Integration Practice

Goal

Add:

• LocalDateTime field for created date.

• File export using BufferedWriter.

• Show how CRUD can combine with File I/O and DateTime.

User.java (extended)

java
import java.time.LocalDateTime;

public class User {
    private int id;
    private String name;
    private String email;
    private LocalDateTime createdAt;

    public User(int id, String name, String email) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.createdAt = LocalDateTime.now();
    }

    public int getId() { return id; }
    public String getName() { return name; }
    public String getEmail() { return email; }
    public LocalDateTime getCreatedAt() { return createdAt; }

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

    @Override
    public String toString() {
        return "User{" +
               "id=" + id +
               ", name='" + name + '\'' +
               ", email='" + email + '\'' +
               ", createdAt=" + createdAt +
               '}';
    }
}

UserRepository.java (extended)

java
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class UserRepository {
    private Map<Integer, User> storage = new HashMap<>();
    private int nextId = 1;

    public User create(String name, String email) {
        User user = new User(nextId++, name, email);
        storage.put(user.getId(), user);
        return user;
    }

    public User read(int id) {
        return storage.get(id);
    }

    public boolean update(int id, String newName, String newEmail) {
        User user = storage.get(id);
        if (user == null) return false;

        user.setName(newName);
        user.setEmail(newEmail);
        return true;
    }

    public boolean delete(int id) {
        return storage.remove(id) != null;
    }

    public void listAll() {
        for (User user : storage.values()) {
            System.out.println(user);
        }
    }

    // EXPORT all users to a text file
    public void exportToFile(String filename) {
        try (BufferedWriter bw = new BufferedWriter(new FileWriter(filename))) {
            for (User user : storage.values()) {
                bw.write(user.toString());
                bw.newLine();
            }
            System.out.println("✅ Export complete: " + filename);
        } catch (IOException e) {
            System.out.println("Error exporting: " + e.getMessage());
        }
    }
}

Main.java (integration practice)

java
public class Main {
    public static void main(String[] args) {
        UserRepository repo = new UserRepository();

        // CREATE users
        repo.create("Alice", "alice@example.com");
        repo.create("Bob", "bob@example.com");
        repo.create("Charlie", "charlie@example.com");

        // LIST all users
        System.out.println("All users:");
        repo.listAll();

        // EXPORT to file
        repo.exportToFile("users.txt");
    }
}

Mini Project Summary

• Learned how to combine multiple topics: CRUD + collections + I/O + LocalDateTime.

• Used Map for simple in-memory database.

• Added export to file using BufferedWriter.

• Added creation timestamps using LocalDateTime.

This small structure can scale to real databases, web APIs, and even Spring Boot REST services.
It’s a perfect starting point for your first real CRUD back-end logic!