Class Libraries

To help programmers be more productive, Java includes predefined class packages (aka Java class libraries) as part of the installation. Some specific package/libraries are: applets java.applet, language extensions java.lang, utilities java.util, formatters java.text, io, GUI java.awt and javax.swing, network services, precision math java.math, database management java.sql, security

Note: Some programmers find that using an interactive development environment (IDE) such as NetBeans, JCreator or JBuilder to be very helpful in accessing and using the many 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.

Note: Unless another library is indicated the following classes are contained in the java.lang package.

Class Libraries:
  • A class library is collection of classes
    • Example : System, Scanner, String
  • Java Standard class library is comes as a part of java instillation.
  • You can create your custom libraries


  • The classes of Java Standard class library  are organized into packages.
  • Few Important packages are :
    • java.lang  :General support
    • java.applet :Web pages
    • java.awt. : Graphics
    • : Networking
    • java.util :Utilities
    • java.swing : Additional graphic capabilities


Type Wrappers

Wrappers are classes used to enclose a simple datatype or a primitive object into an object. This is sometimes necessary because:

  • Simple datatypes are not part of the object hierarchy.
  • Simple datatypes are passed by value and not by reference.
  • Two methods can't refer to the same instance of a simple type.
  • Some classes can only use members of another class and not a simple type.
  • The wrapped object lacks advanced methods needed for object manipulation

The Number class wrapper has subclasses of Byte, Short, Integer, Long, Double and Float. Each of these subclasses have methods for converting from and to strings such as: parseInt(str[, radix]), valueOf(str[, radix]), and toString(value).

The Boolean class wrapper allows passing Boolean values (true and false) by reference.

The Character class wrapper has many methods available, some of which are:

  • isLowerCase(char c)
  • isUpperCase(char c)
  • isDigit(char c)


  • isLetter(char c)
  • isLetterOrDigit(char c)
  • isWhitespace(char c)


  • toLowerCase(char c)
  • toUpperCase(char c)
  • compareTo(String g)


Note: Java 1.5 has introduced automatic type conversion (casting) between primitives and wrapper objects. For example the following are equivalent:

Integer num = 17;
Integer num = new Integer(17);

The System Class

The System class provides access to the native operating system's environment through the use of static methods. As an example System.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 System.getProperty("property_name") allows access to the following system properties:

  • file.separator
  • line.separator
  • path.separator
  • os.arch
  • os.version
  • user.dir
  • user.home
  • java.class.path
  • java.class.version
  • java.home
  • java.vendor
  • java.vendor.url
  • java.version
  • java.vm.vendor
  • java.vm.version

Note: Many System class methods can throw a SecurityException exception error for safety reasons.

The System class also provides very basic io streams for console read, write and error operations. reads a keystroke and returns an integer value. It can also throw an IOException exception error. System.out.println(string) displays a string to the current output device. There are much better ways for user interaction. Refer to io streams for methods of reading more than one keystroke at a time or refer to file choosers for visual interfaces using AWT and Swing objects.

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), round(x)
Miscellaneous IEEEremainder(x,y), random(), toDegrees(x), toRadians(x)

Note: To generate a random integer between 1 and x you can use the following.

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

The NumberFormat Class [java.text library]

The NumberFormat class is used to display numbers in a locale sensitive fashion. The methods getInstance(), getIntegerInstance(), getCurrencyInstance() and getPercentInstance() create objects formatted using local varients. 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
% 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.*; import java.util.*;
public class test
public static void main (String args[])
Date date = new Date(); String rptDate;
SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd @ hh:mm");
rptDate = sdf.format(date); System.out.println(rptDate+"\n");

The DecimalFormat class methods are as follows:

Group Methods
Constructor DecimalFormat (), DecimalFormat(pattern), DecimalFormat(pattern,symbols)
Accessor getDecimalFormatSymbols (), getGroupingSize(), getMultiplier(), getNegativePrefix(),,getNegativeSuffix(), getPositivePrefix(), getPositiveSuffix()
Mutator setDecimalFormatSymbols(newSymbols), setDecimalSeparatorAlwaysShown(newValue), setGroupingSize(newValue), setMaximumFractionDigits(newValue), setMaximumIntegerDigits(newValue), setMinimumFractionDigits(newValue), setMinimumIntegerDigits(newValue), setMultiplier(newValue), setNegativePrefix(newValue), setNegativeSuffix(newValue), setPositivePrefix(newValue), setPositiveSuffix(newValue)
Boolean equals(obj), isDecimalSeparatorAlwaysShown()
Instance applyLocalizedPattern(pattern), applyPattern(pattern), format(number),
String toLocalizedPattern(), String toPattern(),

The Locale Class [java.util library]

The Locale 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 and SimpleDateFormat are locale-sensitive. By default the locale is determined by the operating system.

Some of the more commonly used locale methods are: setDefault(loc_obj), getDisplayCountry(), getDisplayLanguage() and getDisplayName().



The Calendar Class [java.util library]

The Calendar class provides a set of methods for formatting dates. getInstance() returns the current date/time as a Calendar object. get(param) returns the specific parameter as a string object. Param can be YEAR, MONTH, DAY_OF_MONTH etc.

Group Methods
Constructor none
Accessor get(), getAvailableLocales, GetInstance(), getTimeZone()
Mutator set(), setTime(), setTimeZone()
Boolean equals(), isSet()

The Date Class [java.util library]

The Date class provides a standard format for any member of the class.The Date class methods are as follows:

Group Methods
Constructor Date()
Accessor long getTime()
Mutator String Date.toString()
Boolean before(Date when), after(Date when), equals(Date when)

The Date Format Class [java.text library]

The abstract class DateFormat and its concrete subclass SimpleDateFormat provides the ability to format and parse dates and times. The constructor normally takes a formatting string made from the following symbols:

Char Meaning
a AM or PM
d Day of month
h Hour (1-12)
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