Java Fundamentals

Goal of this lecture

• 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

• The console format
• The file format
• The data base format

This is featured is usually called Logging

What a Log class should do ?

• 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)

• 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

• 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

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 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

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

• 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

Formatting Dates with DateTimeFormatter

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

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

• 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

Exercise 2: Age Calculator

• 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

2nd part: The String object

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)

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

Exercise 3: Modern Logger Implementation

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

• 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

• 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