What happens when you put a key-value pair in a HashMap where the key already exists?

When you put a key-value pair in a HashMap where the key already exists, the HashMap will replace the existing value with the new value associated with the key. Here are the steps that occur:

1. The HashMap checks if the key already exists in its internal data structure.
2. If the key is found, the HashMap updates the value associated with that key to the new value provided.
3. If the key is not found, the HashMap adds the new key-value pair to its internal data structure.
4. The HashMap uses the key's hash code to determine the bucket where the key-value pair should be stored.
5. If there is already a key-value pair in the bucket with the same key, the HashMap replaces the existing value with the new value.
6. If there are no conflicts, the HashMap adds the key-value pair to the bucket.
7. The HashMap may also resize its internal data structure if necessary to maintain a good balance between the number of elements and the number of buckets.

Overall, when a key already exists in a HashMap, the existing value is replaced with the new value.