GC roots and the reachability lattice

Three questions decide when an object can die: who can reach it? (the collector starts from GC roots — stacks, statics, live threads), how urgently might it be needed? (the reachability lattice), and when will it be collected? (a policy question, never a guarantee).

Object strong  = new Object();               // reachable — lives
WeakReference<Object> weak = new WeakReference<>(new Object());
System.gc();                                 // weak target: unreachable → gone
weak.get()                                   // now null (usually immediately;
                                             // soft refs get breathing room first)

The lattice: strong (an ordinary reference — never collected while reachable) → soft (collected only under memory pressure; good for caches that may survive) → weak (collected at the next GC; good for metadata and canonical maps) → phantom (enqueues cleanup after death; replacement for finalization).

Two disciplines follow. First: System.gc() is a hint — tests and programs must be written so that correctness never depends on a collection happening, only on what collection means when it does. Second: value caches belong in WeakHashMap/weak keys precisely because keys reachable nowhere else should not pin their values.