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The System Class

Tutorial 6 - Class Libraries

To aid programmer productivity, the Java installation includes several predefined

class packages (aka Java class libraries). Packages/libraries discussed in this set of tutorials are: applets java.applet , language extensions java.lang , utilities java.util ,

formatters java.text , file streams java.io, GUIs java.awt and javax.swing , network

services java.net , new io (ie. memory mapped) java.nio and remote methodinvocation java.rmi . Other libraries include beans java.beans, communication

ports java.comm, precision math java.math, database management java.sql and

security java.security. The import reserved word is used to access classes from thelibraries (except for java.lang). Unless another library is indicated the following

classes are contained in the java.lang package.

Syste

m

Class

Math

Class

Local

e

Class

Calend

ar &

Date

Exampl

e:

timeSp

an()

DateFormat

Class

NumberForm

at Class

DecimalForm

at Class

Note 1: For information on creating your own class packages or using 3rd-party

libraries see the Java Appendix.

Note 2: Some programmers find that using an interactive development

environment (IDE) such as NetBeans to be very helpful in accessing and using themany extended features that these class libraries provide. Others stay with a basic text

editor and develop a more complete understanding of the libraries. It is your call but I

recommend a basic text editor for beginning programmers.

The System Class

The S ystem class provides redirectable standard io streams for console read, write

and error operations.

The S ystem class also provides access to the native operating system's environmentthrough the use of static methods. As an

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example S ystem.currentTimeMillis() retrieves the system clock setting (as a long, in

milliseconds counted from January 1, 1970). Some of the available methods are:

currentTime()

freeMemory()

gc()

totalMemory()

exit(int

status)

exec(String

cmd)

execin(String

cmd)

getenv(String

var)

getCWD()

getOSName()

arraycopy(src[],

srcpos, dest[],

destpos, len)

Another method called S ystem.getProperty("property_name") allows access to the

following system properties:

file.separator

line.separator

path.separator

os.arch

os.name

os.version

user.dir

user.home

user.name

java.class.path

java.class.version

java.home

java.vendor

java.vendor.url

java.version

java.vm.name

java.vm.vendor

java.vm.version

Note: Many System class methods can throw a S ecurityException exception error for

safety reasons.

The Math Class

The Math class provides the important mathematical constants E and PI which are of

type double. It also provides many useful math functions as methods.

Group Methods

Transcendental acos(x), asin(x),

atan(x), atan2(x,y),

cos(x), sin(x), tan(x)

Exponential exp(x), log(x),

pow(x,y), sqrt(x)

Rounding abs(x), ceil(x),

floor(x), max(x,y),

min(x,y), rint(x),

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round(x)

Miscellaneous IEEEremainder(x,y),

random(),

toDegrees(x),

toRadians(x)

N ote: random() generates double in range of 0 to < 1.

intRnd=(int) (Math.random() * x) + 1; // from 1 to x

intRnd=(int) (Math.random() * 10); // random digit

The Locale Class [java.util library]

The ocale class produces object that describe a geographical or cultural region. For

example dates, times and numbers are displayed differently through the world.Calendar, GregorianCalendar, DateFormat, SimpleDateFormat and SimpleTimeZone

are all locale-sensitive. By default the locale is determined by the operating system.

Some of the more commonly used locale methods are: setDefault(loc_obj),

getDefault(), getDisplayCountry(), getDisplayLanguage() and getDisplayName().

Locale constants include: CA N ADA, CA N ADA_FRE N CH, CHI N A, CHI N E S E, E NGL I S H, FRA N CE, FRE N CH, G ERMA N , G ERMA NY , ITA L IA N , ITA LY ,

JAPA N , JAPA N E S E, KOREA, KOREA N , PRC, S IMP L IFIED_CHI N E S E,

TAIWA N , TRADITIO N A L _CHI N E S E, UK A N D U S . Locale.CANADA would give

the loc_obj for Canada.

Calendar & Date Classes [java.util library]

The abstract class Calendar provides methods for formatting and comparing

dates. getInstance() returns the current date/time in Calendar objectformat. get(fld) returns the specific field as a string object. fld can be YEAR,

MONTH, DAY_OF_MONTH, etc. getTime() returns the time in DATEformat. G regorianCalendar is a concrete subclass which adds a boolean isLeapYear()

test.

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Group Methods

C onstr uct or getInstance()

Accessor get(fld),

getAvailableLocales,

getInstance(),

getTime(),

getTimeZone()

Mutat or add(whichField,

intVal), clear(),

set(fld,intVal),

setTime(),

setTimeZone()

C omparison after(), before(),

equals(), isSet()

The wrapper class Date is used primarily to convert objects into longs for math

operations with the getTime() method.

N ote: Calendar.getTime() converts to Date objects, Date.getTime() converts to long

primitives.

Example: timeSpan()

A common request is for a time S pan() method that computes the difference betweentwo Date or Calendar objects. Since Date has been downgraded to a wrapper with no

setField operations, my example uses Calendar objects. The user will have to provide

his own input (either sio or file) and output reporter.

import java.util.*;

public class Timer{

public static void main(String args[])

{

// create start and end calendar objects

Calendar sTime=Calendar.getInstance();

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Calendar eTime=Calendar.getInstance();

// now set times -- add routines to get from sio or file

// be sure to verify times are in range !!

// adjust times for early start and late finish

sTime.set(Calendar.HOUR_OF_DAY,8);sTime.set(Calendar.MINUTE,0);

eTime.set(Calendar.HOUR_OF_DAY,16);eTime.set(Calendar.MINUTE,0);

long span=timeSpan(sTime,eTime);

// adjust time for lunch hour here

long secs=span/1000;long mins=secs/60;long hours=mins/60;

System.out.println(hours);

System.out.println(mins);

}

// timeSpan (calendarObject,calendarObject) returns long milliseconds

public static long timeSpan(Calendar calStart,Calendar calEnd)

{

Date sTime1,eTime1;long interval,sTime2,eTime2;

sTime1=calStart.getTime();eTime1=calEnd.getTime(); // to Date objects

sTime2=sTime1.getTime();eTime2=eTime1.getTime(); // to long objects

interval=eTime2-sTime2;return interval;

}

}

The DateFormat Class [java.text library]

The abstract class DateFormat and its concrete subclass S impleDateFormat providesthe ability to format and parse dates and times. The constructor normally takes a

formatting string made from the following symbols:

Char M

eaning

a AM or PM

d Day of month

h Hour (1-12)

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k Hour (1-24)

m Minute

s Second

w Week of year

y Year

z Timezone

: Separator

Char Meaning

D Day of year

E Day of week

F Day of week in month

G Era (AD or BC)

H Hour in Day (0-23)

K Hour in Day (0-11)

M Month

S Millisecond

W Week of month

/ Escape character

Here is an example of how SimpleDateFormat can be used:

import java.text.*; import java.util.*;

public class test

{

public static void main (String args[])

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{

Date date=new Date(); String rptDate;

SimpleDateFormat sdf=new SimpleDateFormat("yyyy MMM dd @ hh:mm aa");

rptDate=sdf.format(date); System.out.println(rptDate+"\n");

}

}

The NumberFormat Class [java.text library]

The N umberFormat class is used to display numbers in a locale sensitive fashion.The methods getInstance(), getIntegerInstance(), getCurrencyInstance() and

getPercentInstance() create objects formatted using local variants. Other useful

methods are setDecimalSeparatorAlwaysOn(bool), setMinimunFractionDigits(i),setMaximunFractionDigits(i), setMinimunIntegerDigits(i),

setMaximunIntegerDigits(i) and setParseIntegerOnly(bool).

The DecimalFormat Class [java.text library]

The DecimalFormat class provides highly customized number formatting and

parsing. Objects are created by passing a suitable pattern to the DecimalFormat()

constructor method. The applyPattern() method can be used to change this pattern. A

DecimalFormatSymbols object can be optionally specified when creating a

DecimalFormat object. If one is not specified, a DecimalFormatSymbols object

suitable for the default locale is used. Decimal format patterns consists of a string of

characters from the following table. For example: "$#,##0.00;($#,##0.00)"

Char Interpretation

0 A digit // leading zeros show as 0

# A digit // leading zeros show as absent

. The locale-specific decimal separator

, The locale-specific grouping separator (comma)

- The locale-specific negative prefix

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% Shows value as a percentage

; Separates a positive number format (on left) from

an optional negative number format (on right)

' Escapes a reserved character so it appears literally in the output

Here is an example of how DecimalFormat can be used:

import java.text.*;

public class test

{

public static void main (String args[])

{

int numb=3; String rptNumb;

DecimalFormat df=new DecimalFormat("000");

rptNumb=df.format(numb); System.out.println(rptNumb+"\n");

}

}

The DecimalFormat class methods are as follows:

Group Methods

C onstr uct or DecimalFormat (), DecimalFormat(pattern),

DecimalFormat(pattern,symbols)

Accessor getDecimalFormatSymbols (), getGroupingSize(),

getMultiplier(), getNegativePrefix(),,getNegativeSuffix(),

getPositivePrefix(), getPositiveSuffix()

Mutat or setDecimalFormatSymbols(newSymbols),

setDecimalSeparatorAlwaysShown(newValue),

setGroupingSize(newValue),

setMaximumFractionDigits(newValue),

setMaximumIntegerDigits(newValue),

setMinimumFractionDigits(newValue),

setMinimumIntegerDigits(newValue), setMultiplier(newValue),

setNegativePrefix(newValue), setNegativeSuffix(newValue),

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setPositivePrefix(newValue), setPositiveSuffix(newValue)

Boolean equals(ob j), isDecimalSeparatorAlwaysShown()

Instance applyLocalizedPattern(pattern), applyPattern(pattern),

format(number),

String toLocalizedPattern(), String toPattern(),

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