Question

I have this class that everytime I call the constructor should increment the id

public class Candidato {
    private String nome;
    private String cognome;
    private final int id;                 // id that tracks number of objects
    private static int counter=0;         //counter that increments the id
    private static int counterd=0;
    private static int counters=0;
    private static int counterp=0;
    private double voti;
    private int doc;
    private int stud;
    private int pta;

    public Candidato(String n,String c){
        this.id=counter++;           //everytime i call the constructor increment me
        this.nome=n;
        this.cognome=c;
        System.out.println(counter+"- -");  // here 
    }
          public int getNumero(){
        //System.out.println(id+"--id--");
            return id+1;
          }
}  

So my doubts arrise here : Suppose I got a class Elections that calls the Candidato constructor, how can i keep track of the Candidato id everytime a new instance of elections gets created ?

Like in this example at testNumeriCandiati() i get some strange numbers as they are not the expected ones, therefore everytime I create an Election class I want to set the Candidato id back to 0

    public class TestR2_Candidati extends TestCase {

        public void testCandidato(){
            Elezione sistema = new Elezione();

            Candidato c = sistema.nuovoCandidato("Marco", "Gilli");      
            assertNotNull("metodo nuovoCandidato() non implemenato",c);
            assertEquals("Marco",c.getNome());
            assertEquals("Gilli",c.getCognome());
            assertEquals(1,c.getNumero());
        }

    public void testGetCandidato(){
        Elezione sistema = new Elezione();

        Candidato c = sistema.nuovoCandidato("Marco", "Gilli");

        Candidato cc = sistema.getCandidato(1);

        assertNotNull("metodo getCandidato() non implemenato",cc);

        assertSame("Non viene restituito lo stesso candidato",c,cc);
    }

    public void testNumeriCandiati(){
        Elezione sistema = new Elezione();

        Candidato c1 = sistema.nuovoCandidato("Marco", "Gilli");
        Candidato c2 = sistema.nuovoCandidato("Francesco", "Profumo");
        Candidato c3 = sistema.nuovoCandidato("Rodolfo", "Zich");

        assertEquals("Non corrisponde il numero",1,c1.getNumero());
        assertEquals("Non corrisponde il numero",2,c2.getNumero());
        assertEquals("Non corrisponde il numero",3,c3.getNumero());
    }
}

Answer 1

Your counter is stored as a static variable of Candidato, so its value does not depend on creation of instances of Elezione. If you want this funcionality you need to manage the counter of Candidato into Elezione:

class Elezione {
    private candidatoCounter = 0;
    ................

    Candidato createCandidato() {
        return new Candidato(candidatoCounter++);
    }
}

class Candidato {
    private int id;
    Candidato(int id) {
        this.id = id;
    }

}