1

I have the following code:

class Test {
   public static void main(String[] args) {
      String a,b,c;
      a = new String(args[0]);
      b = a;
      a = null;
      b = null;
   }
}

Can someone tell me when a will be eligible for garbage collection. I think it's after b is made null because don't a and b reference the same object ?

  • `a` is not an object. It's a reference to an object. – JB Nizet Jun 21 '12 at 17:12
  • http://stackoverflow.com/a/3315292/1403780 That answer should answer your question. http://stackoverflow.com/questions/3315189/when-does-javas-garbage-collection-free-a-memory-allocation possible duplicate? – Sephallia Jun 21 '12 at 17:12

5 Answers5

1

The object you've created, new String(args[0]);, will be eligible for collection once there are no longer any references to it. So let's step through the code:

a = new String(args[0]);

a points to your String, it is not eligible for collection.

b = a;

a and b point to your String, not eligible. 

a = null;

b points to your String, not eligible.

b = null;

No references to your String, Garbage Colelctor is happy!

cklab
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Pitching my answer too. As the other answers say, the String/ object is available for Garbage Collection once it is no longer accessible (you no longer have a handle to it).

So if you had a one-direction linked list... [1] -> [2] -> [3] and you had a handle to [1] (which has a handle to [2] and onwards). If you set your handle to [1] to null, you would put the entire list available to the Garbage collector. As this answer says, you are able to call System.gc() to request the Garbage Collector run, but it is not guaranteed that it will.

I believe the main focus to this answer is that objects are available to the garbage collector when they are inaccessible and that does not necessarily mean that there are no references to it. In my above example, even though [1] had a handle to [2], [2] was available for the garbage collector because there was no handle to [1].

Community
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Sephallia
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What do you mean by a? a is a reference, and only objects are garbage collected.

The string referenced by a, and then b, is eligible for garbage collection when nothing references it anymore. In this case, this is after both references to the string (a and b) have been changed to reference something else; in this case, null.

thecoop
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    Although this is true in this particular case, an object, generally, can become eligible to GC even if it still has references to it. It must not be reachable anymore to become eligible to GC. – JB Nizet Jun 21 '12 at 17:13
  • @JBNizet You reminded me of those fun times when I was learning about linked lists. Especially one direction linked lists. n.n – Sephallia Jun 21 '12 at 17:15
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You must reason in terms of allocated objects: the String you allocated is not referenced by anyone as soon as you nullify both a and b. From thereon the garbage collector is entitled to do its job.

Luca Geretti
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class Test {
   public static void main(String[] args) {
      String a,b,c; // you dont have objects yet
      a = new String(args[0]); // here u make a new object
      b = a; // b references to the same object as a
      a = null; // a points to null, reference is removed.
      b = null; // b point to null, reference is removed
   }
}

in your program the object that was created and assigned to a and b will be garbage collected after references are removed.

Garbage collector will clean up the object that has no roots and not being referenced by anything.

DarthVader
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  • I am confused. After a is null doesn't b still point to a? So then after a = null and before b = null will "a" be eligible for GC ? –  Jun 21 '12 at 17:35
  • a is not an object, a is reference to an oobject. b point to the object that a was pointing to. – DarthVader Jun 21 '12 at 17:36