Question

I have two sets:

 Set<Attribute> set1 = new HashSet<Attribute>(5);
 Set<Attribute> set2 = new HashSet<Attribute>(5);

 //add 5 attribute objects to each of them. (not necessarily the same objects)


 assertEquals(set1,set2); //<--- returns false, even though 
                         //the added attribute objects are equal

The equals method of Attribute is overridden, according to my requirements:

public abstract class Attribute implements Serializable{

public int attribute;

public abstract boolean isNumerical();

@Override
public boolean equals(Object other){
    if(!(other instanceof Attribute)){
        return false;
    }

    Attribute otherAttribute = (Attribute)other;
    return (this.attribute == otherAttribute.attribute && 
            this.isNumerical() == otherAttribute.isNumerical());
}

}

when debugging, the equals method is not even called!

Any ideas?

Answer 1

You’re not overriding hashCode(), which means the default implementation will be used. HashSet checks for matching hash codes first, before calling equals – that’s how it manages to find potential matches so efficiently. (It’s easy to “bucket” an integer.)

Basically, you need to override hashCode in a manner which is consistent with your equals method.