Hashcode and Equals for Hashset [duplicate]

Answer 1

according jdk source code from javasourcecode.org, HashSet use HashMap as its inside implementation, the code about put method of HashSet is below :

public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

The rule is firstly check the hash, then check the reference and then call equals method of the object will be putted in.

Answer 2

1. There's no need to call equals if hashCode differs.
2. There's no need to call hashCode if (obj1 == obj2).
3. There's no need for hashCode and/or equals just to iterate - you're not comparing objects
4. When needed to distinguish in between objects.

Answer 3

I think your questions will all be answered if you understand how Sets, and in particular HashSets work. A set is a collection of unique objects, with Java defining uniqueness in that it doesn't equal anything else (equals returns false).

The HashSet takes advantage of hashcodes to speed things up. It assumes that two objects that equal eachother will have the same hash code. However it does not assume that two objects with the same hash code mean they are equal. This is why when it detects a colliding hash code, it only compares with other objects (in your case one) in the set with the same hash code.

Answer 4

Because in 2nd case you adding same reference twice and HashSet have check against this in HashMap.put() on which HashSet is based:

        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }

As you can see, equals will be called only if hash of key being added equals to the key already present in set and references of these two are different.

Answer 5

You should read up on how to ensure that you've implemented equals and hashCode properly. This is a good starting point: What issues should be considered when overriding equals and hashCode in Java?