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need clarification on garbage collection

 
dhana rangu
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1)How can you force garbage collection?
[a] Garbage collection cannot be forced
[b] Call System.gc().
[c] Call Runtime.gc().
[d] Set all references to null.

The answe i feel is (b) call System.gc()
But the answer shown in someother site is (a) garbage collection cannot be forced

2). From the following code how many objects are garbage collected?
String string1 = "Test";
String string2 = "Today";
string1 = null;
string1 = string2;
[a] 1
[b] 2
[c] 3
[d] 0
plz let me know the answer for this...
help is appreciated
 
Ernest Friedman-Hill
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Hi,
Welcome to JavaRanch.
The answer to the first question is indeed, a. System.gc() is just a suggestion to the Java Virtual Machine, which can be ignored.
The answer to the second question is 0. The only string objects here are String literals, and they are stored in an internal table, and are not GC'd.
 
David Crossett
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If I could try to clarify to make sure I understand... when an object has been released ( either set to null or GC suggested, whatever ), Java will/can clean this stuff up on it's next pass. All you can do is mark stuff for collection - right? There is no way to force it whatsoever?
 
Howard Kushner
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Sorry David buts that's what the JVM spec says. You can set a reference to null, and the object is eligible for garbage collection, but you cannot force it. There are books you can read and probably online refences on Sun's web site.
The implementation is generally a mark and sweep implementation on a low priority thread, and if you code simple experiments, you can see garbage collection in action, by overriding the finalize method and then callng System.gc after setting references on instances to null.
Regards,
 
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