How does the automatic memory management mechanism determine that a variable is no longer used and can be freed?

The automatic memory management mechanism, such as the garbage collector in Java, determines that a variable is no longer used when there are no active references from root objects to the object it points to.

This happens as follows:

• The garbage collector starts with root objects (call stack, static variables, active threads).
• It marks all objects reachable through references from these roots.
• Objects that are not marked are considered unreachable and can be removed.

Thus, if a variable or object is not accessible from active code, the garbage collector will free the memory occupied by them.

Example:

Object obj = new Object();
obj = null; // now the object is unreachable and can be collected