Recognize the point in a piece of source code at which an object becomes eligible for garbage collection.
Read subsections "Reachable References" and "Facilitating Garbage Collection" in Section 8.1.
An object becomes eligible for garbage collection when it has no references from running code. If a reference is reassigned, the object previously denoted by the reference now has one less reference, increasing this object's chances of becoming eligible for garbage collection. It is important to be able to identify where in the code an object will have no references during execution.