How do garbage collector mechanisms determine which object should be freed from memory?

Garbage collector (GC) mechanisms in Java determine which objects can be freed from memory based on their reachability from root references.

The main principle is reachability-based garbage collection:

• The collector starts with root objects, such as local variables in the stack, static fields, and JNI references.
• Then it recursively traverses all objects reachable from these roots.
• All objects that cannot be reached from the roots are considered unreachable and are eligible for removal.

Example: if an object is not linked to any variable or other object accessible from the current context, it is considered garbage.

Some garbage collectors use additional techniques, such as reference counting or generational tracing, but the basic criterion is an object's reachability from the roots.