Garbage Collection and String Literals

[Sunita Vontel]

Please see the code

public class TestClass 
{
   public static void main(String args[ ] ) 
   {
      String str0 = null;
      String str1 = "some string";
      String str2 = "second string";
      String str3 = "Third string";
      str0 = str1;    // 1
      str1 = null;    //2
      str2 = str0;    //3
      str3 = str2;    // 4
   }
}

How many objects are elgible for garbage cllection at the end of line 4. I say None.
Can somebody please explain the answer.
Thanks
Sunita

[Jane Griscti]

Hi Sunita,
Think the following 3 items are available for garbage collection:
str1 - set to 'null' in Line 2. Any object set to 'null' is eligible for garbage collection.
The literal strings "second string" and "third string" as they are no longer being referenced. (Not 100% sure of these, as they would be in the string pool and I'm not clear on gc and the string pool).

------------------
Jane

[ricky gonzalez]

Hi, I think all are String literals...and they should be saved in String pools. There is no new String() created so no String object gc ed?

[Khurram Fakhar]

I agree with Ricky

Regards
Khurram

[Kalpesh Pandya]

I am bit confused. Why GC is not considering String pool ?

[Manish Singhal]

Hi Friends,
Please see following URL ---
http://www.javaranch.com/maha/Discussions/Garbage_Collection/Strings_-_Khalid_-_JavaRanch_Big_Moose_Saloon.htm
Since String pool is not in the objectives of SCJP ( as mentioned in the above link) so....
I think there will only be two objects available for GC. i.e. "second string" and "Third string".
Maha Anna please say something about this code.
regards,
Manish
[This message has been edited by Manish Singhal (edited October 08, 2000).]

[Santosh Jaiswal]

I am thinking all string objects str0, str1, str2, str3 will be available for GC, after line 4.
any comments?
Thanks
Santosh Jaiswal

[mohit joshi]

I would agree with Manish.
also Jane, even if we are setting str1 to null in line 2, the String object that str1 was refering to ( ie. "some string") is still being refered to by str0 now. So that object will not be Garbage Collected.

[Shameen MK]

String str0 = null;
String str1 = "some string";
String str2 = "second string";
String str3 = "Third string";
str0 = str1; // 1
str1 = null; //2
str2 = str0; //3
str3 = str2; // 4
According to me, at line 4 , this will be the case:
str1 str0 str2 str3
| \ | / null Some string
therefore, second string and third string will be ready for garbage collection.

[Ajith Kallambella]

For your information, string literals( constructed using the = operator instead of new() ) are stored in the String pool. These strings are re-used if the program tries to create identical strings.
Strings in the string pool live longer than the objects created using the new() operator. The JVM garbage collects the strings in the pool when the class is unloaded.
Please note that you are not required to know about GC of string literals for the exam. The GC-related questions in the exam almost always deal with regular new()'d objects. However, if in case questions do appear about GC of string literals, treat them as any regular object.
Hope that helps,
Ajith