Garbage Collection Problem Explain Pls Dan Chisholm single topic

[Rob Kiu]

Hi!
Have problem understanding the answer to question below Anyone can give a line by line explanation how i1, i2, i3 ends up referring to each other? What abt i4?
Your help most appreciated.
Q from Dan Chisholm single topic exam on GC.
http://www.danchisholm.net/dec20/topic/section3/gc1.html
Question 4
class I {
private I other;
public void other(I i) {other = i;}
}
class J {
private void m1() {
I i1 = new I(), i2 = new I();
I i3 = new I(), i4 = new I();
i1.other(i3); i2.other(i1);
i3.other(i2); i4.other(i4);
}
public static void main (String[] args) {
new J().m1();
}}
Which object is not eligible for garbage collection after method m1 returns? Ans: g. None of the above
Ans:
Please note that this question asks which object is NOT eligible for garbage collection after method m1 returns. The objects referenced by i1, i2 and i3 form a ring such that each object is referenced by another. Even so, nothing outside of method J.m1 references any of those objects. When method J.m1 returns, the ring becomes an island of isolated objects that are not reachable by any part of the user program. A key point to remember is that an object that is referenced by another object can be eligible for garbage collection if the two objects form an island of isolated objects.

[Bojan Knezovic]

After the

I i1 = new I(), i2 = new I();
I i3 = new I(), i4 = new I();

you have four objects on the heap, referenced by i1, i2, i3, i4.
And after this part of the code:

i1.other(i3); i2.other(i1);
i3.other(i2); i4.other(i4);  

you'll have i1's field "other" pointing to i3 (i1.other->i3), i2's to i1 and i3's to i2 (these 3 will form a ring-like structure). i4.other will reference the same object the i4 itself is referencing.
But the key point here is to read the question well:
"Which object is not eligible for garbage collection after method m1 returns?"
So the answer is "None" (double negation!) or to be more clear "They're all eligible for GC AFTER the method m1 returns".

HTH.
[ February 24, 2004: Message edited by: Bojan Knezovic ]

[Corey McGlone]

Right - I think you're being hung up on the grammar of the question, not on the question itself. You obviously understand garbage collection in this instance, but the semantics of the question seem to have gotten in your way.
The question asks which of the following is NOT eligible for garbage collection. Therefore, in order for you to answer a, b, c, or d (which equate to i1, i2, i3, and i4, respectively), one of those objects would have to be ineligible for garbage collection. Obviously, that's not the case. Therefore, g is the correct answer.

[Rob Kiu]

Dear Bojan & Corey,
Thanks a lot, your explanations certainly helps.
Got a bit confused by i1.other(i3); i2.other(i1); i3.other(i2); i4.other(i4);part and how they end up referring to each other, luckily not by the NOT eligible part. Bojan cleared it up with his "ring like structure" explanation.
Will try more GC questions, seems my weak area.
Cheers,
Rob

[Corey McGlone]

When dealing with garbage collection, I find that the old cliche, "A picture is worth a thousand words" is very true. Almost any time I'm faced with a question about garbage collection, I find that drawing a picture is the easiest thing to do in order to understand what is going on.
I drew a quick example of this question and put it online here. If you look at that picture, you can see very easily how each object is caught in a circular reference and is, therefore, eligible for garbage collection.
I hope that helps,
Corey

[Shahfazal Mohammed]

Hello everyone-
The following is from the same exam- SectionIII- q8

 
class I 
{
    private String name;
    public String name() {return name;}
    public I(String s) {name = s;}
}
 
class J 
{
    public static void m1(I i) {i = null;}
    public static void main (String[] args) 
    {
        I i = new I("X");           // 1
        m1(i);                      // 2
        System.out.print(i.name()); // 3
    }
}
 
 
 
Which of the following is a true statement? 
 
a.  The object created a line 1 is eligible for garbage collection after line 2. 
b.  A NullPointerException is generated at line 3. 
c.  The program compiles, runs and prints X. 
d.  The program fails to compile. 
e.  None of the above 
 
 
 
And the answer is 'c. the program compiles and prints X'

now, when i'm passing an object reference to a method, the method receives a copy of the reference and any changes done to the reference reflect to the object whose reference was actually passed right?? the following is from Kathy's book

1. import java.awt.Dimension;
2. class ReferenceTest {
3.     public static void main (String [] args) {
4.         Dimension d = new Dimension(5,10);
5.         ReferenceTest rt = new ReferenceTest();
6.         System.out.println("Before modify() d.height = " + d.height);
7.         rt.modify(d);
8.         System.out.println("After modify() d.height = " + d.height);
9.     }
 
10. void modify(Dimension dim) {
11.     dim.height = dim.height + 1;
12.     System.out.println("dim = " + dim.height);
13.     }
14. }
 
 
 
When we run this class, we can see that the modify() method was indeed able
to modify the original (and only) Dimension object created on line 4.
 
C:\Java Projects\Reference>java ReferenceTest
 
Before modify() d.height = 10
dim = 11
After modify() d.height = 11
 
Notice when the Dimension object on line 4 is passed to the modify()
method, [B] any changes to the object that occur inside the method are 
being made to the object whose reference was passed. In the preceding 
example, reference variables d and dim both point to the same object[/B]. 

now the bold part says any changes to the object inside the method are being made to the actual object reference passed. now in Dan's exam, why does the progam print out "X" when in m1() the object reference was set to null and the output should've been a NullPointerException. (I ran both the programs, they run as they say. and also for kathy's example, i set

 dim = null 

in the modify method and it prints out

 After modify() d.height = 10 

meaning it has not nulled the reference.
can anyone explain plz
Edited by Corey McGlone: Cleaned up formatting
[ February 24, 2004: Message edited by: Corey McGlone ]

[Shahfazal Mohammed]

and sorry for messing up the format of this thread..i just copied and pasted into the textarea..dunno what went wrong

[Corey McGlone]

First of all, Shahfazal, it is generally considerate to start a new thread when asking a new question. I realize that this question comes from the same mock exam, but it's really a new question and warrants a new thread.
However, you already have the answer to your question - you just don't realize it. You said it yourself - when you pass a reference variable to a method, that method receives a copy of the reference.
At that point, both the calling block and the called block have a reference to the same object, but those references are different variables - they are copies of one another. Therefore, if you modify the object referred to by either reference, you'll see that change reflected via the other reference because they both point to the same object.
However, if you change the reference itself (as is the case when you set it to null), you are only impacting one copy of the reference, not the object that the reference refers to.
I think you might find the following demonstration useful: Link.
I hope that helps,
Corey

[Shahfazal Mohammed]

Ouch! deserved that rap. forgot that this is a different concept frmo the actual question that started this thread.
Anyway, thanx a lot.

[Roopa Gowda]

Hello everyone,
I am new to the forum and this is my first post.
I would really appreciate if someone can help me .
Question 4
class I {
private I other;
public void other(I i) {other = i;}
}
class J {
private void m1() {
I i1 = new I(), i2 = new I();
I i3 = new I(), i4 = new I();
i1.other(i3); i2.other(i1);
i3.other(i2); i4.other(i4);
}
public static void main (String[] args) {
new J().m1();
}}
Which object is not eligible for garbage collection after method m1
returns? Ans: g. None of the above
********

i did understand that
I i1 = new I(), i2 = new I();
I i3 = new I(), i4 = new I();
creates 4 objects on the heap of type I, refered by i1,i2,i3,i4.
i1.other(i3); i2.other(i1);
i3.other(i2); i4.other(i4);
now i don't get this part.
i1.other referes to the same object as i3 does
i2.other referes to the same object as i1 does
i3.other referes to the same object as i2 does
i4.other referes to itself
but does it mean i1,i2,i3,and i4 do not refere to any object now?
these references still refer to the original objects right? [/LIST][LIST]

[Roopa Gowda]

class I {
private I other;
public void other(I i) {other = i;}
}
class J {
private void m1() {
I i1 = new I();
I i2 = new I();
I i3 = new I();
I i4 = new I();
System.out.println(i1);
i1.other(i3); 
i2.other(i1);
i3.other(i2);
i4.other(i4);
System.out.println(i1);
}
public static void main (String[] args) {
new J().m1();
}}
  

the output of i1 was same .
does it mean that i1 is still refering to the object originally created?

[Corey McGlone]

Originally posted by Roopa Gowda:
i did understand that
I i1 = new I(), i2 = new I();
I i3 = new I(), i4 = new I();
creates 4 objects on the heap of type I, refered by i1,i2,i3,i4.
i1.other(i3); i2.other(i1);
i3.other(i2); i4.other(i4);
now i don't get this part.
i1.other referes to the same object as i3 does
i2.other referes to the same object as i1 does
i3.other referes to the same object as i2 does
i4.other referes to itself
but does it mean i1,i2,i3,and i4 do not refere to any object now?
these references still refer to the original objects right?

There is a subtle yet very distinct difference between what is going on and what you said.
i1, i2, i3, and i4 all refer to objects of type I. However, each of those objects contains a reference variable which refers to some object of type i.
Therefore, after we have invoked these lines:

i1.other(i3);
i2.other(i1);
i3.other(i2);
i4.other(i4);

We have a case where i1, i2, i3, and i4 all still refer to the same objects they referred to in the beginning. However, i1.other refers to the object referred to be i3, i2.other refers to the object referred to by i1 and so on.
Be sure to look at the picture I drew of the situation here. Hopefully, looking at that picture can help you understand what is going on. If you have more questions, let me know.

[Roopa Gowda]

Thanks a ton.
so i1,i2,i3,i4 are local to the method and the objects r left with no references outside the method,hence eligible for gc
hope i am correct this time.