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Garbage Collection

John Paterson
Ranch Hand

Joined: Mar 12, 2012
Posts: 132
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:

Question:


Given:

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.

regards
John
Stephan van Hulst
Bartender

Joined: Sep 20, 2010
Posts: 3649
    
  17

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.
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.


MySQL Blog
http://mysqlearner.blogspot.com/
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18978
    
  40

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...

http://www.coderanch.com/t/568723/java-programmer-SCJP/certification/Garbage-Collection
http://www.coderanch.com/t/561000/java-programmer-SCJP/certification/Garbage-collector
http://www.coderanch.com/t/561429/java-programmer-SCJP/certification/possible-glitch-garbage-collector-example

Henry


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

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

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
author
Sheriff

Joined: Oct 14, 2002
Posts: 8883
    
    5
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.


Spot false dilemmas now, ask me how!
(If you're not on the edge, you're taking up too much room.)
raju salla
Greenhorn

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
author
Sheriff

Joined: Oct 14, 2002
Posts: 8883
    
    5
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
Greenhorn

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 : http://radio.javaranch.com/corey/2004/03/25/1080237422000.html


Sincerely
Rahul Parakkat
 
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subject: Garbage Collection