Ajout d'une condition pour accepter des entrées spécifiques dans HashMap

Question

Mon programme est une base de données de gestion des étudiants simple qui recueille le nom, le sujet et les numéros de téléphone des étudiants et les ajoute à une base de données. J'ai pu réaliser les principales opérations logiques comme l'ajout, la suppression et la recherche d'étudiants dans la base de données. Je ne peux pas empêcher les étudiants d'entrer uniquement des sujets approuvés comme par exemple "Anglais", "Maths" et "Informatique" lors de l'ajout dans une collection HashMap. Toute aide serait appréciée. Voici le code principal:

import java.io.BufferedReader; 
import java.io.BufferedWriter; 
import java.io.File; 
import java.io.FileReader; 
import java.io.FileWriter; 
import java.util.HashMap; 
import java.util.Scanner; 

public class Menu { 
    private HashMap<String, Student> students; 

    public Menu() { 
     students = new HashMap<String, Student>(); 
    } 

    private void eventLoop() { 
     Scanner scanner = new Scanner(System.in); 
     int choice = 0; 
     boolean exit = false; 
     this.readFromFile(); 

     while (!exit) { 
      System.out.println("Welcome to Student Management System"); 
      System.out.println("=============================="); 
      System.out.println("(1) Add new student"); 
      System.out.println("(2) Delete a student"); 
      System.out.println("(3) Find Student By Name"); 
      System.out.println("(4) List Students By Subject"); 
      System.out.println("(5) List All Students"); 
      System.out.println("(6) Exit System"); 
      System.out.println("Choose an option: "); 

      try { 
       choice = Integer.parseInt(scanner.nextLine()); 
       System.out.print('\u000C'); 
       if (choice < 1 || choice > 6) { 
        System.err.println("Error : Choose an option between 1 and 6"); 
        choice = 0; 
       } 
      } catch (NumberFormatException e) { 
       System.err.println("Error : Choose an option between 1 and 6"); 
       choice = 0; 
      } 

      switch (choice) { 
       case 1: 
       this.addStudent(scanner); 
       break; 
       case 2: 
       this.deleteStudent(scanner); 
       break; 
       case 3: 
       this.findStudentByName(scanner); 
       break; 
       case 4: 
       this.findStudentsBySubject(scanner); 
       break; 
       case 5: 
       this.listStudents(); 
       break; 
       case 6: 
       this.writeToFile(); 
       exit = true; 
      } 
     } 
     scanner.close(); 
    } 

    private void findStudentsBySubject(Scanner scanner) { 
     System.out.println("Enter the exact name of the subject:"); 
     String subjectStr = scanner.nextLine(); 
     boolean atleastOne = false; 
     for (String name : students.keySet()) { 
      if (students.get(name).getSubject().getName().toLowerCase().equals(subjectStr.toLowerCase())) { 
       System.out.println(students.get(name)); 
       atleastOne = true; 
      } 
     } 
     if (!atleastOne) { 
      System.err.println("No students have enrolled for this subject."); 
     } 
    } 

    private void findStudentByName(Scanner scanner) { 
     System.out.println("Enter the exact name of the Student to search:"); 
     String name = scanner.nextLine(); 
     if (students.get(name.toLowerCase()) != null) { 
      System.out.println("Student details:"); 
      System.out.println(students.get(name.toLowerCase())); 
     } else { 
      System.err.println(name + " not found in the database."); 
     } 
    } 

    private void deleteStudent(Scanner scanner) { 
     System.out.println("Enter the exact name of the Student to delete:"); 
     String name = scanner.nextLine(); 
     if (students.get(name.toLowerCase()) != null) { 
      students.remove(name.toLowerCase()); 
      System.err.println("Student " + name + " deleted from the database."); 
     } else { 
      System.err.println(name + " not found in the database."); 
     } 
    } 

    private void addStudent(Scanner scanner) { 
     System.out.println("The information should be comma separated and in a single line."); 
     System.out.println("If the name is not unique, the system will throw an error."); 
     System.out.println("Enter the name, phone and subject of the new student."); 

     String line = scanner.nextLine(); 
     System.out.print('\u000C'); 
     String[] info = line.split(","); 
     if (info.length != 3) { 
      System.err.println("Please enter the information in the proper format."); 
      return; 
     } 
     String name = info[0]; 
     String phone = info[1]; 
     String subjectStr = info[2]; 

     if (students.get(name.toLowerCase()) != null) { 
      System.err.println("This student already exists in the database."); 
      return; 
     } 
     if (phone.length() != 9) { 
      System.err.println("The phone number must contain exactly 9 digits."); 
      return; 
     } 
     if (phone.charAt(0) != '9') { 
      System.err.println("The phone number must start with '9'."); 
      return; 
     } 
     if (!phone.matches("^[0-9]*$")) { 
      System.err.println("The phone number must contain only numbers."); 
      return; 
     } 
     students.put(name.toLowerCase(), new Student(name, new Subject(subjectStr), phone)); 
     System.err.println("Student added successfully"); 
    } 

    private void listStudents() { 
     for (String name : this.students.keySet()) { 
      System.out.println(this.students.get(name)); 
     } 
    } 

    private void readFromFile() { 
     try { 
      BufferedReader br = new BufferedReader(new FileReader(new File("./students.txt"))); 
      String line; 
      while ((line = br.readLine()) != null) { 
       String[] info = line.split(","); 
       String name = info[0]; 
       String phone = info[1]; 
       String subjectName = info[2]; 

       if (students.get(name.toLowerCase()) == null) { 
        Subject subject = new Subject(subjectName); 
        students.put(name.toLowerCase(), new Student(name, subject, phone)); 
       } else { 
        System.err.println("There seems to be a duplicate student in the file."); 
       } 
      } 
      br.close(); 
     } catch (Exception e) { 
      e.printStackTrace(); 
     } 
    } 

    private void writeToFile() { 
     try { 
      BufferedWriter bw = new BufferedWriter(new FileWriter(new File("./students.txt"))); 
      for (String name : students.keySet()) { 
       bw.write(students.get(name).toString()); 
       bw.newLine(); 
      } 
      bw.close(); 
     } catch (Exception e) { 
      e.printStackTrace(); 
     } 
    } 

    public static void main(String[] args) { 
     Menu menu = new Menu(); 
     menu.eventLoop(); 
    } 

} 

Sujet Classe:

public class Subject { 
    private String name; 
    public Subject(String subjectName) { 
     this.setName(subjectName); 
    } 

    public String getName() { 
     return name; 
    } 

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

    public String toString() { 
     return this.getName(); 
    } 
} 

Answer 1

Lorsque vous avez une liste restreinte de valeurs possibles que vous pouvez utiliser un Enum par exemple

enum Subject { 
    English, Maths, Computing 
} 

Une collection de ceux-ci est juste un EnumSet et vous pouvez vérifier sa valeur en appelant

EnumSet<Subject> subjects = EnumSet.of(Subject.class); 

Subject s = Subject.valueOf(subjectName); 
subjects.add(s); 

Answer 2

En utilisant stream et filter vous pouvez facilement atteindre cet objectif

    students.entrySet() 
         .stream() 
         .filter(.. some predicate...) 
         .map(Map.Entry::getValue) 
         .collect(Collectors.toList()) 

Si vous êtes sûr que vous allez obtenir au plus un élément unique qui a passé le filtre (qui est garanti par votre filtre), vous pouvez utiliser findFirst:

Optional<List> o = students.entrySet() 
         .stream() 
         .filter(e -> e.getKey() == 1) 
         .map(Map.Entry::getValue) 
         .findFirst(); 

Dans le cas général, si le filtre peut correspondre à plusieurs listes, vous pouvez les chercher à une liste des listes:

List<List> list = students.entrySet() 
        .stream() 
        .filter(.. some predicate...) 
        .map(Map.Entry::getValue) 
        .collect(Collectors.toList());