Reg. GC from JavaCaps

[Angela Narain]

I have two questions regarding GC:
1. You can call the garbage collector whenever you want to.( True/False )

When we invoke System.gc() , does it mean we are calling the
garbage collector whenever we want to.

12. From the following code how many objects are eligible for garbage collection?  
1.String string1 = "Test";  
2.String string2 = "Today";  
3.string1 = null;  
4.string1 = string2;  
A) 1  
B) 2  
C) 3  
D) 0  

The correct answer is 1.
Is is because the String2 value "Today" is lost after line 4.
Can someone explain it to me in better terms.
thanks

[Cristian Negresco]

Hi,
Is it that in line 3, the "Test" object is lost so it is eligible for gc, and in line 4 the reference string1 will point to the same object as reference string2?
I dont't think that "Today" will be lost.
Just an idea,
Cristian

[Amit Agrawal]

Originally posted by Angela Narain:
I have two questions regarding GC:
1. You can call the garbage collector whenever you want to.( True/False )

When we invoke System.gc() , does it mean we are calling the
garbage collector whenever we want to.

False. (No you can not call garbage collector on your wish. when you invoke System.gc(), you are just 'requesting' JVM for the calling Garbage Collector but cann't make/force JVM to do so).
Remember, GC is the lowest priority thread for JVM.

Originally posted by Angela Narain:
[B]

12. From the following code how many objects are eligible for garbage collection?  
1.String string1 = "Test";  
2.String string2 = "Today";  
3.string1 = null;  
4.string1 = string2;  
A) 1  
B) 2  
C) 3  
D) 0  

The correct answer is 1.
Is is because the String2 value "Today" is lost after line 4.
Can someone explain it to me in better terms.
thanks [/B]

This is because in line no 3 you are assigning null to String1, making one String object ("Test") elligible for garbage collection as it is not referenced any more.
(But even though if line no 3 wouldn't have been there, answer would have been same as in line 4 you are assigning String2 to String1..making "Test" elligible for Garbage Collection).

[Valentin Crettaz]

I agree with you but since the String object are created using String literals and not dynamic object allocation, the String should be kept in the private String pool of the class and not garbage collected. Is this behaviour compiler-specific ? Keeping String literals in the pool is not a requirement, but just a possible optimization, right ??
Val

[tvs sundaram]

Hai Angela,
String Literals are not GCed.
Sun Exam wont be testing u like this. Look for objects that are created with new operator, String concatenation etc for GC questions.
HTH
tvs sundaram
SCJP

[Valentin Crettaz]

I was almost sure that String literals were not gced. Thanks TVS you made my mind clear !!
Val

[Muhammad Farooq]

Does it mean that no objects will be gc'ed in the above example. Can somebody tell me whether String litertal is object or not.
--Farooq.

[Valentin Crettaz]

There are two main ways of creating String instances:
1. Create a String instance using a String literal in which case the private String pool of the class will be asked whether it already contains such a String. If it contains one then it is returned and affected to the variable. If not a new is created and added to the pool
String s = "String1";
2. Allocate it dynamically
String s = new String("String1");
in which case a new String instance will be created in any case

The first way is provided for optimization purposes, in order to avoid creating multiple identical String instances. Since a String is immutable, there is no way one could be modified, so why not using an existing one ?
To sum up, in the example above, no String instance should be gced. Anyone correct me if I'm wrong !
Val

[Anonymous]

...In other words...The answer is Zero.
Shyam

[Angela Narain]

This is with reference to Valentin's post :
Below is what i know about String literals :
1. Suppose we have a statement as :

 String s = "Test"; //1 

A new string will be created in the string pool.
2. Now if again we have a statement as :

 String s1 = "Test"  //2 

then as the contents is same both s and s1 will share
the same string object.
What if the code had been as :

From the following code how many objects are eligible for garbage collection?  
1. String string1 = new String("Test");  
2. String string2 = new String("Today");  
3. string1 = null;  
4. string1 = string2;  
A) 1  
B) 2  
C) 3  
D) 0  

Pls. correct me if wrong .
thanks

[This message has been edited by Angela Narain (edited September 25, 2001).]

[Valentin Crettaz]

Angela,
when you create a String instance using a String literal, that String will automatically be added to the String pool of the class. However, when you create a String dynamically, the String is NOT added to the String pool unless the method intern() is invoked on it !
In the code you mentioned, one String object will be garbage collected, namely the String "Test".
Val

[Angela Narain]

Thanks Valentin.
Will the string object will gced after line4 ?
I am not clear of the sequence,
that is how it is made null first and then assigned
a new reference.
Can you explain to me.

[Valentin Crettaz]

As soon as statement on line 3 has been executed the String "Test" may be gced anytime depending on the will of the garbage collector
Val

[Cristian Negresco]

1.String string1 = "Test";
2.String string2 = "Today";
3.string1 = null;
4.string1 = string2;
I think this is also about my old "instance" problem,
My opinion is that it works like that:
line 1. the "string1" handle points to "Test" object,
line 2. the "string2" handle points to "Today" object,
line 3. the "string1" handle points to null, so there is no handle to point to "Test" object so he is eligible for gc
line 4. "string1" handle will point to the same object as "string2".
BTW, why the Strings Literals are not gc. ?

[Valentin Crettaz]

Cristian, the String "Test" will NOT be gced since the String instance is created using a literal and thus the String "Test" is automatically interned in the internal String pool of the class !!
If the String was created like this
String s = new String("Test");
then it would have been eligible for garbage collection as soon as s is set to null !!!
Val

[Jose Botella]

Because they are always pointed to by the String pool.