I read that an object can be strongly, weakly or even phantom (!!!!) reachable..

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Question: I read that an object can be strongly, weakly or even phantom (!!!!) reachable... ???

Answer: I found the definitions in API. I think it is clear enough to be cited here:

Going from strongest to weakest, the different levels of reachability reflect the life cycle of an object. They are operationally defined as follows:

• An object is strongly reachable if it can be reached by some thread without traversing any reference objects. A newly-created object is strongly reachable by the thread that created it.
• An object is softly reachable if it is not strongly reachable but can be reached by traversing a soft reference.
• An object is weakly reachable if it is neither strongly nor softly reachable but can be reached by traversing a weak reference. When the weak references to a weakly-reachable object are cleared, the object becomes eligible for finalization.
• An object is phantom reachable if it is neither strongly, softly,
  nor weakly reachable, it has been finalized, and some phantom
  reference refers to it.
• Finally, an object is unreachable, and therefore eligible for
  reclamation, when it is not reachable in any of the above ways.

About phantom reference please read more in API (java.lang.ref.PhantomReference)

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