Java Fundamentals

Modern Date/Time API and String Manipulation

java.time package, LocalDate, LocalDateTime, DateTimeFormatter, and String APIs

Goal of this lecture

This lecture will help in the realization of your project, we will see how to :

Keep our data, even if the application is closed (File approach)
Organize textual information thanks to the String APIs
Manage collections of object
Setup a configuration for the application and choose an appropriate structure to store it

Record data in a file, usual need

A recurrent need in all programs is to perform technical traces, while the program is running

Those traces can be output in several format :

The console format
The file format
The data base format

This is featured is usually called Logging

What a `Log class` should do ?

A Log class (or a Logger) should output messages giving precise information on what is currently happening in the program

Regarding a particular event, the following information should be available

A precise temporal data indicating when the event has occurred
The message representing the event
If an error occurred, the location of that error could be a great indication

1st part: The Modern Date/Time API (java.time)

Java 8 introduced a comprehensive Date/Time API in the java.time package

Replaces the old java.util.Date and Calendar classes (which are deprecated)
Immutable and thread-safe by design
Clear separation between dates, times, and timestamps
Much easier to use and understand

Key Classes in java.time

The main classes you'll work with:

LocalDate - Date without time (e.g., 2025-10-06)
LocalTime - Time without date (e.g., 14:30:00)
LocalDateTime - Date and time without timezone (e.g., 2025-10-06T14:30:00)
ZonedDateTime - Date and time with timezone
Instant - Timestamp (point on timeline in UTC)
Period - Amount of time in years, months, days
Duration - Amount of time in hours, minutes, seconds

Working with LocalDate

Creating and manipulating dates without time:

package fr.tbr.exercises.example;

import java.time.LocalDate;
import java.time.Month;

public class LocalDateExample {
    public static void main(String[] args) {
        // Get current date
        LocalDate today = LocalDate.now();
        System.out.println("Today: " + today);

        // Create specific date
        LocalDate birthday = LocalDate.of(1990, Month.JANUARY, 15);
        System.out.println("Birthday: " + birthday);

        // Parse from string
        LocalDate parsed = LocalDate.parse("2025-10-06");
        System.out.println("Parsed: " + parsed);
    }
}

LocalDate Operations

LocalDate provides many useful methods for date manipulation:

LocalDate today = LocalDate.now();

// Add/subtract days, months, years
LocalDate nextWeek = today.plusDays(7);
LocalDate nextMonth = today.plusMonths(1);
LocalDate lastYear = today.minusYears(1);

// Get specific values
int year = today.getYear();
int month = today.getMonthValue();
int day = today.getDayOfMonth();
String dayOfWeek = today.getDayOfWeek().toString();

// Check relationships
boolean isBefore = today.isBefore(nextWeek);  // true
boolean isAfter = today.isAfter(lastYear);     // true

Working with LocalDateTime

LocalDateTime combines date and time (without timezone):

import java.time.LocalDateTime;
import java.time.Month;

public class LocalDateTimeExample {
    public static void main(String[] args) {
        // Get current date and time
        LocalDateTime now = LocalDateTime.now();
        System.out.println("Now: " + now);

        // Create specific date-time
        LocalDateTime meeting = LocalDateTime.of(2025, Month.OCTOBER, 6, 14, 30);
        System.out.println("Meeting: " + meeting);

        // Manipulate date-time
        LocalDateTime nextHour = now.plusHours(1);
        LocalDateTime yesterday = now.minusDays(1);

        System.out.println("Next hour: " + nextHour);
        System.out.println("Yesterday: " + yesterday);
    }
}

Aside - The Exception

Notice the try{]catch(Exception ex){} construct : this is the way errors are handled in Java.

As a quick concept description, the try{} block symbolizes a part of code critical enough to throw an exception.
The catch(Exception e){} block allows to react while concretely facing that exception

We will dedicate a full lecture on that subject, for now consider that it is an error case

Formatting Dates with DateTimeFormatter

Use DateTimeFormatter to format and parse dates:

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

// Predefined formatters
LocalDateTime now = LocalDateTime.now();
String iso = now.format(DateTimeFormatter.ISO_DATE_TIME);
System.out.println(iso); // 2025-10-06T14:30:00

// Custom formatters
DateTimeFormatter customFormatter = DateTimeFormatter.ofPattern("yyyy/MM/dd - HH:mm:ss");
String formatted = now.format(customFormatter);
System.out.println(formatted); // 2025/10/06 - 14:30:00

// Parsing from String
LocalDate parsed = LocalDate.parse("2025-10-06");
LocalDateTime parsedCustom = LocalDateTime.parse("2025/10/06 - 14:30:00", customFormatter);

Calculating Periods Between Dates

Use Period to calculate date-based differences:

import java.time.LocalDate;
import java.time.Period;

LocalDate birthday = LocalDate.of(1990, 1, 15);
LocalDate today = LocalDate.now();

// Calculate age
Period age = Period.between(birthday, today);
System.out.println("Age: " + age.getYears() + " years, "
                 + age.getMonths() + " months, "
                 + age.getDays() + " days");

// Calculate days between dates
long daysBetween = birthday.until(today).getDays();
System.out.println("Days since birth: " + daysBetween);

Exercise 1: Date Input Validation

Create a Java program that:

Asks the user to enter a date in the format yyyy-MM-dd
Parses the input using LocalDate.parse()
Validates that the date is valid (catches DateTimeParseException)
Displays the day of the week for that date

Hint: Use try-catch to handle invalid input

Exercise 2: Age Calculator

Create a program that:

Asks the user for their birth date (format: yyyy-MM-dd)
Calculates their exact age in years, months, and days
Displays how many days until their next birthday

Hint: Use Period.between() and LocalDate.plusYears()

2nd part: The String object

We now need to format properly a String variable. You already got an overview of what could be the usage of the String objects

Creation

String firstString = "Sample String";
String secondString = "Second String";

Concatenation

System.out.println("Testing concatenation");
String concatResult = firstString + " " + secondString;
System.out.println(concatResult);

2nd part: The String object (2)

Replacement

System.out.println("Testing replacement");
concatResult = concatResult.replaceAll("St" , "" );
System.out.println(concatResult);

Immutability : While other objects are "mutable" (their internal values can change during the program execution), Strings are immutable. Format

//Full spec here
//http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html
String formattedString = String.format("%tD", new Date());
System.out.println(formattedString);

2nd part: The String object (3)

You may also need one convenience class: StringBuilder, which allows the developer to "build" a String

StringBuilder sb = new StringBuilder(concatResult);
stringBuilder.append("blah").append(" ").append(new Date());

Mutability : What is convenient with the StringBuilder, is that it is mutable. It allows to pass the StringBuilder to methods, and it is modified by the operations done in the method Body

Text Blocks (Java 15)

Text blocks make multi-line strings much cleaner:

Traditional

String json = "{\n" +
    "  \"name\": \"Alice\",\n" +
    "  \"age\": 30,\n" +
    "  \"city\": \"Paris\"\n" +
    "}";

Text Block

String json = """
    {
      "name": "Alice",
      "age": 30,
      "city": "Paris"
    }
    """;

Text blocks use triple quotes """ and preserve formatting automatically!

Text Blocks: Common Use Cases

// SQL queries
String query = """
    SELECT id, name, email
    FROM users
    WHERE status = 'ACTIVE'
    ORDER BY created_at DESC
    """;

// HTML templates
String html = """
    <html>
      <body>
        <h1>Welcome, %s!</h1>
      </body>
    </html>
    """.formatted(userName);

// JSON responses
String response = """
    {"status": "success", "data": %s}
    """.formatted(data);

Text Blocks: Indentation Rules

// Closing """ determines the left margin
String text = """
    Line 1    ← 4 spaces from margin
    Line 2    ← 4 spaces from margin
    """;      ← This position is the left edge

// Result has no leading spaces
// because content aligns with closing quotes

Escape	Effect
`\` at end of line	Suppress newline (continue line)
`\s`	Preserve trailing whitespace
`\"\"\"`	Include triple quotes in text

Exercise 3: Modern Logger Implementation

Write a class ModernLogger with the following method:

public void log(String message){
 //Implement using LocalDateTime and DateTimeFormatter
}

Requirements:

Use LocalDateTime.now() to get the current timestamp
Format the timestamp as yyyy/MM/dd - HH:mm:ss
Output format: 2025/10/06 - 14:30:00 : Your message here

Exercise 4: Booking System

Create a hotel booking system that:

Asks for check-in and check-out dates (yyyy-MM-dd format)
Calculates the number of nights
Validates that check-out is after check-in
Calculates the total cost (assume 100 euros per night)
Displays a booking confirmation with all details

Java Fundamentals

Modern Date/Time API and String Manipulation

Goal of this lecture

Record data in a file, usual need

What a Log class should do ?

1st part: The Modern Date/Time API (java.time)

Key Classes in java.time

Working with LocalDate

LocalDate Operations

Working with LocalDateTime

Aside - The Exception

Formatting Dates with DateTimeFormatter

Calculating Periods Between Dates

Exercise 1: Date Input Validation

Exercise 2: Age Calculator

2nd part: The String object

2nd part: The String object (2)

2nd part: The String object (3)

Text Blocks (Java 15)

Traditional

Text Block

Text Blocks: Common Use Cases

Text Blocks: Indentation Rules

Exercise 3: Modern Logger Implementation

Exercise 4: Booking System

Slide Overview

What a `Log class` should do ?