When we try to put new values, how does it understand whether to add a new record or update an existing one?

In Java, when we add a new value to a Map collection using the put(key, value) method, the behavior depends on whether an entry with the same key already exists.

Mechanism of determination:

• Map uses the hashCode() method of the key to find the bucket for storage.
• Within the bucket, it searches for the key using the equals() method.

If the key already exists (an element with the same hashCode() is found and equals() returns true), then:

• The old value is replaced with the new one.
• The put method returns the old value.

If the key does not exist:

• A new entry with this key and value is created.
• The put method returns null.

Thus, the logic of adding or updating depends on the implementation of hashCode() and equals() for the key. Proper implementation of these methods is critical for the correct operation of collections.

Example:

Map<String, Integer> map = new HashMap<>();
map.put("key1", 1); // adds a new entry
map.put("key1", 2); // updates the value for "key1"