java.util Part 2: More Utility Classes

Simple, practical, book-style explanation of the remaining major java.util utilities.

Chapter idea: The previous chapter focused on the Collections Framework. This one continues java.util with utilities for text parsing, bits, optional values, dates, time zones, random numbers, scheduling, formatting, input, localization, identifiers, and functional interfaces.

1. What This Chapter Covers

Text
StringTokenizer, StringJoiner

Bits
BitSet

Optional values
Optional, OptionalInt, OptionalLong, OptionalDouble

Date & time
Date, Calendar, GregorianCalendar, TimeZone

Random & scheduling
Random, Timer, TimerTask

Formatting & input
Formatter, Scanner

Localization
Locale, Currency, ResourceBundle

Other utilities
Base64, UUID, Objects, HexFormat

2. StringTokenizer

StringTokenizer divides a string into tokens. A token is a discrete piece of text. It implements the legacy Enumeration interface.

String text = "Java is easy"; StringTokenizer st = new StringTokenizer(text); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); }

Method	Purpose
hasMoreTokens()	Checks whether another token exists.
nextToken()	Returns the next token.
countTokens()	Returns the number of remaining tokens.

3. BitSet

BitSet represents a collection of bits. Individual bits can be set, cleared, tested, or combined with another BitSet.

BitSet bits = new BitSet(16); bits.set(2); bits.set(5); System.out.println(bits);

{2, 5}

Method	Purpose
set(int)	Sets a bit.
clear(int)	Clears a bit.
get(int)	Tests a bit.
flip(int)	Changes a bit's state.
and(BitSet)	Bitwise AND.
or(BitSet)	Bitwise OR.
xor(BitSet)	Bitwise XOR.
cardinality()	Number of set bits.

4. Optional, OptionalInt, OptionalLong, OptionalDouble

Beginning with JDK 8, these classes represent a value that may or may not be present. They provide an explicit alternative to using null to represent absence.

Optional<T>

↙ value present empty ↘

Creating Optional

Optional<String> a = Optional.of("Ravi"); Optional<String> b = Optional.empty(); Optional<String> c = Optional.ofNullable(null);

Method	Purpose
of(value)	Creates Optional containing a non-null value.
ofNullable(value)	Creates Optional containing value or empty for null.
empty()	Creates an empty Optional.
isPresent()	Tests whether a value exists.
isEmpty()	Tests whether no value exists.
get()	Returns the contained value when present.
orElse(value)	Returns value or a default.
orElseGet(Supplier)	Computes a default when empty.
orElseThrow()	Throws when empty.
ifPresent(Consumer)	Runs an action when present.
filter(Predicate)	Keeps the value only when a condition is true.
map(Function)	Transforms the contained value.
flatMap(Function)	Transforms using a function returning Optional.
or(Supplier)	Supplies another Optional when empty.

String value = null; String result = Optional.ofNullable(value) .orElse("Unknown"); System.out.println(result);

Remember: Optional is a value-based class. Its main purpose is to make the possible absence of a value explicit.

5. Optional Primitive Types

The primitive-specialized classes avoid using a normal reference Optional for an optional primitive value.

OptionalInt result = OptionalInt.of(100); if (result.isPresent()) { System.out.println( result.getAsInt() ); }

Value access methods are getAsInt(), getAsLong(), and getAsDouble().

6. Date

Date encapsulates date and time information. The chapter explains that many old Java 1.0 Date methods were moved to Calendar/date-formatting APIs and deprecated.

Date date = new Date(); System.out.println(date); long millis = date.getTime(); System.out.println(millis);

Method	Purpose
after(Date)	Checks whether this date is later.
before(Date)	Checks whether this date is earlier.
compareTo(Date)	Compares two Date values.
getTime()	Returns milliseconds since January 1, 1970.
setTime(long)	Sets the date from milliseconds.
toInstant()	Converts to Instant.
toString()	Returns a string representation.

7. Calendar

Calendar is an abstract class that exposes date/time components such as year, month, day, hour, minute, and second.

Calendar calendar = Calendar.getInstance(); int year = calendar.get(Calendar.YEAR); int month = calendar.get(Calendar.MONTH); int day = calendar.get(Calendar.DATE);

Method	Purpose
getInstance()	Creates Calendar for default locale/time zone.
get(int)	Gets a calendar field.
set(int,int)	Sets a calendar field.
add(int,int)	Adds or subtracts a field value.
before(Object)	Tests whether the date is earlier.
after(Object)	Tests whether the date is later.
getTime()	Returns an equivalent Date.
getTimeZone()	Returns the Calendar time zone.
clear()	Clears time fields.

Important: Calendar's traditional month field is zero-based; January is Calendar.JANUARY.

8. GregorianCalendar

GregorianCalendar is a concrete Calendar implementation using Gregorian calendar rules.

GregorianCalendar cal = new GregorianCalendar(); int year = cal.get(Calendar.YEAR); System.out.println( cal.isLeapYear(year) );

Constructors

GregorianCalendar() GregorianCalendar(year, month, day) GregorianCalendar(year, month, day, hour, minute) GregorianCalendar(year, month, day, hour, minute, second) GregorianCalendar(Locale locale) GregorianCalendar(TimeZone zone) GregorianCalendar(TimeZone zone, Locale locale)

isLeapYear(int) checks whether a year is a leap year. The chapter also describes conversion methods that connect this traditional API with java.time.

9. TimeZone

TimeZone represents time-zone information and can be used by Calendar.

TimeZone zone = TimeZone.getDefault(); System.out.println( zone.getID() );

Method	Purpose
getDefault()	Gets the default time zone.
getTimeZone(String)	Gets a zone by ID.
getID()	Returns the zone ID.
getAvailableIDs()	Returns available zone IDs.
getRawOffset()	Returns the raw offset.

10. Random

Random generates pseudorandom values.

Random random = new Random(); int n = random.nextInt(100); double d = random.nextDouble(); System.out.println(n); System.out.println(d);

Example: nextInt(100) produces an integer from 0 through 99.

Method	Purpose
nextInt()	Pseudorandom int.
nextInt(bound)	Int from 0 inclusive to bound exclusive.
nextLong()	Pseudorandom long.
nextDouble()	Double from 0.0 inclusive to 1.0 exclusive.
nextFloat()	Pseudorandom float.
nextBoolean()	Pseudorandom boolean.
nextBytes(byte[])	Fills an array with pseudorandom bytes.

JDK 8 also added doubles(), ints(), and longs() for streams of pseudorandom values.

11. Timer and TimerTask

Timer schedules a task. TimerTask contains the code that executes. TimerTask implements Runnable, so its run() method contains the task logic.

Timer

→ schedules →

TimerTask

→

run()

class MyTask extends TimerTask { @Override public void run() { System.out.println( "Task executed" ); } } Timer timer = new Timer(); timer.schedule( new MyTask(), 1000 );

Operation	Purpose
schedule(task, delay)	Runs after a delay.
schedule(task, date)	Runs at a specified date.
schedule(task, delay, period)	Repeats at a period.
scheduleAtFixedRate(...)	Repeating fixed-rate scheduling.
cancel()	Stops timer scheduling.

12. Currency

Currency encapsulates currency information. It uses factory methods rather than public constructors.

Currency c = Currency.getInstance(Locale.US); System.out.println( c.getCurrencyCode() ); System.out.println( c.getSymbol() );

Method	Purpose
getInstance(Locale)	Gets currency for a locale.
getInstance(String)	Gets currency by code.
getCurrencyCode()	ISO 4217 currency code.
getSymbol()	Currency symbol.
getDisplayName()	Display name.
getDefaultFractionDigits()	Normal fractional digits.

13. Formatter

Formatter creates formatted output using format specifiers.

Formatter fmt = new Formatter(); fmt.format( "Name: %s, Age: %d, Score: %f", "Ravi", 25, 98.5 ); System.out.println(fmt); fmt.close();

Specifier	Meaning
%c	Character
%s	String
%d	Decimal integer
%f	Floating point
%e	Scientific notation
%g	General floating format
%o	Octal
%x	Hexadecimal
%a	Hexadecimal floating point
%n	Newline
%%	Percent sign

Field width and precision

fmt.format("|%f|%n|%12f|%n|%012f|", 10.12345, 10.12345, 10.12345); fmt.format("%.2f", 98.7654);

A width such as %12f gives a minimum field width. A leading zero such as %012f requests zero padding. Precision such as %.2f controls decimal places for floating-point output.

Date/time formatting

Calendar cal = Calendar.getInstance(); Formatter fmt = new Formatter(); fmt.format( "Today is day %te of %tB, %tY", cal, cal, cal );

The %t conversion uses suffixes for particular date/time components. Relative indexing such as %<tB can reuse the previous argument.

14. printf() Connection

The chapter explains that printf() is a convenient console-oriented alternative to directly creating a Formatter. It uses Formatter-style formatting.

System.out.printf( "Name: %s, Age: %d%n", "Ravi", 25 );

15. Scanner

Scanner is the complement of Formatter. It reads formatted input and converts it into values. It can read from console input, files, strings, and other supported sources.

Scanner scanner = new Scanner(System.in); System.out.print("Age: "); int age = scanner.nextInt(); System.out.println(age); scanner.close();

Method	Purpose
hasNext()	Checks for another token.
next()	Reads the next token.
nextLine()	Reads a complete line.
hasNextInt()	Checks for an integer token.
nextInt()	Reads an int.
hasNextDouble()	Checks for a double token.
nextDouble()	Reads a double.
useDelimiter()	Changes the token delimiter.

Scanner scanner = new Scanner("10 99.88 Java"); int n = scanner.nextInt(); double d = scanner.nextDouble(); String s = scanner.next(); scanner.close();

16. Locale

Locale represents language/cultural or regional settings and is used by utilities such as Calendar, Currency, Formatter, and ResourceBundle.

Locale us = Locale.US; Locale germany = Locale.GERMANY; System.out.println(us); System.out.println(germany);

17. ResourceBundle

ResourceBundle supports localization by keeping resources such as user-visible strings separate from application logic.

ResourceBundle bundle = ResourceBundle.getBundle("SampleRB"); String title = bundle.getString("title"); System.out.println(title);

ResourceBundle german = ResourceBundle.getBundle( "SampleRB", Locale.GERMAN ); System.out.println( german.getString("title") );

Method	Purpose
getBundle(name)	Loads a bundle.
getBundle(name, locale)	Loads a locale-specific bundle.
getString(key)	Gets a String resource.
getObject(key)	Gets an Object resource.
getStringArray(key)	Gets a String array.
getKeys()	Returns resource keys.
getLocale()	Returns the bundle locale.

18. StringJoiner

StringJoiner joins character sequences using a delimiter and can also add a prefix and suffix.

StringJoiner joiner = new StringJoiner( ", ", "[", "]" ); joiner.add("Java"); joiner.add("Python"); joiner.add("C++"); System.out.println(joiner);

[Java, Python, C++]

Important methods include add(), merge(), setEmptyValue(), length(), and toString().

19. UUID

UUID encapsulates and manages Universally Unique Identifiers.

UUID id = UUID.randomUUID(); System.out.println(id);

20. Base64

Base64 supports Base64 encoding and decoding. It provides nested Encoder and Decoder classes.

String text = "Java"; String encoded = Base64.getEncoder() .encodeToString( text.getBytes() ); System.out.println(encoded);

Important: Base64 is an encoding representation; it is not encryption.

21. Summary Statistics

IntSummaryStatistics and DoubleSummaryStatistics collect statistics such as count, sum, minimum, maximum, and average.

IntSummaryStatistics stats = new IntSummaryStatistics(); stats.accept(10); stats.accept(20); stats.accept(30); System.out.println(stats.getCount()); System.out.println(stats.getAverage()); System.out.println(stats.getSum());

22. java.util.function

The java.util.function package defines predefined functional interfaces used heavily with lambda expressions and method references.

Interface	Simple meaning
Consumer<T>	Accepts a value and performs an action.
Supplier<T>	Supplies a value.
Function<T,R>	Accepts a value and returns a result.
Predicate<T>	Tests a condition and returns boolean.
BiConsumer<T,U>	Consumes two values.
BiFunction<T,U,R>	Accepts two values and returns a result.
BiPredicate<T,U>	Tests two values.
UnaryOperator<T>	Input and result have the same type.
BinaryOperator<T>	Two same-type inputs produce the same type.

Predicate<Integer> positive = n -> n > 0; System.out.println( positive.test(10) );

23. Other java.util Utilities

Class	Purpose
Objects	Utility methods that operate on objects.
HexFormat	Conversions to and from hexadecimal representation.
ServiceLoader	Finding service providers.
PropertyPermission	Property permissions.
EventObject	Superclass for event objects.
EventListenerProxy	Proxy support for event listeners.
FormattableFlags	Formatting flags for Formattable.

24. java.util Subpackages

Subpackage	Purpose
java.util.concurrent	Concurrent programming and Fork/Join support.
java.util.concurrent.atomic	Atomic operations.
java.util.concurrent.locks	Explicit lock mechanisms.
java.util.function	Predefined functional interfaces.
java.util.jar	Read/write JAR files.
java.util.logging	Logging facilities.
java.util.prefs	Preferences.
java.util.random	Random-number APIs.
java.util.regex	Regular expressions.
java.util.spi	Service-provider interfaces.
java.util.stream	Stream APIs.
java.util.zip	Compression and ZIP utilities.

25. Complete Mental Map

26. Which Utility Should I Choose?

Problem	Think of
Break text into tokens	StringTokenizer
Read formatted input	Scanner
Create formatted output	Formatter
Represent possibly absent value	Optional
Optional primitive value	OptionalInt / Long / Double
Work with bits	BitSet
Traditional date/time	Date / Calendar
Gregorian rules	GregorianCalendar
Time-zone information	TimeZone
Pseudorandom values	Random
Schedule a task	Timer + TimerTask
Currency information	Currency
Localization	Locale + ResourceBundle
Join strings	StringJoiner
Unique identifier	UUID
Base64 representation	Base64

27. Perspective

Do not memorize all of java.util.

Learn the requirement → utility relationship:

Missing value → Optional
Token input → Scanner / StringTokenizer
Formatted output → Formatter
Date components → Calendar
Time zone → TimeZone
Random values → Random
Scheduled task → Timer + TimerTask
Localization → Locale + ResourceBundle
Currency → Currency
Unique ID → UUID

28. Key Rules

Optional represents a value that may be absent or present.
Date represents date/time information using the traditional API.
Calendar exposes date/time components.
GregorianCalendar is a concrete Calendar implementation.
TimeZone represents time-zone information.
Random generates pseudorandom values.
Timer schedules; TimerTask executes.
Formatter creates formatted output; Scanner reads formatted input.
Locale, Currency, and ResourceBundle support internationalization/localization.
StringJoiner joins strings with delimiters.
UUID manages universally unique identifiers.
java.util.function supplies standard functional interfaces for lambdas.

29. Summary

This chapter completes the major utility-class portion of java.util. The most important skill is knowing which utility matches a programming requirement. Together with the Collections Framework chapter, these give you a strong foundation for Java's collection and general utility APIs.