wj. wj. - 3 months ago 9
Java Question

Overriding equals() & hashCode() in sub classes ... considering super fields

Is there a specific rule on how Overriding

equals()
&
hashCode()
in sub classes considering super fields ?? knowing that there is many parameters : super fields are private/public , with/without getter ...

For instance, Netbeans generated equals() & hashCode() will not consider the super fields ... and

new HomoSapiens("M", "80", "1.80", "Cammeron", "VeryHot").equals(
new HomoSapiens("F", "50", "1.50", "Cammeron", "VeryHot"))


will return true :(

public class Hominidae {

public String gender;
public String weight;
public String height;

public Hominidae(String gender, String weight, String height) {
this.gender = gender;
this.weight = weight;
this.height = height;
}
...
}

public class HomoSapiens extends Hominidae {
public String name;
public String faceBookNickname;

public HomoSapiens(String gender, String weight, String height,
String name, String facebookId) {
super(gender, weight, height);
this.name = name;
this.faceBookNickname = facebookId;
}
...
}


If you want to see the Netbeans generated equals() & hashCode() :

public class Hominidae {

...

@Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final Hominidae other = (Hominidae) obj;
if ((this.gender == null) ? (other.gender != null) : !this.gender.equals(other.gender)) {
return false;
}
if ((this.weight == null) ? (other.weight != null) : !this.weight.equals(other.weight)) {
return false;
}
if ((this.height == null) ? (other.height != null) : !this.height.equals(other.height)) {
return false;
}
return true;
}

@Override
public int hashCode() {
int hash = 5;
hash = 37 * hash + (this.gender != null ? this.gender.hashCode() : 0);
hash = 37 * hash + (this.weight != null ? this.weight.hashCode() : 0);
hash = 37 * hash + (this.height != null ? this.height.hashCode() : 0);
return hash;
}

}


public class HomoSapiens extends Hominidae {

...

@Override
public boolean equals(Object obj) {
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final HomoSapiens other = (HomoSapiens) obj;
if ((this.name == null) ? (other.name != null) : !this.name.equals(other.name)) {
return false;
}
if ((this.faceBookNickname == null) ? (other.faceBookNickname != null) : !this.faceBookNickname.equals(other.faceBookNickname)) {
return false;
}
return true;
}

@Override
public int hashCode() {
int hash = 7;
hash = 89 * hash + (this.name != null ? this.name.hashCode() : 0);
hash = 89 * hash + (this.faceBookNickname != null ? this.faceBookNickname.hashCode() : 0);
return hash;
}
}

Answer

Children should not examine the private members of their parents

But obviously, all significant fields should be taken into account for equality and hashing.

Fortunately, you you can easily satisfy both rules.

Assuming you're not stuck using the NetBeans-generated equals and hashcode, you can modify Hominidae's equals method to use instanceof comparison rather than class equality, and then use it straightforwardly. Something like this:


    @Override  
    public boolean equals(Object obj) {  
        if (obj == null) { return false; }  
        if (getClass() != obj.getClass()) { return false; }  
        if (! super.equals(obj)) return false;
        else {
           // compare subclass fields
        }

Of course, hashcode is easy:


    @Override     
    public int hashCode() {     
        int hash = super.hashCode();
        hash = 89 * hash + (this.name != null ? this.name.hashCode() : 0);     
        hash = 89 * hash + (this.faceBookNickname != null ? this.faceBookNickname.hashCode() : 0);     
        return hash;     
    }     

Seriously, though: what's up with NetBeans not taking superclass fields into account by calling the superclass methods?