java built in classes

8/13/2019 Java Built in Classes

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1

Working with JavaWorking with Java

ClassesClasses



2

Objectives

At the end of the lesson, the student should be able to:

● Explain object-oriented programming and some of itsconcepts

● Differentiate between classes and objects

● Differentiate between instance variables/methods andclass(static variables/methods

● Explain what methods are and how to call and pass

parameters to methods

● !dentif" the scope of a variable

● #ast primitive data t"pes and objects

●

#ompare objects and determine the class of an objects



3

Brief IntroductionBrief Introductionon OOPon OOP



4

Introduction to OOP

● $bject-$riented programming or $$%

– &evolves around the concept of objects as the basic elementsof "our programs'

– hese objects are characteri)ed b" their properties and

behaviors'



5

Introduction to OOP

● Example of objects

objects in the ph"sical world can easil" be modeled assoftware objects using the properties as data and thebehaviors as methods



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Encapsulation

● Encapsulation

– he scheme of hiding implementation details of a class'

– he caller of the class does not need to *now the

implementation details of a class – he implementation can change without affecting the caller of

the class



7

Classes andClasses and

Objects (ObjectObjects (ObjectInstances)Instances)



8

Classes and Objects

● #lass

– can be thought of as a template, a protot"pe or a blueprint of anobject

– is the fundamental structure in object-oriented programming

● wo t"pes of class members:

– +ields (properties, variables

● specif" the data t"pes defined b" the class

– ethods (behavior

● specif" the operations



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Classes and Objects

● $bject

– An object is an instance of a class - we will call it object instance

– he propert" values of an object instance is different from theones of other object instances of a same class

– $bject instances of a same class share the same behavior(methods



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Classes and Objects

● o differentiate between classes and objects, let usdiscuss an example:



11

Classes and Objects

● #lasses provide the benefit of reusabilit"'

● oftware programmers can use a class over and over

again to create man" object instances'



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Creation of ObjectCreation of Object

Instances withInstances with“new ke!word“new ke!word



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Creation of Object Instance

● o create an object instance of a class, we use the new operator'

● +or example, if "ou want to create an instance of theclass tring, we write the following code,

String str2 = new String(“Hello world!”);or also e.uivalent to,

String str2 = "Hello";

● tring class is a special (and onl" class "ou can create aninstance without using new *e"word as shown above



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Creation of Object Instance

● he new operator

– allocates a memor" for that object and returns a reference ofthat memor" location to "ou'

–

hen "ou create an object, "ou actuall" invo*e the class0constructor'

● he constructor

– is a method where "ou place all the initiali)ations, it has the

same name as the class'



15

"ethods"ethods

(Instance #ethods $(Instance #ethods $%tatic #ethods)%tatic #ethods)



17

Why Use Methods?

● ethods contain behavior of a class (business logic

– he heart of effective problem solving is in problemdecomposition'

–

e can do this in 1ava b" creating methods to solve a specificpart of the problem'

– a*ing a problem and brea*ing it into small, manageablepieces is critical to writing large programs'



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Two Types of Methods

● !nstance (non-static methods

– hould be called after object instance is created

– ore common than static methods

● tatic methods – hould be called in the form of 2#lass3ame4'2method3ame4



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Callin Instance Methods

● 6et0s ta*e two sample methods found in the tring class



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Callin Instance Methods

● Example

String str1 = "Hello";

char x = str1.charAt(0); //will return the character H

//and store it to variable x

String str2 = "hello";

//this will return a boolean value true

boolean result = str1.equalsIgnoreCase( str2 );



22

Callin $tatic Methods

● tatic methods

– methods that can be invo*ed without instantiating a class(means without invo*ing the new *e"word'

– tatic methods belong to the class as a whole and not to a

certain instance (or object of a class' – tatic methods are distinguished from instance methods in a

class definition b" the *e"word static'

● o call a static method, just t"pe,

Classname.staticMethodName(params);



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Callin $tatic Methods

● Examples of static methods, we0ve used so far in ourexamples are,

//prints data to screenSystem.out.println(“Hello world”);

//converts the String 10 to an integerint i ! "nteger.parse"nt(“10”);

//#eturns a String representation o$ the integer argument as an//unsigned integer %ase 1&String he'uivalent ! "nteger.toHe'String( 10 );



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Para#eter PassingPara#eter Passing

(Pass&b!&value $(Pass&b!&value $Pass&b!&reference)Pass&b!&reference)



25

Para%eter Passin

● %ass-b"-7alue

– when a pass-b"-value occurs, the method ma*es a cop" ofthe value of the variable passed to the method' he methodcannot accidentall" modif" the original argument even if it

modifies the parameters during calculations'

– all primitive data t"pes when passed to a method are pass-b"-value'



26

Pass"by"&alue



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Para%eter Passin

● %ass-b"-&eference

– hen a pass-b"-reference occurs, the reference to an objectis passed to the calling method' his means that, the methodma*es a cop" of the reference of the variable passed to the

method'

– 8owever, unli*e in pass-b"-value, the method can modif" theactual object that the reference is pointing to, since, althoughdifferent references are used in the methods, the location ofthe data the" are pointing to is the same'



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Pass"by"'eference



29

Pass"by"'eference



30

'ariables (ields'ariables (ieldsPro*erties)Pro*erties)



31

&ariables

● Data identifiers

● Are used to refer to specific values that are generatedin a program--values that "ou want to *eep around



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Three Types of &ariables● 6ocal (automatic variable

– declared within a method bod"

– visible onl" within a method bod"

– maintained in a stac*

● !nstance variable

– declared inside a class bod" but outside of an" method bodies

– per each object instance

– cannot be referenced from static context

●

#lass (static variable – declared inside a class bod" but outside of an" method bodies

– prepended with the static modifier

– per each class

– shared b" all object instances



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%co*e of a 'ariable%co*e of a 'ariable



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$cope of a &ariable

● he scope

– determines where in the program the variable is accessible'

– determines the lifetime of a variable or how long the variablecan exist in memor"'

– he scope is determined b" where the variable declaration isplaced in the program'

● o simplif" things, just thin* of the scope as an"thingbetween the curl" braces 9'''' he outer curl" bracesare called the outer bloc*s, and the inner curl" bracesare called inner bloc*s'



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$cope of a &ariable

● A variable0s scope is

– inside the bloc* where it is declared, starting from the pointwhere it is declared



36

E(a%ple )



38

E(a%ple *



39

E(a%ple *

● !n the main method, the scopes of the variables are,

– ages24 - scope A

– i in < - scope <

– i in # = scope #

● !n the test method, the scopes of the variables are,

– arr24 - scope D

– i in E - scope E



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$cope of a &ariable

● hen declaring variables, onl" one variable with agiven identifier or name can be declared in a scope'

● hat means that if "ou have the following declaration,

{

int test = 10;

int test = 20;

}

"our compiler will generate an error since "ou shouldhave uni.ue names for "our variables in one bloc*'



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$cope of a &ariable

● 8owever, "ou can have two variables of the samename, if the" are not declared in the same bloc*' +orexample,

int test = 0;System.out.print( test ); // prints 0

//..some code here

if ( x == 2) {

int test = 20;

System.out.print( test );// prints 20}

System.out.print( test ); // prints 0



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$cope of &ariables

● 6ocal (automatic variable

– onl" valid from the line the" are declared on until the closingcurl" brace of the method or code bloc* within which the" aredeclared

– most limited scope

● !nstance variable

– valid as long as the object instance is alive

● #lass (static variable

– in scope from the point the class is loaded into the 17 untilthe the class is unloaded' #lass are loaded into the 17 thefirst time the class is referenced



43

Codin +uidelines

● Avoid having variables of the same name declaredinside one method to avoid confusion'



44

+!*e Casting+!*e Casting



45

Type Castin

● "pe #asting

– apping t"pe of an object to another

● o be discussed

– #asting data with primitive t"pes

– #asting objects



46

Castin Pri%itive Types

● #asting between primitive t"pes enables "ou to convertthe value of one data from one t"pe to another primitivet"pe'

● #ommonl" occurs between numeric t"pes'

● here is one primitive data t"pe that we cannot docasting though, and that is the boolean data t"pe'

● "pes of #asting:

– !mplicit #asting

– Explicit #asting



47

I%plicit Castin

● uppose we want to store a value of int data t"pe to avariable of data t"pe double'

int numInt = 10;

double numDouble = numInt; //implicit cast

!n this example, since the destination variable0s data

t"pe (double holds a larger value than the value0s datat"pe (int, the data is implicitl" casted to the destinationvariable0s data t"pe double'



48

I%plicit Castin

● Another example:

int numInt1 = 1;

int numInt2 = 2;

//result is implicitly casted to type doubledouble numDouble = numInt1/numInt2;



49

E(plicit Castin

● hen we convert a data that has a large t"pe to asmaller t"pe, we must use an explicit cast'

● Explicit casts ta*e the following form:

(Type)value

where,

"pe - is the name of the t"pe "ou0re converting tovalue -is an expression that results in the value of the source t"pe



50

E(plicit Castin E(a%ples

double valDouble = 10.12;

int valInt = (int)valDouble;

//convert valDouble to int type

double x = 10.2;int y = 2;

int result = (int)(x/y); //typecast result of operation to int



51

Castin Objects

● !nstances of classes also can be cast into instances ofother classes, with one restriction: he source anddestination classes must be related b" inheritance> oneclass must be a subclass of the other'

– e0ll cover more about inheritance later'

● #asting objects is analogous to converting a primitivevalue to a larger t"pe, some objects might not need to

be cast explicitl"'



52

Castin Objects

● o cast,

(classname)object where,

classname is the name of the destination class ?

object is a reference to the source object



53

Castin Objects E(a%ple

● he following example casts an instance of the class7ice%resident to an instance of the class Emplo"ee>7ice%resident is a subclass of Emplo"ee with moreinformation, which here defines that the 7ice%resident

has executive washroom privileges'

Employee emp = new Employee();

VicePresident veep = new VicePresident();

// no cast needed for upward use

emp = veep;

// must cast explicitly

veep = (VicePresident)emp;



54

Pri#itives $Pri#itives $Wra**er +!*esWra**er +!*es

Convertin Pri%itive types to



55

Convertin Pri%itive types toObjects and vice versa

● $ne thing "ou can0t do under an" circumstance is castfrom an object to a primitive data t"pe, or vice versa'

● As an alternative, the java'lang pac*age includes

classes that correspond to each primitive data t"pe:+loat, <oolean, <"te, and so on' e call them rapper classes'



56

Wrapper Classes

● ost of these classes have the same names as thedata t"pes, except that the class names begin with acapital letter

– !nteger is a wrapper class of the primitive int

– Double is a wrapper class of the primitive double

– 6ong is a wrapper class of the primitive long

● @sing the classes that correspond to each primitivet"pe, "ou can create an object that holds the same

value'

Convertin Pri%itive types to



57

ypObjects !Wrapper# and vice versa

● he following statement creates an instance of the !nteger classwith the integer value BC

"nteger data*ount ! new "nteger(+,01);

●

he following statement converts an !nteger object to its primitivedata t"pe int' he result is an int with value BC

int new*ount ! data*ount.int-alue();

● A common translation "ou need in programs is converting a tringto a numeric t"pe, such as an int ($bject-primitive

String pennsylvania ! &000;

int penn ! "nteger.parse"nt(pennsylvania);



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Co#*aring ObjectsCo#*aring Objects

C i Obj t



59

Co%parin Objects

● !n our previous discussions, we learned aboutoperators for comparing valuesFe.ual, not e.ual, lessthan, and so on' ost of these operators wor* onl" onprimitive t"pes, not on objects'

● he exceptions to this rule are the operators fore.ualit": GG (e.ual and HG (not e.ual' hen applied toobjects, these operators don0t do what "ou might firstexpect' !nstead of chec*ing whether one object has thesame value as the other object, the" determine whetherboth sides of the operator refer to the same object'

Co%parin Objects



60

Co%parin Objects

● Example:1 class EqualsTest2 {3 public static void main(String[] arguments) {4 String str1, str2;5 str1 = "Free the bound periodicals.";

6 str2 = str1;7 System.out.println("String1: " + str1);8 System.out.println("String2: " + str2);9 System.out.println("Same object? " + (str1 == str2));10 str2 = new String(str1);11 System.out.println("String1: " + str1);12 System.out.println("String2: " + str2);

13 System.out.println("Same object? " + (str1 == str2));14 System.out.println("Same value? " + str1.equals(str2));15 }16 }

C i Obj t



61

Co%parin Objects

● his program0s output is as follows:

String1: Free the bound periodicals.


Same object? true



Same object? false

Same value? True

Co%parin Objects



62

Co%parin Objects

● 3$E on trings: – ;iven the code:

String str1 ! “Hello”;String str ! “Hello”;

– hese two references str and strI will point to the sameobject'

– tring literals are optimi)ed in 1ava> if "ou create a stringusing a literal and then use another literal with the samecharacters, 1ava *nows enough to give "ou the first tring

object bac*'

– <oth strings are the same objects> "ou have to go out of "ourwa" to create two separate objects'



63

getClass() #ethod $getClass() #ethod $instanceof O*erator instanceof O*erator

,eter%inin the class of an object



64

,eter%inin the class of an object

● ant to find out what an object0s class isJ 8ere0s the wa"to do it'

● uppose we have the following object:

SomeClassName key = new SomeClassName();

3ow, we0ll discuss two wa"s to *now the t"pe of theobject pointed to b" the variable key'

etClass!# %ethod



65

etClass!# %ethod

● he get#lass( method returns a #lass object instance(where #lass is itself a class

● #lass class has a method called get3ame('

– get3ame( returns a string representing the name of the class'

● +or Example,

String name = key.getClass().getName();

instanceof operator



66

instanceof operator

● he instanceof has two operands: a reference to anobject on the left and a class name on the right'

● he expression returns true or false based on whether

the object is an instance of the named class or an" ofthat class0s subclasses'

● +or Example,

boolean ex1 = "Texas" instanceof String; // trueObject pt = new Point(10, 10);

boolean ex2 = pt instanceof String; // false



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%u##ar!%u##ar!

$u%%ary

#l d $bj t



68

● #lasses and $bjects

– !nstance variables

– #lass 7ariables

● #lass !nstantiation

● ethods

– !nstance methods

– %assing 7ariables in ethods (%ass-b"-value, %ass-b"-reference

– tatic methods

● cope of a variable

●

#asting (object, primitive t"pes● #onverting %rimitive "pes to $bjects and 7ice 7ersa

● #omparing $bjects

● Determining the #lass of an $bject

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