How does the garbage collection mechanism determine which objects are no longer used in memory?

The garbage collection mechanism (Garbage Collector) determines which objects are no longer in use by tracking reachability of objects from root references. If an object is not reachable from any root reference (for example, from the stack, global variables, or registers), it means there are no references to it in the program, and it is considered garbage.

An example of a method is the depth-first search algorithm (mark-and-sweep):

1. All objects reachable from the roots are marked.
2. All unmarked objects are considered unused and are freed.

Thus, the garbage collector automatically frees the memory occupied by objects that the program no longer uses.