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Java Date/Time APIs, Exception Handling, and Optional Class Guide

Tech May 27 4

1. Date and Time Classes

1.1 The Date Class

Epoch Reference Point

The epoch in computing begins at January 1, 1970, 00:00:00 UTC. All timestamps are calculated relative to this point.

Time Conversion

1 second equals 1000 milliseconds.

Date Class Overview

The Date class represents a specific moment in time, accurate to milliseconds.

Constructors

Method Description
public Date() Creates a Date object initialized to the current time
public Date(long date) Creates a Date object initialized to the specified milliseconds since epoch

Example

public class DateDemo01 {
    public static void main(String[] args) {
        Date d1 = new Date();
        System.out.println(d1);

        long milliseconds = 1000 * 60 * 60;
        Date d2 = new Date(milliseconds);
        System.out.println(d2);
    }
}

1.2 Common Date Methods

Method Description
public long getTime() Returns milliseconds since epoch to the Date object
public void setTime(long time) Sets the time using milliseconds

Example

public class DateDemo02 {
    public static void main(String[] args) {
        Date d = new Date();

        long currentMillis = System.currentTimeMillis();
        d.setTime(currentMillis);

        System.out.println(d);
    }
}

1.3 SimpleDateFormat Class

Overview

SimpleDateFormat is a concrete class for formatting and parsing dates in a locale-sensitive manner.

java.text.SimpleDateFormat formats dates to text and parses text to dates independently of language conventions.

Constructors

Method Description
public SimpleDateFormat() Creates using default pattern and date format
public SimpleDateFormat(String pattern) Creates using specified pattern and default date format

Common Methods

  • Formatting (Date → String): public final String format(Date date)
  • Parsing (String → Date): public Date parse(String source)

Example

public class SimpleDateFormatDemo {
    public static void main(String[] args) throws ParseException {
        Date current = new Date();
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
        String formatted = formatter.format(current);
        System.out.println(formatted);
        System.out.println("--------");

        String input = "2048-08-09 11:11:11";
        SimpleDateFormat parser = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date parsed = parser.parse(input);
        System.out.println(parsed);
    }
}

Calendar Class

Important Notes

  • Month values: January is 0, February is 1, and so on (December is 11)
  • Day of week values: Sunday is 1, Monday is 2, and so on (Saturday is 7)

Example

import org.junit.Test;
import java.util.Calendar;
import java.util.TimeZone;

public class CalendarTests {
    @Test
    public void testBasicCalendar() {
        Calendar cal = Calendar.getInstance();
        System.out.println(cal);

        int year = cal.get(Calendar.YEAR);
        int month = cal.get(Calendar.MONTH) + 1;
        int day = cal.get(Calendar.DATE);
        int hour = cal.get(Calendar.HOUR_OF_DAY);
        int minute = cal.get(Calendar.MINUTE);

        System.out.println(year + "-" + month + "-" + day + " " + hour + ":" + minute);
    }

    @Test
    public void testTimeZone() {
        TimeZone tz = TimeZone.getTimeZone("America/Los_Angeles");
        Calendar cal = Calendar.getInstance(tz);
        int year = cal.get(Calendar.YEAR);
        int month = cal.get(Calendar.MONTH) + 1;
        int day = cal.get(Calendar.DATE);
        int hour = cal.get(Calendar.HOUR_OF_DAY);
        int minute = cal.get(Calendar.MINUTE);

        System.out.println(year + "-" + month + "-" + day + " " + hour + ":" + minute);
    }

    @Test
    public void testManipulation() {
        Calendar calendar = Calendar.getInstance();
        Date date = calendar.getTime();

        date = new Date(234234235235L);
        calendar.setTime(date);
        calendar.set(Calendar.DAY_OF_MONTH, 8);
        System.out.println("After setting day to 8: " + calendar.getTime());

        calendar.add(Calendar.HOUR, 2);
        System.out.println("After adding 2 hours: " + calendar.getTime());

        calendar.add(Calendar.MONTH, -2);
        System.out.println("After subtracting 2 months: " + calendar.getTime());
    }
}

1.4 Date and Time Practice

Requirement

Flash sale starts at November 11, 2020 00:00:00 and ends at November 11, 2020 00:10:00. User A orders at 00:03:47, User B orders at 00:10:11. Determine which users successfully participated.

Implementation Steps

  1. Check if order time falls within the start and end range
  2. Convert string time to milliseconds

Code

public class FlashSaleCheck {
    public static void main(String[] args) throws ParseException {
        String startTime = "2020年11月11日 0:0:0";
        String endTime = "2020年11月11日 0:10:0";
        String userAOrder = "2020年11月11日 0:03:47";
        String userBOrder = "2020年11月11日 0:10:11";

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
        long startMillis = sdf.parse(startTime).getTime();
        long endMillis = sdf.parse(endTime).getTime();
        long userAMillis = sdf.parse(userAOrder).getTime();
        long userBMillis = sdf.parse(userBOrder).getTime();

        if (userAMillis >= startMillis && userAMillis <= endMillis) {
            System.out.println("User A participated successfully");
        } else {
            System.out.println("User A did not participate");
        }

        System.out.println("------------------------");

        if (userBMillis >= startMillis && userBMillis <= endMillis) {
            System.out.println("User B participated successfully");
        } else {
            System.out.println("User B did not participate");
        }
    }
}

2. JDK 8 Date and Time API

Expressing time as "1502643933071" is simple but lacks context. We need human-readable time tied to days and seasons, which complicates things. JDK 1.0 had java.util.Date, but most of its methods were deprecated when Calendar was introduced in JDK 1.1. Calendar wasn't much better:

  • Mutability: Date and time classes should be immutable
  • Offsets: Year values start from 1900, months from 0
  • Formatting: Only works with Date, not Calendar
  • Thread Safety: Not thread-safe; no leap second handling

A leap second is added to keep UTC close to UT1. Due to Earth's irregular rotation, when the difference exceeds ±0.9 seconds, UTC is adjusted by one second (positive or negative). 27 leap seconds have occurred globally, all positive.

The third date-time API introduction in Java 8 fixed these issues with java.time.

New API packages:

  • java.time – Core value types
  • java.time.chrono – Alternative calendar systems
  • java.time.format – Formatting and parsing
  • java.time.temporal – Framework extensions
  • java.time.zone – Time zone support

2.1 New Date Classes

  • LocalDate: Date only (year, month, day)
  • LocalTime: Time only (hours, minutes, seconds)
  • LocalDateTime: Date and time combined

2.2 Creating LocalDateTime

Method Description
public static LocalDateTime now() Gets current system time
public static LocalDateTime of(year, month, day, hour, minute, second) Creates with specified values

Example

public class JDK8DateDemo2 {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();
        System.out.println(now);

        LocalDateTime specific = LocalDateTime.of(2020, 11, 11, 11, 11, 11);
        System.out.println(specific);
    }
}

2.3 Getting LocalDateTime Values

Method Description
public int getYear() Gets year
public int getMonthValue() Gets month (1-12)
public int getDayOfMonth() Gets day of month (1-31)
public int getDayOfYear() Gets day of year (1-366)
public DayOfWeek getDayOfWeek() Gets day of week
public int getMinute() Gets minutes
public int getHour() Gets hours

Example

public class JDK8DateDemo3 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 11, 11, 11, 11, 20);

        System.out.println("Year: " + dt.getYear());
        System.out.println("Month: " + dt.getMonthValue());
        System.out.println("Day of month: " + dt.getDayOfMonth());
        System.out.println("Day of year: " + dt.getDayOfYear());
        System.out.println("Day of week: " + dt.getDayOfWeek());
        System.out.println("Hour: " + dt.getHour());
        System.out.println("Minute: " + dt.getMinute());
    }
}

2.4 Converting LocalDateTime

Method Description
public LocalDate toLocalDate() Converts to LocalDate
public LocalTime toLocalTime() Converts to LocalTime

Example

public class JDK8DateDemo4 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 12, 12, 8, 10, 12);

        LocalDate date = dt.toLocalDate();
        System.out.println(date);

        LocalTime time = dt.toLocalTime();
        System.out.println(time);
    }
}

2.5 Formatting and Parsing LocalDateTime

Method Description
public String format(DateTimeFormatter formatter) Formats to string
public static LocalDateTime parce(CharSequence text, DateTimeFormatter formatter) Parses from string
public static DateTimeFormatter ofPattern(String pattern) Creates formatter

Example

public class JDK8DateDemo5 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 11, 12, 13, 14, 15);

        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy年MM月dd日 HH:mm:ss");
        String result = dt.format(fmt);
        System.out.println(result);

        String input = "2020年11月12日 13:14:15";
        LocalDateTime parsed = LocalDateTime.parse(input, fmt);
        System.out.println(parsed);
    }
}

2.6 Adding Time to LocalDateTime

Method Description
public LocalDateTime plusYears(long years) Adds/subtracts years
public LocalDateTime plusMonths(long months) Adds/subtracts months
public LocalDateTime plusDays(long days) Adds/subtracts days
public LocalDateTime plusHours(long hours) Adds/subtracts hours
public LocalDateTime plusMinutes(long minutes) Adds/subtracts minutes
public LocalDateTime plusSeconds(long seconds) Adds/subtracts seconds
public LocalDateTime plusWeeks(long weeks) Adds/subtracts weeks

Example

public class JDK8DateDemo6 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 11, 11, 13, 14, 15);
        LocalDateTime adjusted = dt.plusYears(-1);
        System.out.println(adjusted);
    }
}

2.7 Subtracting Time from LocalDateTime

Method Description
public LocalDateTime minusYears(long years) Subtracts/adds years
public LocalDateTime minusMonths(long months) Subtracts/adds months
public LocalDateTime minusDays(long days) Subtracts/adds days
public LocalDateTime minusHours(long hours) Subtracts/adds hours
public LocalDateTime minusMinutes(long minutes) Subtracts/adds minutes
public LocalDateTime minusSeconds(long seconds) Subtracts/adds seconds
public LocalDateTime minusWeeks(long weeks) Subtracts/adds weeks

Example

public class JDK8DateDemo7 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 11, 11, 13, 14, 15);
        LocalDateTime adjusted = dt.minusYears(-1);
        System.out.println(adjusted);
    }
}

2.8 Modifying LocalDateTime

Method Description
public LocalDateTime withYear(int year) Sets year directly
public LocalDateTime withMonth(int month) Sets month directly
public LocalDateTime withDayOfMonth(int day) Sets day of month
public LocalDateTime withDayOfYear(int day) Sets day of year
public LocalDateTime withHour(int hour) Sets hour
public LocalDateTime withMinute(int minute) Sets minute
public LocalDateTime withSecond(int second) Sets second

Example

public class JDK8DateDemo8 {
    public static void main(String[] args) {
        LocalDateTime dt = LocalDateTime.of(2020, 11, 11, 13, 14, 15);
        LocalDateTime modified = dt.withMonth(20);
        System.out.println(modified);
    }
}

2.9 Instant Class

Instant represents a point on the timeline without timezone context. It stores seconds since epoch (January 1, 1970 00:00:00 UTC).

Method Description
now() Returns Instant for default UTC zone
ofEpochMilli(long millis) Creates Instant from milliseconds
atOffset(ZoneOffset offset) Creates OffsetDateTime with offset
toEpochMilli() Returns milliseconds since epoch

China (UTC+8) is 8 hours ahead of UTC. Beijing is in the eastern 8th time zone.

2.10 DateTimeFormatter Class

Three formatting approaches:

  • Predefined standard formats: ISO_LOCAL_DATE_TIME, ISO_LOCAL_DATE, ISO_LOCAL_TIME
  • Localized formats: ofLocalizedDate(FormatStyle.LONG)
  • Custom patterns: ofPattern("yyyy-MM-dd HH:mm:ss")
Method Description
ofPattern(String pattern) Creates formatter with pattern
format(TemporalAccessor temporal) Formats to string
parse(CharSequence text) Parses from string

Example

import org.junit.Test;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.FormatStyle;

public class FormatterTests {
    @Test
    public void testStandardFormat() {
        DateTimeFormatter formatter = DateTimeFormatter.ISO_LOCAL_DATE_TIME;
        LocalDateTime dt = LocalDateTime.now();
        String result = formatter.format(dt);
        System.out.println(result);

        TemporalAccessor parsed = formatter.parse("2022-12-04T21:02:14.808");
        LocalDateTime fromParse = LocalDateTime.from(parsed);
        System.out.println(fromParse);
    }

    @Test
    public void testLocalizedFormat() {
        LocalDateTime dt = LocalDateTime.now();

        DateTimeFormatter fmt1 = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG);
        String str1 = fmt1.format(dt);
        System.out.println(str1);

        DateTimeFormatter fmt2 = DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL);
        String str2 = fmt2.format(java.time.LocalDate.now());
        System.out.println(str2);
    }

    @Test
    public void testCustomPattern() {
        DateTimeFormatter customFmt = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
        String formatted = customFmt.format(LocalDateTime.now());
        System.out.println(formatted);

        TemporalAccessor parsed = customFmt.parse("2022/12/04 21:05:42");
        LocalDateTime dt = LocalDateTime.from(parsed);
        System.out.println(dt);
    }
}

2.11 Time Zone Handling

ZoneId and ZonedDateTime

  • ZoneId: Contains all timezone information (e.g., Europe/Paris)
  • ZonedDateTime: Date-time with timezone in ISO-8601 calendar (e.g., 2007-12-03T10:15:30+01:00)

Common Zone IDs

Asia/Shanghai
UTC
America/New_York

Example

import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.Set;

public class ZoneTests {
    @org.junit.Test
    public void listAllZones() {
        Set<String> zones = ZoneId.getAvailableZoneIds();
        for (String zone : zones) {
            System.out.println(zone);
        }
    }

    @org.junit.Test
    public void useZones() {
        ZonedDateTime local = ZonedDateTime.now();
        System.out.println(local);

        ZonedDateTime ny = ZonedDateTime.now(ZoneId.of("America/New_York"));
        System.out.println(ny);
    }
}

2.12 Period and Duration

Period - Date-based intervals

Method Description
static Period between(LocalDate start, LocalDate end) Calculates interval
int getYears() Gets years
int getMonths() Gets months
int getDays() Gets days
long toTotalMonths() Total months

Example

public class JDK8DateDemo9 {
    public static void main(String[] args) {
        LocalDate date1 = LocalDate.of(2020, 1, 1);
        LocalDate date2 = LocalDate.of(2048, 12, 12);
        Period period = Period.between(date1, date2);

        System.out.println(period);
        System.out.println("Years: " + period.getYears());
        System.out.println("Months: " + period.getMonths());
        System.out.println("Days: " + period.getDays());
        System.out.println("Total months: " + period.toTotalMonths());
    }
}

Duration - Time-based intervals

Method Description
static Duration between(Temporal start, Temporal end) Calculates interval
long toSeconds() Total seconds
int toMillis() Total milliseconds
int toNanos() Total nanoseconds

Example

public class JDK8DateDemo10 {
    public static void main(String[] args) {
        LocalDateTime dt1 = LocalDateTime.of(2020, 1, 1, 13, 14, 15);
        LocalDateTime dt2 = LocalDateTime.of(2020, 1, 2, 11, 12, 13);
        Duration duration = Duration.between(dt1, dt2);

        System.out.println(duration);
        System.out.println("Seconds: " + duration.toSeconds());
        System.out.println("Millis: " + duration.toMillis());
        System.out.println("Nanos: " + duration.toNanos());
    }
}

2.13 Conversion with Legacy APIs

New Class To Legacy From Legacy
Instant ↔ java.util.Date Date.from(instant) date.toInstant()
Instant ↔ java.sql.Timestamp Timestamp.from(instant) timestamp.toInstant()
ZonedDateTime ↔ GregorianCalendar GregorianCalendar.from(zdt) cal.toZonedDateTime()
LocalDate ↔ java.sql.Date Date.valueOf(localDate) date.toLocalDate()
LocalTime ↔ java.sql.Time Time.valueOf(localTime) time.toLocalTime()
LocalDateTime ↔ java.sql.Timestamp Timestamp.valueOf(ldt) ts.toLocalDateTime()
ZoneId ↔ java.util.TimeZone TimeZone.getTimeZone(id) tz.toZoneId()

3. Exception Handling

3.1 Exception Overview

An exception is an abnormal condition during program execution.

3.2 Checked vs Unchecked Exceptions

Checked Exceptions

  • All subclasses of Exception
  • Must be handled explicitly, otherwise compilation fails

Unchecked Exceptions

  • All subclasses of RuntimeException
  • Optional handling, can be treated like checked exceptions

3.3 JVM Default Exception Handling

When an exception occurs with no handling:

  1. Exception name, cause, and location printed to console
  2. Program execution stops

3.4 Exception Information

Console output shows: exception class name, reason, and location. Use this to locate and fix bugs.

3.5 Handling with throws

Declaration Format

public void method() throws ExceptionType {
}

Example

public class ExceptionDemo {
    public static void main(String[] args) throws ParseException {
        System.out.println("Start");
        method2();
        System.out.println("End");
    }

    public static void method2() throws ParseException {
        String dateStr = "2048-08-09";
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Date d = sdf.parse(dateStr);
        System.out.println(d);
    }

    public static void method() throws ArrayIndexOutOfBoundsException {
        int[] arr = {1, 2, 3};
        System.out.println(arr[3]);
    }
}

Notes

  • throws follows the method parentheses
  • Checked exceptions must be handled (try-catch or throws)
  • Unchecked exceptions may not be declared, handled by JVM by default

3.6 Throwing Exceptions

Format

throw new Exception();

Code after throw does not execute.

throws vs throw

throws throw
In method declaration, followed by exception class In method body, followed by exception object
Declares possible exception, caller may need handling Manually throws exception object

Example

public class ExceptionDemo8 {
    public static void main(String[] args) {
        int[] arr = null;
        printArray(arr);
    }

    private static void printArray(int[] array) {
        if (array == null) {
            throw new NullPointerException();
        }
        for (int i = 0; i < array.length; i++) {
            System.out.println(array[i]);
        }
    }
}

3.7 try-catch Handling

Format

try {
    // Code that may throw exception
} catch (ExceptionType variable) {
    // Exception handling
}

Flow

  1. Execute code inside try
  2. On exception, jump to corresponding catch
  3. Continue execution after try-catch block

Example

public class ExceptionDemo01 {
    public static void main(String[] args) {
        System.out.println("Start");
        process();
        System.out.println("End");
    }

    public static void process() {
        try {
            int[] arr = {1, 2, 3};
            System.out.println(arr[3]);
            System.out.println("Can this print?");
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("Index out of bounds, please fix");
        }
    }
}

Key Points

  1. If try completes without exception, catch is skipped
  2. If exception occurs in try, remaining try code is skipped, catch executes
  3. Uncaught exceptions are handled by JVM
  4. Multiple catch blocks handle different exceptions; parent classes should come last

3.8 Throwable Methods

Method Description
String getMessage() Returns detailed exception message
String toString() Returns short description
void printStackTrace() Prints full stack trace to console

Example

public class ExceptionDemo02 {
    public static void main(String[] args) {
        System.out.println("Start");
        process();
        System.out.println("End");
    }

    public static void process() {
        try {
            int[] arr = {1, 2, 3};
            System.out.println(arr[3]);
        } catch (ArrayIndexOutOfBoundsException e) {
            e.printStackTrace();
        }
    }
}

3.9 Exception Practice

Requirement

Input student name and age (18-25). Invalid ages require re-entry until valid.

Student Class

public class Student {
    private String name;
    private int age;

    public Student() {}

    public Student(String name, int age) {
        this.name = name;
        setAge(age);
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        if (age >= 18 && age <= 25) {
            this.age = age;
        } else {
            throw new RuntimeException("Age out of range");
        }
    }
}

Test Class

public class StudentInputDemo {
    public static void main(String[] args) {
        Student student = new Student();
        Scanner scanner = new Scanner(System.in);

        System.out.println("Enter name:");
        student.setName(scanner.nextLine());

        while (true) {
            System.out.println("Enter age:");
            String ageInput = scanner.nextLine();
            try {
                int age = Integer.parseInt(ageInput);
                student.setAge(age);
                break;
            } catch (NumberFormatException e) {
                System.out.println("Please enter a valid integer");
            } catch (AgeOutOfBoundsException e) {
                System.out.println(e.toString());
                System.out.println("Enter age within valid range");
            }
        }
        System.out.println(student);
    }
}

3.10 Custom Exceptions

When to Use

When built-in exceptions don't meet requirements.

Steps

  1. Create exception class
  2. Extend appropriate base class
  3. Provide no-arg constructor
  4. Provide constructor with message

Custom Exception

public class AgeOutOfBoundsException extends RuntimeException {
    public AgeOutOfBoundsException() {}

    public AgeOutOfBoundsException(String message) {
        super(message);
    }
}

Student Class with Custom Expection

public class Student {
    private String name;
    private int age;

    public Student() {}

    public Student(String name, int age) {
        this.name = name;
        setAge(age);
    }

    public void setAge(int age) {
        if (age >= 18 && age <= 25) {
            this.age = age;
        } else {
            throw new AgeOutOfBoundsException("Age out of range");
        }
    }
}

4. Optional Class

4.1 Creating Optional Objects

Overview

Optional is a container that may or may not hold a non-null value.

Method Description
static <T> Optional<T> of(T value) Creates Optional with non-null value
static <T> Optional<T> ofNullable(T value) Creates Optional that may be null

Example

public class OptionalDemo1 {
    public static void main(String[] args) {
        Student s = null;
        Optional<Student> optional = Optional.ofNullable(s);
        System.out.println(optional);
    }
}

4.2 Common Methods

Method Description
T get() Returns value if present, throws NoSuchElementException otherwise
boolean isPresent() Returns true if value exists, false otherwise

Example

public class OptionalDemo2 {
    public static void main(String[] args) {
        Student s = null;
        Optional<Student> optional = Optional.ofNullable(s);

        if (optional.isPresent()) {
            Student student = optional.get();
            System.out.println(student);
        } else {
            System.out.println("Optional is empty");
        }
    }
}

4.3 Handling Null Values

Method Description
T orElse(T other) Returns value if present, otherwise returns other
T orElseGet(Supplier<? extends T> supplier) Returns value if present, otherwise returns supplier result
void ifPresent(Consumer<? super T> action) Executes action if value present
void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) Executes action if present, else executes emptyAction

Example

public class OptionalDemo3 {
    public static void main(String[] args) {
        Student s = null;
        Optional<Student> optional = Optional.ofNullable(s);

        // orElse: returns default if empty
        Student result1 = optional.orElse(new Student("lisi", 24));
        System.out.println(result1);

        // orElseGet: returns supplier result if empty
        Student result2 = optional.orElseGet(() -> new Student("wangwu", 25));
        System.out.println(result2);

        // ifPresent: execute if not empty
        optional.ifPresent(student -> System.out.println(student));

        // ifPresentOrElse: execute different actions based on presence
        optional.ifPresentOrElse(
            student -> System.out.println(student),
            () -> System.out.println("No value present")
        );
    }
}
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