How can I take into consideration the object itself when calculating a hash for an object in Java?-CodePudding

I was working on some algorithmic problems when I got to this and it seemed interesting to me. If I have two lists (so two different objects), with the same values, the hashcode is the same. After some reading, I understand that this is how it should behave. For example:

        List<String> lst1 = new LinkedList<>(Arrays.asList("str1", "str2"));
        List<String> lst2 = new LinkedList<>(Arrays.asList("str1", "str2"));
        System.out.println(lst1.hashCode()   " "   lst2.hashCode());
        ...........
        Result: 2640541 2640541

My purpose would be to differentiate between lst1 and lst2 in a list for example.

Is there a structure (like a HashSet for example) that takes into consideration the actual object and not only the values inside the object when calculating the hashcode for something?

CodePudding user response：

Yes, you can use java's java.util.IdentityHashMap, or guava's identity hash set.

The hashes of the two lists must be equal, because the objects are equal. But the identity map and set above are based on the identity of the list objects, not their hash.

CodePudding user response：

Not generally in collections, because you generally want two collections with all the same items to be equal (which is why they implement it like this- equals will return true and the hash codes are the same).

You can subclass a list and have it not do that, it would just not be widely useful and would cause a lot of confusion if other programmers read your code. In that case, you'd just want equals to return the result of == and hashCode to return the integer value of the reference (the same thing that Object.equals does).