Multiple values ​​in the keys of a TreeMap

TreeMapand Map, are collections that do not allow repeated values ​​in their keys, for a key there can only be one value. The difference is that it TreeMapis a class that implements the interface Mapbut works with the natural order of the keys instead of using equalsand hashCodeas they do HashMapand LinkedHashMap. In order for it to TreeMapknow the natural ordering of the keys, the key class must either implement the interface Comparableor be assigned an instance of Comparatorin the constructor that tells it how to compute the order.

In addition Map, they accept all types of objects, so I would change your Map<Integer, String> datos = new TreeMap<>();to Map<Integer, Alumnado> datos = new TreeMap<>();y in your method darDeAlta()is when I would put the object in the collection using the registration (which I understand will be unique for each object) as a key for each student.

....
alum = new Alumnado(n, m, d);
datos.put(m,alum);
System.out.println("Se han dado de alta los datos :)");
....

Also mention that your Map, should not be instantiated in the class Alumnado, the class Alumnadois a class that represents an object of type student, but in a student, you do not want information from more students, only theirs, that is why the collection should be in the class Ejercicio, which is the one that is going to be in charge of doing all the high/low/modification operations.

class Alumnado {

private String nombre;
private int matricula;
private double calificaciones[];


Alumnado() {
}

Alumnado(String nombre, int matricula, double calificaciones[]) {
    this.calificaciones = calificaciones;
    this.matricula=maticula;
    this.calificaciones=calificaciones;

}

String getNombre() {
    return nombre;
}

int getMat() {
    return matricula;
}

double[] getCalif() {
    return calificaciones;
}

void setCalif(double calif[]) {
    calificaciones = calif;
}

}

public class Ejercicio {

Map<Integer, Alumnado> datos = new TreeMap<Integer, Alumnado>();

public static boolean darDeAlta() {
    Scanner y = new Scanner(System.in);
    boolean b;
    double d[] = null;
    Alumnado alum;
    System.out.println("Introduzca el nombre del estudiante");
    String n = y.nextLine();
    System.out.println("Introduzca su matricula (de 5 digitos)");
    int m = y.nextInt();
    System.out.println("Introduzca las calificaciones \n");
    for (int i = 0; i < 5; i++) {
        System.out.println("Introduzca la calificacion");
        d[i] = y.nextDouble();
    }
    alum = new Alumnado(n, m, d);
    datos.put(m,alum);
    System.out.println("Se han dado de alta los datos :)");

    System.out.println("Desea ingresar mas alumnos?");
    String s = y.nextLine();
    if (s.equals("si")) {
        b = true;
    } else {
        b = false;
    }
    return b;

}

public static void darDeBaja(String b) {
    Scanner y = new Scanner(System.in);
    System.out.println("Introduzca la matricula del alumno");
    int m = y.nextInt();
    if (datos.containsKey(m)) {
        datos.remove(m);
        System.out.println("Se ha dado de baja al alumno");
    } else {
        System.out.println("No existe esa matricula");
    }

}

public static void consultas() {
    Scanner y = new Scanner(System.in);
    System.out.println("Introduzca la matricula del estudiante a 
   buscar");
    int k = y.nextInt();
    if (datos.containsKey(k)) {
        System.out.println("Los datos son:" + datos.get(k).getNombre() + " " + 
 datos.get(k).getCalif());
    } else {
        System.out.println("El alumno no se encuentra");
    }

}

public static void modificaciones() {
    Scanner y = new Scanner(System.in);
    System.out.println("Introduzca la matricula del estudiante");
    int k = y.nextInt();

    if (datos.containsKey(k)) {

        double calificacionNva[] = null;

        System.out.println("Cuales son las nuevas calificaciones?");
        for (int i = 0; i < 4; i++) {
            double calif = y.nextDouble();
            calificacionNva[i] = calif;
        }
        datos.get(k).setCalif(calificacionNva);
        System.out.println("Se han modificado las calificaciones 
  correctamente");
    } else {
        System.out.println("No existe esa matricula");
    }

}

public static void main(String[] args) {
    Scanner x = new Scanner(System.in);
    boolean bandera;


    if (bandera = true) {

        System.out.println("[a]ltas" + "\n" + "[b]ajas" + "\n" + "
   [c]onsultas" + "\n" + "[m]odificaciones" + "\n" + "[s]alir");
        System.out.println("\n Seleccione una opcion");
        String opcion = x.next();
        if (opcion.compareTo("a") == 0) {
            bandera = darDeAlta();
        }else if (opcion.compareTo("b") == 0) {
            darDeBaja("b");
        }else if (opcion.compareTo("c") == 0) {
            consultas();
        }else if (opcion.compareTo("m") == 0) {
            modificaciones();
        }else if (opcion.compareTo("s") == 0) {
            bandera = false;

        }
    }
}

}

I have made a couple of modifications, see if you see it well.

To start with, from the definition of Mapin the documentation (translated and highlighted by me):

An object that maps keys to values. A Mapcannot contain duplicate keys; each key can be mapped to at most one value

That means your key cannot have two values ​​assigned to it . That is impossible by design. I think that in your case, the key is the enrollment and the multiple values ​​are the characteristics of a specific student... Doesn't it sound more like a mapping of the style matricula -> alumno?

Having said that, I think you have a planning error... You have your Students map in the student class, what is the one you instantiate every time you create a new student?

Think about how it would be in real life: You don't want a group of students for each student there is, you want a single group, in which to put each new student that is created.

Refactor the student class

Your student class has a couple of design errors, in my opinion:

• Shouldn't have a student map: A student has no students inside.
• If it's only going to represent one student, don't call it Student, call it Student. You are not instantiating a group of students, you are instantiating a single student. Names are important.

Your class Alumnowould look like this:

class Alumno {

    private String nombre;
    private int matricula;
    private double calificaciones[];
    
    Alumno() {
    }

    Alumno(String nombre, int matricula, double calificaciones[]) {
        this.calificaciones = calificaciones;
    }

    // Getters y setters...
}

Refactor the Exercise class

This is your main class, but that doesn't mean it can't have attributes. You can either make the attributes static (not a very good idea) or create a new instance of the method mainand call a non-static method on that instance. That is your decision.

In this case, the Mapstudent one is perfect for you. You would have something like this:

public class Ejercicio {
    
    Map<Integer, Alumno> alumnos = new TreeMap<>();

    public boolean darDeAlta() {
        
        // ... Pedir los datos del alumno    
        
        if (alumnos.containsKey(matricula)) {
            return false;
        } else {
            alumnos.put(matricula, new Alumno(nombre, matricula, calificaciones));
            return true;
        }
    }

    // ... otros metodos
}