Java two Set objects - hashCode() are the same but .equals() fails

Question

I have two sets (LinkedHashSet) of a custom objects which overrides the hashCode() and equals() methods.

When comparing these two sets the equal() method fails even though the hash codes are the same.

    System.out.println("cmContacts.equals(cm2Contacts):" + cmContacts.equals(cm2Contacts));
    System.out.println("cm2Contacts.equals(cmContacts):" + cm2Contacts.equals(cmContacts));
    System.out.println("compare hash codes:" + ( cm2Contacts.hashCode() == cmContacts.hashCode() ) );

This returns:

cmContacts.equals(cm2Contacts):false

cm2Contacts.equals(cmContacts):false

compare hash codes:true

According to the documentation if the hash codes are the same the equals method should return true.

The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object.hashCode().

https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#equals(java.lang.Object)

Any ideas why this is happening?

equals() does not *depend* on hashCode(). It's up to the implementer to ensure consistent behaviour. AKA there's something wrong with your custom objects. — Oliver Charlesworth, Mar 06 '16 at 11:11
Due to hash collisions, unequal objects may very well have the same hash code. — Mick Mnemonic, Mar 06 '16 at 11:19
The documentation you quoted says exactly the opposite of what you have claimed. — user207421, Mar 06 '16 at 11:22
I get it now, there was a mistake in the overridden equals method. Will accept an answer as soon as I can — DavidA, Mar 06 '16 at 11:23

score 5 · Accepted Answer · answered Mar 06 '16 at 11:12

5

You've got it backwards: if two objects are equal, they must have the same hash code. Having the same hash code does not imply that the objects should be equal.

answered Mar 06 '16 at 11:12

Joni

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Yassin Hajaj · Answer 2 · 2016-03-06T11:17:35.213

1

Same hashcode() does not mean equals.

You can compare that to ZipCode. You and your neighbhour have the same but are not the same.

edited Mar 06 '16 at 11:17

answered Mar 06 '16 at 11:12

Yassin Hajaj

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Java two Set objects - hashCode() are the same but .equals() fails

2 Answers2