Final answer:
The extra work at the end of the clone method is to perform a deep copy of objects to prevent the cloned object from sharing references with the original object. Problems arise from a shallow copy when changes to the clone's sub-object also affect the original due to shared references.
Step-by-step explanation:
When implementing the clone method in object-oriented programming, particularly in Java, it is important to perform a deep copy of objects that have references to other objects. If this extra step is omitted, only a shallow copy is performed, leading to the problem of the cloned object still holding references to the same sub-objects as the original. This can result in unintended side-effects, as modifications to the sub-objects in the cloned instance would affect the original instance as well.
Deep vs. Shallow Copy Issue
The essential extra work at the end of the clone method is to ensure that all mutable objects that the original object references are also cloned, thereby creating a unique and separate state for the cloned object. Without this, both the original and the clone will point to the same references for these mutable fields, which often leads to errors and bugs that are difficult to trace when data is changed in one instance and unexpectedly affects the other.
Example of Potential Problems
Consider an object called Book that has a reference to a mutable object called Author. If we only perform a shallow copy, both the original Book object and its clone will reference the same Author object. If the clone modifies the Author's name, this will also change the Author's name in the original Book, which is usually undesired behavior.