How does the garbage collection mechanism determine that an object is no longer accessible and should be deleted?

The garbage collection (GC) mechanism determines that an object is no longer accessible by using reachability tracking algorithms.

The main idea:

• The garbage collector starts with the so-called "root" objects, such as global variables, call stacks, and registers.
• It scans all objects that are referenced from the roots and marks them as reachable.
• Then it recursively marks all objects reachable from the already marked objects.
• Objects that are not marked are considered unreachable (inaccessible) and are subject to removal.

Thus, if there are no references to an object from the roots or reachable objects, it is considered garbage and can be freed.

In Android (for example, in ART), a similar approach is used with additional optimizations to improve performance.