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Garbage Collection

John Paterson
Ranch Hand

Joined: Mar 12, 2012
Posts: 168
Hi Friends,

I have some difficulty understanding the answer of the first question from chapter 3 of the Kathy Seira SCJP 6 book. The question, given answer and my doubt are as follow:



When // doStuff is reached, how many objects are eligible for GC?
A. 0
B. 1
C. 2
D. Compilation fails
E. It is not possible to know
F. An exception is thrown at runtime

Given answer:

C is correct. Only one CardBoard object (c1) is eligible, but it has an associated Short
wrapper object that is also eligible.

and my doubt is, how did the Short wrapper object, become an 'associated' object?? Hope someone can help. Thanks.

Stephan van Hulst

Joined: Sep 20, 2010
Posts: 4200

Well, the Cardboard object holds on to a Short instance, right? So when the Cardboard instance is eligible, so is the Short instance, because there are no other variables that hold a reference to the Short object.

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Zeeshan Sheikh
Ranch Hand

Joined: Nov 20, 2011
Posts: 144

An object is eligible for garbage collection when there are no more references to that object. The way I would solve these questions is to dry run. Just by looking at the code c3 & c1 will have NULL. Please check out Oracle tutorial.

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Henry Wong

Joined: Sep 28, 2004
Posts: 20370

Please SearchFirst. This question has been asked many times in the past. In fact, a quick search yield about three pages of hits. The first three hits on the first page (not counting this topic) were...


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Hasitha Su

Joined: May 15, 2011
Posts: 3
But what is the reason for that c3 is not eligible for GC?
Hasitha Su

Joined: May 15, 2011
Posts: 3
Hasitha Su wrote:But what is the reason for that c3 is not eligible for GC?
varun Dosapati
Ranch Hand

Joined: Nov 18, 2010
Posts: 31

Post Today 9:57:27 AM Subject: Garbage Collection
But what is the reason for that c3 is not eligible for GC?


I believe you are asking why c3 is not eligible for garbage.
Reason: Because c3 has no reference to existing object and also have not created any instance of new object. So there nothing present for Garbage collection.
Bert Bates

Joined: Oct 14, 2002
Posts: 8898
Terminology is EXTREMELY IMPORTANT. Strictly speaking, c3 is a reference variable. Reference variables are NEVER eligible for garbage collection. As a reference variable, c3 might be referring to an object. At some point, when that object has no "live" reference variables pointing to it, that object can become eligible, but reference variables are never eligible.

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raju salla

Joined: Jan 05, 2012
Posts: 18
Here two objects are eligible for Garbage collection. First of all we are explicitly assigning null to the C1. That's why c1 is eligible for Garbage Collection. Then C3 is assigned by calling method on C1. As c1 is null, c3 is also eligible for garbage collection. So totally 2 objects are eligible for garbage collection.

Thank you.
Bert Bates

Joined: Oct 14, 2002
Posts: 8898
you MUST come up with a concrete way to refer to objects. given that references to objects can be re-assigned, you have to say things like:

"the object referred to by var c1 at line x"

it's NOT sufficiently precise to say "c1 is eligible for GC".

c1 is a reference var, it's NOT an object. as the code runs, c1 might start off by referring to one object, and then later on it might be changed to refer to a different object.

does that help?
Rahul Parakkat

Joined: Feb 13, 2012
Posts: 8

When ever i took any practise scjp test, i got vey low score in garbage collection. After reading this post, i am lot more clear on the idea of GC.

Thanks a lot Bert Bates & Kathy Sierra. I am reading your book on SCJP 1.6

Its little more fun here :

Rahul Parakkat
I agree. Here's the link:
subject: Garbage Collection
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