How does the garbage collector determine that an object can be destroyed?

The garbage collector in Android (JVM) uses the Reachability Analyzers algorithm. It determines the reachability of objects from "roots" (GC roots).

An object cannot be destroyed if it is reachable from any GC root. An object can be destroyed if it is unreachable from any GC root.

Examples of GC roots:

• Local variables in active thread methods.
• Static variables of classes.
• References from JNI (Java Native Interface) code.
• Threads.

The algorithm works in several stages:

1. Marking: The garbage collector traverses the object graph starting from GC roots and marks all reachable objects.
2. Sweeping: After marking, the garbage collector finds all unmarked (unreachable) objects and deletes them, freeing memory.

More modern garbage collectors may use other techniques (e.g., generations), but the basic principle of reachability determination remains key.