How does the garbage collection mechanism determine that an object needs to be removed from memory?

The Garbage Collector mechanism determines that an object should be removed from memory based on whether this object is accessible to the program. If there are no more references to the object from active code (i.e., it is unreachable), then it is considered unnecessary and can be deleted.

The main method of determination is to traverse the object graph starting from root references (such as the call stack, static variables). All objects reachable through references are considered alive. Objects that cannot be reached are considered garbage and are subject to removal.

An example of a simplified algorithm:

• Start with root objects.
• Mark all objects reachable from the roots.
• All unmarked objects are considered garbage.
• Free the memory occupied by unmarked objects.

In Android (based on JVM), algorithms like Mark-and-Sweep and other optimizations are used.