About Question enthuware.ocpjp.v11.2.3616 :

[ravi2519]

The Buddy object created at line number 2 is set into instance variable upper at line number 3.
Now the object that will be returned from following constructor will remain till the program ends.

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public Buddy(String name, Buddy upper)

So i am confused as to why and how this option is correct:

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Object created at line //1 will be eligible for garbage collection after line //2

And this option incorrect:

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Object created at line //1 will be eligible for garbage collection after line //5

[admin]

>number 2 is set into instance variable upper at line number 3.
No, at //2 the number variable refers to the method parameter and not the instance variable.
This has now been added to the explanation.
thank you for your feedback!

[pblanchardie]

There is a typo, the variable name is upper (instead of number)

[admin]

You are right. Fixed.
thank you for your feedback!

[lucash9]

I think that should be

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b = upper; //2

instead of

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upper = b; //2

to to satisfy the correctness of first answer.

[admin]

No, it is correct. At //2, when you set upper = b, the reference to the Buddy object created at //1 is not stored in the instance variable upper. It is stored in the method parameter named upper, which is lost after the method returns (i.e. after //2 ).

[lucash9]

I'm still not convinced. upper = b; what it actually does is set upper reference at the same as b which is Buddy object. Now we have two references to Buddy: b and upper. No reference to Buddy is lost. Take a look at this code executed at https://www.jdoodle.com/online-java-compiler/

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public class Buddy {
    public static void main(String args[]) {
        Buddy b1 = new Buddy("A"); // 3
		Buddy b2 = new Buddy("B", b1); // 4
		//System.out.println(b1); // 5
    }
    
   	Buddy upper;
	String name;

	public Buddy() {}

	public Buddy(String name) {
		this.name = name;
	}

	public Buddy(String name, Buddy upper){        
		this.name =  name;         
		Buddy b = new Buddy(upper.name);//1
		System.out.println("b reference before line 2: "+b);
 		upper = b; //2
        System.out.println("b reference after line 2: "+b);
        System.out.println("upper reference: " +upper);
	}
}

As a result we will see something like this

b reference before line 2: Buddy@3b192d32
b reference after line 2: Buddy@3b192d32
upper reference: Buddy@3b192d32

which indicates that b still referenced to Buddy.

[lucash9]

Maybe you mean a situation after method is called so that local Buddy pointed by b and upper is not stored anywhere so it will be eligible for GC?

[lucash9]

It certainly is. Silly me.

[admin]

Right, b and upper will both reference to the same Buddy object that was created at //1. But after line //2, both of these references are not used and so this object will be eligible for gc after that line because the method ends there.

[pavvel]

Why Object created at line //3 will be eligible for garbage collection after line //5? Is this because the program is end?
In this case Object created at line //1 will be eligible for garbage collection after line //5. is also a correct case.

[admin]

Yes, you could say that but option 1 is better. We will update the problem statement to say that what is the "earliest" line after which the object is eligible for gc.
thank you for your feedback!