Java: Intro to Variables

8/19/2019 Java: Intro to Variables

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Variables

Chapter 2.1


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Variables• Variables store data such as numbers and

letters. – Think of them as places to store data. –

They are implemented as memory locations.• The data stored by a variable is called its

value. – The value is stored in the memory location.

• Its value can be changed.


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Variables and Values• Variables

numberOfBasketseggsPerBasket

totalEggs• Assigning values

eggsPerBasket = 6;eggsPerBasket = eggsPerBasket - 2;

• What is the value of eggs er!asket no"#


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$aming and %eclaringVariables

• Choose names that are helpful &moreabout documentation in a later "eek'

•

When you declare a variable( you provideits name and type as a statement.int numberOfBaskets,eggsPerBasket ;

• A variable)s type determines "hat kinds ofvalues it can hold & int , double , char , etc .'.

• A variable must be declared before it is

used.


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*ynta+ and ,+amples• *ynta+

type ariable!", ariable!2, #;• ,+amples

int style$hoice, numberOf$hecks;double balance, interest%ate;char &ointOr'ndi idual;


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%ata Types• A class type is used for a class of ob-ects

and has both data and methods. – ()a a is fun( is a value of class type

*tring• A primitive type is used for simple(

nondecomposable values such as anindividual number or individual character.

– int , double , and char are primitive types.


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rimitive Types


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ava Identi/ers• An identifer is a name( such as the name

of a variable.• Identi/ers may contain only

– 0etters – %igits & through ' – The underscore character &3' – And the dollar sign symbol &4' "hich has a

special meaning• The /rst character cannot be a digit.


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,+ercise5• Identi/ers may not contain any spaces(

dots & . '( asterisks & * '( or other characters6+-"" oracle com util &not allo"ed'

• Identi/ers can be arbitrarily long.• *ince ava is case sensitive ( stuff ,

*tuff , and *./00 are di7erent identi/ers.


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,+ercise5• What is the value of totalValue#stu7 8 19

*tu7 8 29*tu7 8 :9stu7 8 ;9

totalValue 8 stu7 < *tu79


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=ey"ords or >eserved

Words• Words such as if are called keywords or

reserved words and have special(

prede/ned meanings. – Cannot be used as identi/ers. – *ee Appendi+ 1 for a complete list of ava

key"ords.•

,+ample key"ords6 int , public , class• If the font changes color in an editor

it’s probably a reserved word and youneed to change the variable name


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$aming Conventions• Class types begin "ith an uppercase letter

&e.g. *tring '.• rimitive types begin "ith a lo"ercase

letter &e.g. int '.• Variables of both class and primitive types

begin "ith a lo"ercase letters&e.g. my1ame , myBalance '.

• ?ulti"ord names are @punctuated@ usinguppercase letters.


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,+ercise5

In a program "e "ant to store6• A person s Bull $ame•

A person s middle initial• Their "hole number age• Their hourly "age

%eclare legal variables in order to dothis.


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Where to %eclare Variables

• %eclare a variable – ust before it is used or – At the beginning of the section of your program

that is enclosed in 3 . public static oid main4*tring5 args7

8 declare ariables here 8 3


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rimitive Types• Bour integer types & byte , short , int ,

and long ' – int is most common

• T"o oatingDpoint types & float anddouble '

– double is more common•

Ene character type & char '• Ene boolean type & boolean '


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,+amples of rimitive 0iteral Values

• Integer types9 -" :6 "2999

• BloatingDpoint types9


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Assignment *tatements• An assignment statement is used to assign

a value to a variable.ans er = >2;

• The @eFual sign@ is called the assignmentoperator.

• We say( @The variable named ans er is

assigned a value of ;2(@ or more simply(@ans er is assigned ;2.@


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Assignment *tatements• *ynta+

ariable = eCpression;"here eCpression can be anothervariable( a literal or constant &such as anumber'( or something more complicated"hich combines variables and literalsusing operators &such as < and D'


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Assignment ,+amples

amount = :


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InitialiGing Variables• A variable that has been declared( but not

yet given a value is said to beuninitialized.

• HninitialiGed class variables have the valuenull .

• HninitialiGed primitive variables may havea default value.

• It)s good practice not to rely on a defaultvalue.


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InitialiGing Variables• To protect against an uninitialiGed variable

&and to keep the compiler happy'( assign avalue at the time the variable is declared.

• ,+amples6int count = 9;char grade = ?@?;


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InitialiGing Variables• synta+

type ariable!" = eCpression!",ariable!2 = eCpression!2, #;


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Assignment ,valuation• The e+pression on the rightDhand side of the

assignment operator & 8 ' is evaluated /rst.• The result is used to set the value of the

variable on the leftDhand side of theassignment operator.score = numberOf$ards handicap;

eggsPerBasket = eggsPerBasket - 2;


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,+ercise5• %eclare and assign values to each of

the variables per line.

Variable Valuefull$ame ohn %oeJ

middleInitial KC

age 21

hourlyWage ; .LfullTime true


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*imple Input

• *ometimes the data needed for acomputation are obtained from the userat run time.

• =eyboard input reFuiresimport &a a util *canner;at the beginning of the /le.


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*imple Input• %ata can be entered from the keyboard

using *canner keyboard =ne *canner4*ystem in7;

follo"ed( for e+ample( byeggsPerBasket = keyboard neCt'nt47;"hich reads one int value from the

keyboard and assigns it to eggsPerBasket .


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*imple *creen Eutput

*ystem out println4(.he count is ( count7;

• Eutputs the sting literal F.he count is ( • Bollo"ed by the current value of the variable

count .


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Constants• 0iteral e+pressions such as 2, : +, or ?y?

are called constants .• Integer constants can be preceded by a

or - sign( but cannot contain commas.• BloatingDpoint constants are "ritten "ith

digits before and after a decimal point


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Imprecision in BloatingD oint$umbers• BloatingDpoint numbers often are only

appro+imations since they are stored "ith a/nite number of bits.

• Mence " 98: 9 is slightly less than 1N:. –

The real mathematical representation is.::::::::::5 – This is truncated to the number of places that

can be stored•

,+ample of issue – As a fraction 1N: < 1N: < 1N: 8 :N: 8 1 – This becomes .::::: < .::::: < .::::: 8 .


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$amed Constants• ava provides mechanism to 5

– %e/ne a variable – InitialiGe it – Bi+ the value so it cannot be changed

public static final .ype Gariable = $onstant;

–

,+ample public static final double P' = : ">"

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Java: Intro to Variables

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