Is it proper equals method for my class Java

Question

As in subject: Is it proper equals method for my class Java? I have generated it automaticly by Eclipse I don't know if it makes vector.remove(pracownik) will work correctrly. Or is it wrong to generate it by Eclipse?

import java.util.Date;
import java.util.Vector;

public class Pracownik extends Osoba {
    private String stanowisko;
    private int pensja;
    private Date dataZatrudnienia;

    public Pracownik(Adres adres, String telefon, String imie, String nazwisko,
            int id, Date dataUrodzenia, String stanowisko, int pensja,
            Date dataZatrudnienia) {
        super(adres, telefon, imie, nazwisko, id, dataUrodzenia);
        this.stanowisko = stanowisko;
        this.pensja = pensja;
        this.dataZatrudnienia = dataZatrudnienia;
    }

    public String getStanowisko() {
        return stanowisko;
    }

    public int getPensja() {
        return pensja;
    }

    public Date getDataZatrudnienia() {
        return dataZatrudnienia;
    }

    @Override
    public String toString() {
        return super.toString() + "\nstanowisko=" + stanowisko + "\npensja="
                + pensja + "\ndataZatrudnienia=" + dataZatrudnienia;
    }

    private static Vector<Pracownik> ekstensja = new Vector<Pracownik>();//kolekcja zawierajaca ekstensje

    private static void dodajPracownik(Pracownik pracownik) { //metoda dodajac aobiekt do ekstensji
        ekstensja.add(pracownik);
    }

    private static void usunPracownik(Pracownik pracownik) {//metoda usuwajaca obiekt z ekstensji
        ekstensja.remove(pracownik);
    }



    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Pracownik other = (Pracownik) obj;
        if (dataZatrudnienia == null) {
            if (other.dataZatrudnienia != null)
                return false;
        } else if (!dataZatrudnienia.equals(other.dataZatrudnienia))
            return false;
        if (pensja != other.pensja)
            return false;
        if (stanowisko == null) {
            if (other.stanowisko != null)
                return false;
        } else if (!stanowisko.equals(other.stanowisko))
            return false;
        return true;
    }

    private static void pokazEkstensje(){ //wyswietlenie ekstensji przy pomocy petli for each
        System.out.println("Ekstensja klasy Pracownik");
        for(Pracownik pracownik: ekstensja)
            System.out.println(pracownik);
        System.out.println();
    }

    public static void main(String[] args){
        Adres adres = new Adres("tara", "588 m.42", "03-422", "Warszawa");
        Pracownik pracownik = new Pracownik(adres, "02-6451-4564", "Ala", "Kotowa", 323, new Date(), "szef", 14000, new Date()); //tworze pracownika
        System.out.println(pracownik);//wyswietlam pracowanika

        //tworze stazyste
        Stazysta stazysta = new Stazysta(adres, "3232 9898", "frajer", "costam", 3232, new Date(), "podawanie kawy", 0, new Umowa(new Date(2010,10,5), new Date(2011,11,8)));
        //wysswietlam stazyste
        System.out.println(stazysta);
    }



}

Answer 1

Generating the equals method using Eclipse is fine. It is important to make sure a field is included in the generation if, and only if, it affects logical equality of your objects.

When you override equals, you should also override hashCode.

In general, when overriding inherited methods you need to ensure the new methods conform to any rules stated in the superclass. The Object hashCode documentation states several rules, including "If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result."

The hashCode you inherit from Object follows that rule - if the equals method is also the one inherited from Object. It does not follow that rule when used with your equals method.

If you let Eclipse do its "Generate hashCode() and equals()" thing, it will get it right. If you write an equals method manually, you need to write your own hashCode to match.

As a practical matter, a class that does not follow the Object hashCode contract is a trap for future reuse. Hashed data structures, such as HashMap and HashSet, may fail to find an object that is actually present if it has a broken hashCode method. One lesson I've learned the hard way is that it is a mistake to depend on "I'll never use this that way.". It is much better to keep things safe as one goes along.

Answer 2

I am not expert on this but I believe double equals are comparing address rather than content of a variable. So, you may change it to .equals(). I hope someone with much more experience correct me if I am wrong.