java assertEquals sets

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?

See also: [Overriding equals and hashCode in Java](http://stackoverflow.com/questions/27581) — McDowell, Sep 04 '12 at 10:32
@McDowell: thanks! I knew that if hashCode returns different values for 2 objects then there is no chance of getting a true from the equals call. I was in a hurry! :) — Razvan, Sep 04 '12 at 10:36

score 13 · Accepted Answer · answered Sep 04 '12 at 10:27

13

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.

answered Sep 04 '12 at 10:27

Jon Skeet

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Which means that if `equals` returns true, `hashCode` has to return true as well (only in this direction). – brimborium Sep 04 '12 at 10:33

score 0 · Answer 2 · answered Sep 04 '12 at 10:59

If you check the source code of HashSet contains() method calls containsKey() which calls getEntry(). As per the below source code it is clear that a proper hashcode() implementation is needed to call equals.

/**
 * Returns the entry associated with the specified key in the
 * HashMap.  Returns null if the HashMap contains no mapping
 * for the key.
 */
final Entry<K,V> getEntry(Object key) {
    int hash = (key == null) ? 0 : hash(key.hashCode());
    for (Entry<K,V> e = table[indexFor(hash, table.length)];
         e != null;
         e = e.next) {
        Object k;
        if (e.hash == hash &&
            ((k = e.key) == key || (key != null && key.equals(k))))
            return e;
    }
    return null;
}

PS: Contains() method is used from equals of AbstractCollection

java assertEquals sets

2 Answers2