I have a quick question about [`TreeSet`][1] collections and [`hashCode`][2] methods. I have a `TreeSet` and I'm adding objects to it, before I add an object, I check to see if it exists in the `TreeSet` using the [`contains`][3] method. I have 2 distinct objects, each of which produce a distinct hashCode using my implementation of the hashCode method, example below: public int hashCode() { int hash = 7; hash = hash * 31 + anAttribute.hashCode(); hash = hash * 31 + anotherAttribute.hashCode(); hash = hash * 31 + yetAnotherAttribute.hashCode(); return hash; } The hashCodes for a particular run are: 76126352 and 76126353 (the objects only differ by one digit in one attribute). The contains method is returning true for these objects, even though the hashCodes are different. Any ideas why? This is really confusing and help would really be appreciated. [1]: http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html [2]: http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode-- [3]: http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html#contains-java.lang.Object-