define aclass

How to createobjects

How toadd data fieldsandmethodsto classes

How toaccess data fieldsandmethodsto classes

Classesare the most important language feature that makeobject-oriented programming( OOP ) possible

Programming inJavaconsists of defining a number ofclasses

Every program is aclass

All helping softwareconsists of classes

All programmer-defined types areclasses

Classesare central toJava

You already know how to useclassesand theobjectscreated from them, and how toinvoke their methods

For example, you have already been using the predefinedStringandScanner classes

Now you will learn how to defineyour own classesand theirmethods , and how to createyour own objectsfrom them

Aclassis a special kind of programmer-defined type, and variables can be declared of aclasstype

A value of aclasstype is called anobjectoran instance oftheclass

If A is aclass , then the phrases " blais of type A," " blais anobjectof theclassA," and " blaisan instance oftheclassA" mean the same thing

Aclassdetermines the types ofdatathat anobjectcan contain, as well as theactionsit can perform

Reminder:aJava filemust be given the same name as theclassit contains with an added.javaat the end

For example, aclassnamedMyClassmust be in a file namedMyClass.java

For now, your program and all theclassesit uses should be in the same directory or folder

AJavaapplication programor "regular"Java programis aclasswith amethod namedmain

When aJava application programis run, therun-time systemautomatically invokes themethodnamedmain

AllJava application programsstart with themain method

Java programswork by having things calledobjectsperformactions

System.out :anobjectused forsending outputto the screen

Theactionsperformed by anobjectare calledmethods

println :themethodoractionthat theSystem.out objectperforms

Invokingorcallingamethod :When anobjectperforms anactionusing amethod

Also calledsending a message to theobject

Methodinvocation syntax (in order):anobject , a dot (period) , themethodname, and apair of parentheses

Arguments :Zero or more pieces of information needed by themethodthat are placed inside the parentheses

Variabledeclarations inJavaare similar to those in other programming languages (C)

Simply give thetypeof thevariablefollowed by its name and a semicolon;

intanswer ;

InJava , the equal sign ( = ) is used as theassignment operator

Thev ariableon the left side of the assignment operator=isassigned the valueof the expression on the right side of the assignment operator=

answer=2+2;

InJava , the plus sign ( + ) can be used to denote addition (as above) orconcatenation

Using+ ,two strings can be connected together

A primitive type value is a single piece ofdata

Aclasstype value orobjectcan have multiple pieces ofdata , as well as actions calledmethods

Allobjectsof aclasshave the samemethods

Allobjectsof aclasshave thesame pieces of data(i.e., name, type, and number)

For a givenobject ,each piece of datacan hold a different value

Anobjectof aclassis named or declared by avariableof theclasstype:

ClassNameclassVar ;

Thenewoperator must then be used to create theobjectand associate it with itsvariablename:

classVar=newClassName();

These can be combined as follows:

ClassNameclassVar=newClassName();

Aclassdefinition specifies thedata itemsandmethodsthat all of itsobjectswill have

Thesedata itemsandmethodsare sometimes calledmembersof theobject

Data itemsare calledfieldsorinstance variables

Instance variable declarationsandmethod definitionscan be placed in any order within theclassdefinition

Instance variablescan be defined as in the following two examples

Note thepublicmodifier (for now):

publicString instanceVar1 ;

public int instanceVar2 ;

In order to refer to a particularinstance variable , preface it with itsobjectname as follows:

objectName . instanceVar1

objectName . instanceVar2

Methoddefinitions are divided into two parts:aheadingand abody :

public voidmyMethod()Heading

codeto perform some actionBody

and/or compute a value

Methodsare invoked using the name of the callingobjectand themethodname as follows:

classVar . myMethod() ;

Invoking amethodis equivalent to executing themethodbody

There are two kinds ofmethods :

Methodsthat compute and return a value

Methodsthat perform an action

This type ofmethod does not return a value, and is called avoid method

Each type ofmethoddiffers slightly in how it is defined as well as how it is (usually) invoked

Amethodthat returns a value must specify thetype of that valuein its heading:

publictypeReturnedmethodName (paramList)

Avoid methoduses the keywordvoidin its heading to show that it does not return a value :

public void methodName (paramList)

Aprogram in Javais just a class that has amain method

When you give a command to run aJava program , the run-time system invokes themethod main

Note thatmainis avoid method , as indicated by its heading:

public static voidmain (String[] args)

The body of both types ofmethodscontains a list of declarations and statements enclosed in a pair of braces

public< voidortypeReturned >myMethod ()

declarationsBody

statements

The body of amethodthat returns a value must also contain one or morereturnstatements

Areturnstatement specifies the value returned and ends themethodinvocation:

return Expression;

Expressioncan be any expression that evaluates to something of the type returned listed in themethodheading

Avoid methodneed not contain areturnstatement, unless there is a situation that requires the method to end before all its code is executed

In this context, since it does not return a value, areturnstatement is used without an expression:

return;

An invocation of amethodthat returns a value can be used as an expression anyplace that a value of thetypeReturnedcan be used:

typeReturned tRVariable;

tRVariable= objectName . methodName() ;

An invocation of avoid methodis simply a statement:

objectName . methodName() ;

Amethodthat returns a value can also perform an action

If you want the action performed, but do not need the returned value, you can invoke themethodas if it were avoid method , and the returned value will be discarded:

objectName . returnedValueMethod() ;

A variable declared within amethoddefinition is called alocal variable

All variables declared in themain methodarelocal variables

Allmethodparameters arelocal variables

If twomethodseach have alocal variableof the same name, they are stilltwo entirely different variables

Some programming languages include another kind of variable called aglobalvariable

TheJava languagedoesnothave global variables

Ablockis another name for a compound statement, that is, a set of Java statements enclosed in braces, {}

Avariabledeclared within a block islocalto that block, and cannot be used outside the block

Once a variable has been declared within a block, its name cannot be used for anything else within the samemethoddefinition

You candeclare one or more variableswithin the initialization portion of aforstatement

Avariableso declared will be local to theforloop, and cannot be used outside of the loop

If you need to use such a variable outside of a loop, then declare it outside the loop

Themethodsseen so far have had no parameters, indicated by an empty set of parentheses in themethodheading

Somemethodsneed to receive additional data via a list ofparametersin order to perform their work

Theseparametersare also calledformal parameters

Aparameter listprovides a description of the data required by amethod

It indicates the number and types of data pieces needed, the order in which they must be given, and the local name for these pieces as used in themethod

publicdoublemyMethod (int p1, int p2, double p3)

When amethodis invoked, the appropriate values must be passed to themethodin the form ofarguments

Arguments are also calledactual parameters

The number and order of the arguments must exactly match that of the parameter list

The type of each argument must be compatible with the type of the corresponding parameter

int a=1,b=2,c=3;

double result= myMethod(a,b,c) ;

In the preceding example,the value of each argument(not the variable name) is plugged into the correspondingmethodparameter

This method of plugging in arguments for formal parameters is known as thecall-by-value mechanism

If argument and parameter types do not match exactly,Java will attempt to make an automatic type conversion

In the preceding example, theintvalue of argumentcwould becastto adouble

A primitive argument can be automaticallytype castfrom any of the following types, to any of the types that appear to its right:

byte short int long float double

A parameter is often thought of as a blank orplaceholderthat is filled in by the value of its corresponding argument

However, a parameter is more than that:it is actually alocal variable

When amethodis invoked, the value of its argument is computed, and the corresponding parameter (i.e.,local variable ) is initialized to this value

Even if the value of aformal parameteris changed within amethod(i.e., it is used as alocal variable ) the value of the argument cannot be changed

Do not be surprised to find that people often use the termsparameterandargumentinterchangeably

When you see these terms, you may have to determine their exact meaning from context

Allinstance variablesare understood to have

< the calling object > .in front of them

If an explicit name for thecalling objectis needed, the keywordthiscan be used

myInstanceVariable always means and is always interchangeable withthis . myInstanceVariable

this mustbe used if aparameteror otherlocal variablewith the same name is used in themethod

Otherwise, allinstances of the variable namewill be interpreted aslocal

int someVariable =this . someVariable

Thethis parameter is a kind ofhidden parameter

Even though it does not appear on theparameter listof amethod , it is still aparameter

When amethodis invoked, thecalling objectis automatically plugged in forthis

An invocation of amethodthat returns a value of typebooleanreturns eithertrueorfalse

Therefore, it is common practice to use an invocation of such amethodto control statements and loops where a Boolean expression is expected

if-elsestatements,whileloops, etc.

Java expects certainmethods , such asequals andtoString , to be in all, or almost all,classes

The purpose ofequals , abooleanvaluedmethod , is to compare twoobjectsof theclassto see if they satisfy the notion of "being equal

Note:You cannot use==to compareobjects

publicbooleanequals (ClassNameobjectName )

The purpose of thetoString methodis to return aStringvalue that represents the data in theobject

publicStringtoString()

Eachmethodshould be tested in aprogramin which it is the only untested program

A program whose only purpose is to testa methodis calledadriver program

Onemethodoften invokes othermethods , so one way to do this is to first test all the methods invoked by thatmethod , and then test themethoditself

This iscalledbottom-up testing

Sometimes it is necessary to test amethodbefore anothermethodit depends on is finished or tested

In this case, use a simplified version of themethod , called astub ,to return a value for testing

Everymethodshould be tested in a program in which every othermethodin the testing program has already been fully tested and debugged

Information hiding is the practice of separating how to use aclassfrom the details of its implementation

Abstractionis another term used to express the concept of discarding details in order to avoid information overload

Encapsulation means that thedataandmethodsof a class are combined into a single unit (i.e., a classobject ), which hides the implementation details

Knowing the details is unnecessary because interaction with theobjectoccurs via a well-defined and simpleinterface

InJava , hiding details is done by marking themprivate

The API ora pplicationp rogrammingi nterfacefor aclassis a description of how to use theclass

A programmer need only read theAPI in order to use a well designedclass

AnADTorabstract data type is a data type that is written using good information-hiding techniques

The modifierpublicmeans that there are no restrictions on where aninstance variableormethodcan be used

The modifierprivatemeans that aninstance variableormethodcannot be accessed by name outside of theclass

It is considered good programming practice to makeall instance variablesprivate

Mostmethodsarepublic , and thus providecontrolled access to theobject

Usually,methodsareprivateonly if used as helpingmethodsfor othermethodsin the class

Accessor methodsallow the programmer to obtain the value of anobject 'sinstance variables

Thedatacan be accessed but not changed

The name of an accessormethodtypically starts with the wordget

Mutator methodsallow the programmer to change the value of anobject 'sinstance variablesin a controlled manner

Incomingdatais typically tested and/or filtered

The name of a mutatormethodtypically starts with the wordset

Within the definition of aclass ,privatemembers ofany objectof theclasscan be accessed, not justprivatemembers of the callingobject

Thepreconditionof amethodstates what is assumed to be true when themethodis called

Thepostconditionof amethodstates what will be true after themethodis executed, as long as the precondition holds

It is a good practice to always think in terms of preconditions and postconditions when designing amethod , and when writing themethodcomment

Overloadingis when two or moremethods in the sameclasshave the samemethodname

To be valid, any two definitions of themethodname must have differentsignatures

A signatureconsists of the name of amethodtogether with its parameter list

Differing signaturesmust have different numbers and/or types of parameters

IfJavacannot find amethod signaturethat exactly matches amethodinvocation, it will try to use automatic type conversion

The interaction ofoverloadingand automatic type conversion can have unintended results

In some cases ofoverloading , because of automatic type conversion, a singlemethodinvocation can be resolved in multiple ways

Ambiguousmethodinvocations will produce an error in Java

The signatureof amethodonly includes themethodname and its parameter types

The signaturedoesnotinclude the type returned

Javadoes not permitmethodswith the same name and different return typesin the sameclass

Although many programming languages, such asC++ , allow you tooverload operators( + ,- , etc.),Javadoes not permit this

You may only use amethodname and ordinarymethodsyntax to carry out the operations you desire

Aconstructoris a special kind ofmethodthat is designed to initialize theinstance variablesfor anobject :

publicClassName (anyParameters)

Aconstructormust have the same name as theclass

Aconstructorhas no type returned, not evenvoid

Constructorsare typically overloaded

Aconstructoris called when anobjectof the class is created usingnew

ClassNameobjectName=new ClassName (anyArgs);

Thename of the constructorand its parenthesized list of arguments (if any) must follow thenewoperator

This is theonlyvalid way to invoke aconstructor :a constructorcannot beinvoked like anordinary method

If aconstructoris invoked again (usingnew ), thefirst objectis discarded and an entirelynew objectis created

If you need tochange the values of instance variablesof theobject , usemutator methodsinstead

The first action taken by aconstructoris to create anobjectwithinstance variables

Therefore, it is legal to invokeanother methodwithin the definition of aconstructor , since it has the newly createdobjectas itscalling object

For example,mutator methodscan be used to set the values of theinstance variables

It is even possible forone constructorto invokeanother

Like any ordinarymethod , everyconstructorhas athis parameter

Thethisparameter can be used explicitly, but is more often understood to be there than written down

The first action taken by aconstructoris to automatically create anobjectwithinstance variables

Then within the definition of aconstructor , thethisparameter refers to theobjectcreated by theconstructor

If youdo not include any constructorsin yourclass ,Javawill automatically create adefaultorno-argumentconstructorthat takes no arguments, performs no initializations, but allows theobjectto be created

If you include even one constructorin yourclass ,Java will not providethisdefault constructor

If you includeany constructorsin yourclass , be sure to provide yourown no-argument constructoras well

Instance variablesare automatically initialized inJava

booleantypes are initialized to false

Other primitives are initialized to the zero of their type

Classtypes are initialized tonull

However, it is a better practice to explicitly initializeinstance variablesin aconstructor

Note:Local variablesare not automatically initialized

TheStringTokenizer classis used torecover the wordsor tokensin a multi-wordString

You can use whitespace characters to separate each token, or you can specify the characters you wish to use as separators

In order to use theStringTokenizer class , be sure to include the following at the start of the file:

importjava.util.StringTokenizer;

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Lecture 2 classes i