How does the garbage collector determine that an object is no longer used and can be removed from memory?

The garbage collector in Java determines that an object is no longer in use based on the concept of reachability. If an object is unreachable from root references (GC roots), such as active threads, static variables, or local variables on the stack, it is considered a candidate for removal.

The process roughly works as follows:

1. The collector starts with root objects and marks all objects reachable through references.
2. All objects not marked as reachable are considered unreachable.
3. Unreachable objects are removed, freeing memory.

Thus, if there are no references to an object from active code, it will be collected.

It is important to note that the garbage collector does not rely on reference counting (although some implementations may use hybrid approaches), but on analyzing the reachability graph, which allows it to correctly handle cyclic references.