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TOPICS

8 1 Introduction to Wrapper Classes

8 2 Character Testing and Conversion with

the

Character

Class

8 3 More about

Str ing

Objects

8 4 The StringBuilder

Class

8 5 Tokenizing Strings

8 6 Wrapper Classes for the Numeric

Data Types

8 7 Focus on Problem Solving: The

TestScoreReader

Class

88 Common

Errors

t Avoid

Introduction

to

Wrapper lasses

CONCEPT Java provides wrapper classes for the primitive data types. The wrapper

class for a given primitive type contains not only a value of that type, but

also methods that perform operations related to the type.

Recall from Chapter 2 that the primitive data types are called primi tive because they

are not created from classes. Instead of instantiating objects, you create variables from the

primitive data types, and variables do not have attributes or methods . They are designed

simply to hold a single value in memory.

Java also provides wrapper classes for all of the primitive data types. A wr pper cl ss

is

a class that is wrapped around a primitive data type and allows you to create objects

instead

of

variables. In addition, these wrapper classes provide methods

that

perform useful

operations on primitive values.

Although these wrapper classes can be used to create objects instead

of

variables, few

programmers use them that way. One reason

is

because the wrapper classes are immu-

table, which means that once you create an object, you cannot change the object's value.

Another reason is because they are not as easy to use as variables for simple operations. For

example, to get the value stored in

an

object you must call a method, whereas variables can

be used directly in assignment statements, passed as arguments to the pr n t

and

pr nt ln

methods, and so forth .

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Chapter

8 Text Processing and Wrapper Classes

Although it is not normally useful to create objects from the wrapper classes, they do provide

static methods that are very useful.

We

examine several of Java s wrapper classes in this chapter.

We begin by looking

at

the

Character

class, which is the wrapper class for the char data type.

haracter Testing and onversion with the

haracter lass

CONCEPT: The haracter class

is

a wrapper class for the

char

data type . t

provides numerous methods for testing and converting character data.

The Character class

is

part of the j ava . l ang package, so no i m

port

statement is necessary

to

use this class. The class provides several static methods for testing the value

of

a ch

ar

variable. Some of these methods are listed in Table 8-1. Each of the methods accepts a single

char argument and returns a boo l ean value.

Table 8 1

Some static

Character

class

methods

for testing

char

values

Method

boolean isDigit char ch

boolean

isLetter char

ch

boolean

isLetterOrDigit char

ch

boolean isLowerCase(char ch

bool ean i sUpperCase(char ch

bool ean i sSpaceChar char ch

bool ean i hiteSpace char ch

Description

Returns true if the

argument

passed

into

ch

is

a

digit from 0 through 9. Otherwise returns fa l se.

Returns true if the argument passed into ch

is

an

alphabetic letter. Otherwise returns fa l se.

Returns

true

if the character passed into

ch

con

tains a digit (0 through 9) or an alphabetic letter.

Otherwise returns

false.

Returns ue if the argument passed into ch

is

a

lowercase letter. Otherwise returns fa l se.

Returns ue if the argument passed into ch

is

an

uppercase letter. Otherwise returns fa l se.

Returns

tru

e if the argument passed into ch

is

a

space character. Otherwise returns fal se.

Returns tr ue if the argument passed into ch

is

a whitespace character (a space, tab,

or

newline

character). Otherwise returns fal se.

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8.2 Character Testing

and

Conversion with the

Character

Class 493

The program in Code Listing 8-1 demonstrates many of these methods.

Code Listing

8

CharacterTest.java)

1

import

java .ut i l .Sca

nner

;

2

1

4

5

6

7

This

program

demonstrates some

of the Character

class ' s

character

test ing methods.

1

8 public class

CharacterTest

9

10 public sta t ic void main(String[] args)

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

String inputLine ;

char inputChar;

II A l ine of input

II A character

II Create a Scanner

object

for keyboard input .

Scanner keyboard = new

Scanner(System.in);

II Get a

character

from the user .

System. out.

print Enter a character: );

inputLine

keyboard

. nextLine() ;

inputChar

= inputLine

.c

harAt(O) ;

II

Test the character.

i (Character.isLetter(inputChar))

System.out.println( Letter ) ;

i f

(Character

.

isDigit(inputChar))

System .o

ut . print ln( Digi t ) ;

i f

(Character.isLowerCase(inputChar))

System .out . println( Lowercase le t ter );

i C

haracter

. isUpperCase(inputChar))

System.out.println(

Uppercase

let ter )

;

i f (Character . isSpaceChar(inputChar))

System.out.println( Space );

i

(Character.isWhitespace(inputChar))

System. out.

print ln Whitespace ) ;

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494 Chapter

8 Text Processing

and

Wrapper Classes

Program Output with Example Input Shown in old

Enter a character :

~ n t e ~

et ter

Lowercase l e t t e r

Program Output

with

Example Input

Shown

in old

Enter a character : A [Enter]

et ter

Uppercase

l e t t e r

Program Output

with

Example Input Shown in old

Enter

a

character : [Enter]

Digi t

Program Output

with

Example Input Shown in old

Enter a character :

[ pace]

[Enter]

Space

Whitespace

character

Program Output

with

Example Input Shown in old

Enter

any

character : [Tab] [Enter]

Whitespace character

Code Listing 8 2 shows a

more

practical application of the character testing methods. t

tests a string to determine whether it is a seven character customer number in the proper

format.

Code Listing

8-2 CustomerNumber.java)

1 import java.ut i l . Scanner;

2

3 1

4

This

program

tests

a

customer

number

to

determine

5

whether i t

is in the proper

format

.

6 1

7

8 public

class

CustomerNumber

9

1 public sta t ic

void

main String[] args)

11

12 String

customer;

To hold

a customer number

13

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16

17

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19

20

21

22

23

24

25

26

27

28

29

30

8.2 Character Testing

and

Conversion with the

Character

Class 95

II Create a Scanner object

for

keyboard input .

Scanner keyboard

= new

Scanner(System.in);

System.out.println( Enter a customer number in "

+

the

form LLLNNNN );

System.out.print( (LLL

= le t ters and NNNN "

+ "= numbers): );

II Get a customer number from the user .

customer =

keyboard

.

nextLine();

II Determine whether i t is valid.

i f (isValid(customer))

System.out.println( That's

a valid

customer

+ number. );

31

else

32

33

34

35

36

37

38

39

40 1

System. out.

print ln(

That is not the proper

+ format. );

System.out.println( "Here is an example: "

+ "ABCI234");

41 The iSValid method

accepts

a

String

as i t s

argument

42 and tes ts

i ts contents

for a valid customer number .

43 1

44

45 private st t ic

boolean isValid(String

custNumber)

46

47

48

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50

51

52

53

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55

56

57

58

59

60

61

boolean

goodSoFar

int

index

= 0;

true;

Flag

II Loop control variable

Is the str ing the correct length?

i f (custNumber.length() = 7)

goodSoFar = fa lse;

II Test the f i r s t three characters for

le t ters

.

while (goodSoFar

index

< 3)

i f ( C h a r a c t e r . i s L e t t e r ( C U S ~ a r A t ( i n d e X ) ) )

goodSoFar = fa lse;

index++;

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96 Chapter 8 Text Processing and Wrapper Classes

6 Test the last four characters for digits .

63

while

(goodSoFar

index