how do I correctly override equals for inheritance in java?

问题

I am using hibernate and id... is used for persistence (which is why it is omitted in comparison). (Also, using google guava helper equals)

HolidayPackageVariant:

public abstract class HolidayPackageVariant {
    private Integer idHolidayPackageVariant;
    private HolidayPackage holidayPackage;
    private String typeHolidayPackage;

@Override
public boolean equals(Object obj) {
    if (obj == this)
        return true;
    if(obj == null)
        return false;

    if (getClass().equals(obj.getClass())) {
        final HolidayPackageVariant otherPackageVariant = (HolidayPackageVariant) obj;
        return Objects.equal(getTypeHolidayPackage(),otherPackageVariant.getTypeHolidayPackage())
                && Objects.equal(getHolidayPackage(),
                        otherPackageVariant.getHolidayPackage());
    }
    return false;
}

@Override
public int hashCode() {
    return Objects.hashCode(getTypeHolidayPackage(), getHolidayPackage());
}

FlightHolidayPackageVariant:

public final class FlightHolidayPackageVariant extends HolidayPackageVariant{
    private Destination originCity;
    public boolean equals(Object obj) {
    // .. 

Should I completely override the equals() or should I be invoking super.equals(...) in some way ?

回答1:

This is how you should implement data objects: no inheritance allowed, composition only.

These are immutable objects, but you could want to modify them, so remove final where needed. Also you may remove final from the class definition because Hibernate doesn't support it, but in that case, you should document that these classes are not eligible for inheritance.

The main advantages are all the ones described in Effective Java plus, in this case, you don't have to manage inheritance through Hibernate which can sometimes be a real pain.

public final class HolidayPackageVariant {
  private final Integer idHolidayPackageVariant;
  private final HolidayPackage holidayPackage;
  private final String typeHolidayPackage;

  ...

  @Override
  public boolean equals(Object obj) {
    if (obj == this)
      return true;
    if (!(obj instanceof HolidayPackageVariant))
      return false;

    HolidayPackageVariant that = (HolidayPackageVariant) obj;
    return Objects.equal(this.typeHolidayPackage, that.typeHolidayPackage)
        && Objects.equal(this.holidayPackage, that.holidayPackage);
  }

  @Override
  public int hashCode() {
    return Objects.hashCode(this.typeHolidayPackage, this.holidayPackage);
  }
}

public final class FlightHolidayPackageVariant {
  private HolidayPackageVariant holidayPackageVariant;
  private Destination originCity;

  ...

  public HolidayPackageVariant asHolidayPackageVariant() {
    return this.holidayPackageVariant;
  }

  public boolean equals(Object obj) {
    if (obj == this)
      return true;
    if (!(obj instanceof FlightHolidayPackageVariant))
      return false;

    FlightHolidayPackageVariant that = (FlightHolidayPackageVariant) obj;
    return Objects.equal(this.holidayPackageVariant, that.holidayPackageVariant)
        && Objects.equal(this.originCity, that.originCity);
  }

  @Override
  public int hashCode() {
    return Objects.hashCode(this.holidayPackageVariant, this.originCity);
  }
}

回答2:

Following the Secret of Equals:

HolidayPackageVariant:

public abstract class HolidayPackageVariant {
    private Integer idHolidayPackageVariant;
    private HolidayPackage holidayPackage;
    private String typeHolidayPackage;

    @Override
    public boolean equals(Object obj) {
        if (obj == this) return true;
        if(obj == null) return false;

        if (getClass().equals(obj.getClass())) {
            final HolidayPackageVariant otherPackageVariant = (HolidayPackageVariant) obj;
            return Objects.equal(getTypeHolidayPackage(),otherPackageVariant.getTypeHolidayPackage())
                    && Objects.equal(getHolidayPackage(),
                            otherPackageVariant.getHolidayPackage());
        }
        return false;
    }
}

---

FlightHolidayPackageVariant:

public final class FlightHolidayPackageVariant extends HolidayPackageVariant{
    private Destination originCity;

    @Override
    public boolean equals(Object obj) {

        if(super.equals(obj)){
            return Objects.equal(getOriginCity(),
                    ((FlightHolidayPackageVariant)(obj)).getOriginCity());
        }
        return false;
    }
}

This will ensure that only the same Type of variants are equal to each other.

来源：https://stackoverflow.com/questions/9885773/how-do-i-correctly-override-equals-for-inheritance-in-java