Does adding a duplicate value to a HashSet/HashMap replace the previous value

Question

In the case of HashMap, it replaces the old value with the new one.

In the case of HashSet, the item isn't inserted.

The first thing you need to know is that HashSet acts like a Set, which means you add your object directly to the HashSet and it cannot contain duplicates. You just add your value directly in HashSet.

However, HashMap is a Map type. That means every time you add an entry, you add a key-value pair.

In HashMap you can have duplicate values, but not duplicate keys. In HashMap the new entry will replace the old one. The most recent entry will be in the HashMap.

Understanding Link between HashMap and HashSet:

Remember, HashMap can not have duplicate keys. Behind the scene HashSet uses a HashMap.

When you attempt to add any object into a HashSet, this entry is actually stored as a key in the HashMap - the same HashMap that is used behind the scene of HashSet. Since this underlying HashMap needs a key-value pair, a dummy value is generated for us.

Now when you try to insert another duplicate object into the same HashSet, it will again attempt to be insert it as a key in the HashMap lying underneath. However, HashMap does not support duplicates. Hence, HashSet will still result in having only one value of that type. As a side note, for every duplicate key, since the value generated for our entry in HashSet is some random/dummy value, the key is not replaced at all. it will be ignored as removing the key and adding back the same key (the dummy value is the same) would not make any sense at all.

Summary:

HashMap allows duplicate values, but not keys. HashSet cannot contains duplicates.

To play with whether the addition of an object is successfully completed or not, you can check the boolean value returned when you call .add() and see if it returns true or false. If it returned true, it was inserted.

The docs are pretty clear on this: HashSet.add doesn't replace:

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

But HashMap.put will replace:

If the map previously contained a mapping for the key, the old value is replaced.

It the case of HashSet, it does NOT replace it.

From the docs:

http://docs.oracle.com/javase/6/docs/api/java/util/HashSet.html#add(E)

"Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false."

Correct me if I'm wrong but what you're getting at is that with strings, "Hi" == "Hi" doesn't always come out true (because they're not necessarily the same object).

The reason you're getting an answer of 1 though is because the JVM will reuse strings objects where possible. In this case the JVM is reusing the string object, and thus overwriting the item in the Hashmap/Hashset.

But you aren't guaranteed this behavior (because it could be a different string object that has the same value "Hi"). The behavior you see is just because of the JVM's optimization.

HashMap basically contains Entry which subsequently contains Key(Object) and Value(Object).Internally HashSet are HashMap and HashMap do replace values as some of you already pointed..but does it really replaces the keys???No ..and that is the trick here. HashMap keeps its value as key in the underlying HashMap and value is just a dummy object.So if u try to reinsert same Value in HashMap(Key in underlying Map).It just replaces the dummy value and not the Key(Value for HashSet).

Look at the below code for HashSet Class:

public boolean  [More ...] add(E e) {

   return map.put(e, PRESENT)==null;
}

Here e is the value for HashSet but key for underlying map.and key is never replaced. Hope i am able to clear the confusion.