What is the difference between a shallow copy and a deep copy, and why does it matter when writing a clone() method?

A shallow copy duplicates an object's top-level fields, but if a field is a reference to a mutable object, both the original and the copy end up pointing at that same shared object. A deep copy goes further and recursively clones any mutable referenced object too, so the copy is fully independent. This matters because a shallow copy creates a hidden coupling: mutating the 'copy' can silently mutate the original, since they're really sharing state underneath. When writing a clone() method, every mutable reference field needs to be explicitly cloned rather than just reassigned, while genuinely immutable fields like Strings or primitives are safe to copy by value directly.