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Strings and String Operations

Overview

Creating String Objects

Substring methods

The Concatenation Operator

Strings are Immutable

Other Methods of the String Class

Example using String Methods

Formatting Floatingpoint !umbers

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Creating String Objects

Strings are Objects String is a se"uence of characters enclosed in

"uotes# E#g# $%ello&

String processing is one of the most important andmost fre"uent applications of a computer

'ava recogni(es this fact and therefore providesspecial support for strings to ma)e their useconvenient

Ever* string is an instance of 'ava+s built in Stringclass, thus, Strings are objects#

Declaration -i)e an* object, a string can be created using new

as in the following example.

String str1 = new String(Hello dear);

%owever, as an extra support, 'ava allows Stringobject to be created without the use of new, as in.

String str2=How are you;

This is inconsistent with the wa* 'ava treats otherclasses#

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Substrings

String objects are represented as a se"uence of

characters indexed from /#

Example. String greeting = Hello, World;

0 common operation on Strings is extracting a

substring from a a given string#

'ava provides two methods for this operation.

Examples:

String sub = greetingsubstring!", #$;$%ell&

String % = greetingsubstring!&, '($;$1orld&

String tail = greetingsubstring!&$;$1orld2&

H e l l o , W o r l d )

/ 3 4 5 6 7 8 9 : ; 3/ 33 34

substring ?eturns the substring from start tothe end of the string

substring ?eturns a substring from start to endbut not including the character atend#

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Concatenation Operator

0nother common operation on String is

concatenation# 0s another special support for String, 'ava

overloaded the @ operator to be used to concatenatetwo String objects to get a bigger one#String *irst+ame = mr;

String last+ame = l-.bra/im;

String *ull+ame = last+ame0 0*irst+ame;

$0lIbrahim 0mr&

If one of the operands of the @ operator is a string,the other is automaticall* converted to string and

the two strings are then concatenated#String course = .1S;

int code = '"(;

String course1ode = course0code$ICS3/4&

1e fre"uentl* use the concatenation operator inprintlnstatements#

S2stemoutprintln!3/e area =0area$;

Aou need to be careful with concatenation operator#For example, what do *ou this the following printB

S2stemoutprintln!Sum =0405$;

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Strings are Immutable

0nother special feature of Strings is that the* are

immutable# That is, once a string object is created,its content cannot be changed# For example,consider the following.

String str1 = Hello World;

str1 = str1.substring(4);

Instead of changing the str3 object, another objectis created# The former is garbage collected, thus,the reference to the former is lost#

The fact that Strings are immutable allows the 'avas*stem to process Strings ver* efficientl*#

For example, consider the following.

String str1 = Hello;

String str2 = Hello;

1e would expect the following

ut in fact, this is what happens

The 'ava s*stem is smart enough to )now that thetwo strings are identical and allocates samememor* location for the two objects

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Methods of the String Class

.n addition to t/e substring met/ods, se6eral

prede*ined met/ods are pro6ided in t/e built-inString class Some o* t/ese are:

nt length() returns the number ofcharacters in this String

String toUpperCase()

String toLowerCase()

returns a new String,

e"uivalent to the Dpperlowercase of this String

boolean equals(s)

booleanequalsIgnoreCase(s)

returns true if sthe same as thisString#

nt compareTo(String s)ntcompareToIngoreCase(s)

returns @ve number, /, or venumber if this String is greaterthan, e"ual to or less than s#

char charAt(index) returns the char in this String,at the indexposition#

nt indexOf(ch)nt lastIndexOf(ch)

?eturns the index of the first last occurrence of chin thisstring, If not fount3 is returned

String trim() returns a String, obtained b*removing spaces from the startand end of this string#

static String valueOf (anyprimitive type)

?eturns a String representationof the argument

String concat(s) equivalent to symbol

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!xample using methods of Stringclass

3/e *ollo%ing program generates a pass%ord *or

a student using /is initials and age

public class a!e"assword #

public static $oid %ain(String&' args) #

String irsta%e = *+%r*;

String %iddlea%e = *Sa%ir*;

String lasta%e = *,bra-i%*;

e/tract initials

String initials =

irsta%e.substring(01)

%iddlea%e.substring(01)

lasta%e.substring(01);

append age

int age = 20;

String password =initials.to3owerase()age;

Syste%.out.println(*5our "assword

=*password);6

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"ormatting #oating$point numbers

Some times %e %ould li7e to print *loating

point numbers onl2 up to certain number o*decimal places

8or example %e %ould %ant print t/e cost o*an item in t/e *orm: S9'54"

3o ac/ie6e t/is, %e use t/eget+umber.nstance met/od o* t/e t/enumber8ormat class o* t/e ja6atextpac7age, to create an object

We t/en use t/e setaximum8ractionDigitsand setinimum8ractionDigits met/ods toset t/e decimal places reuired

8inall2, %e use t/e *ormat met/od to *ormatour number

3/e *ormat met/od returns a string, so %ecan print it

3/e example in t/e next slide demonstratet/is process

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"ormatting #oating$point numbers

i%port 7a$a.te/t.u%ber8or%at;

public class u%ber8or%atting #public static $oid %ain(String&' args) #

u%ber8or%at or%atter =u%ber8or%at.getu%ber,nstance();

or%atter.seta/i%u%8raction9igits(2);

or%atter.setini%u%8raction9igits(2);

double annualSalary = :200;

double %ont-lySalary =annualSalary12;

Syste%.out.println(**%ont-lySalary);

Syste%.out.println(*8or%atted SalaryS>*or%atter.or%at(%ont-lySalary));

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"ormatting #oating$point numbers

1e can also use the Decimal8ormatclass of the

same pac)age to format floatingpoint numbers# In this approach, we create an object of the

ecimalFormat class, specif*ing the format we

want as a parameter#

The difference between this approach and theprevious is that the comma is not used b* default#

import ja6atextDecimal8ormat;

public class Decimal8ormatting args$