Explain the four states of entities in Hibernate and how it relates to the need to call save().

In Hibernate, entities can be in four states:

1. Transient — the object is created in memory but not associated with a Hibernate session and not saved to the database.
2. Persistent — the object is associated with a session, changes are tracked and will be synchronized with the database.
3. Detached — the object was previously persistent but the session is closed, and the object is no longer tracked.
4. Removed — the object is marked for deletion from the database.

The save() method is used to transition a transient object to persistent, i.e., to save a new object to the database. If the object is already persistent, save() is not needed as changes are automatically tracked. For detached objects, update() or merge() can be used to re-associate them with a session.

Thus, the need to call save() depends on the current state of the entity: it is mandatory for new (transient) objects, but not for already managed (persistent) ones.