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

The Garbage Collector in Android (based on JVM or ART) determines that an object is no longer in use if there are no active references to it from root objects, such as thread stacks, static fields, etc.

The process is roughly as follows:

• The collector starts with root objects and marks all objects reachable through references.
• All objects that are not marked are considered unreachable and can be deleted.

Thus, if an object is not reachable from active code, it is considered garbage and eligible for collection.

This is called the depth-first traversal or mark-and-sweep algorithm.