Garbage collection

[Jason Attin]

Guys, here is another question whose answer I don't understand, from
Glenn, Mitchell. OCAJP Oracle Certified Associate Java SE 8 Programmer Practice Exams (Kindle Locations 11419-11425). Enthuware. Kindle Edition.

Consider the following code:

class MyClass { }
 public class TestClass{ 
MyClass getMyClassObject(){ 
MyClass mc = new MyClass(); // 1 
return mc; // 2 
} 
public static void main( String[] args){ 
TestClass tc = new TestClass(); // 3
 MyClass x = tc.getMyClassObject(); // 4 
System.out.println(" got myclass object"); // 5
 x = new MyClass(); // 6 
System.out.println(" done"); // 7 
} 
} 

After what line the MyClass object created at line 1 will be eligible for garbage collection? Select 1 option
A. 2
B. 5
C. 6
D. 7
E. Never till the program ends.

My answer was E but it's apparently C. I answered E because even when x is made to point to a different object, a new one that it, mc still points at the object created at line 1. I thought that by doing

MyClass mc = new MyClass();

and returning its reference to x we effectively ended up with two references to the same object and removing one would not remove the other one, hope it makes sense.

[Henry Wong]

Jason Attin wrote: ... and returning its reference to x we effectively ended up with two references to the same object and removing one would not remove the other one, hope it makes sense.

Hint: What happens to the local variables of a method when the method completes / returns?

Henry

[Jason Attin]

They cease to exist, but here we're returning that reference and save it onto a variable

[Jeanne Boyarsky]

Let's try this with hands since there are only two references. In getMyClassObject(), i give you a ball and tell you to hold it on your right hand. So one reference. When the method ends, you pass the ball to your left hand. Your right hand (also known as variable mc) is free because the method is over. But the ball is now in your left (also known as variable x) so no worries.

Now on line 6, I give you a different ball to hold in your left hand. The original ball is now on the floor and rolling away for the garbage collector to take care of.

[Henry Wong]

Just to finish my train of discussion (even though I really like Jeanne's analogy ) ...

Jason Attin wrote:They cease to exist, but here we're returning that reference and save it onto a variable

So, if the local reference variable "cease to exist", meaning that it goes out of scope, then...

Jason Attin wrote: and returning its reference to x we effectively ended up with two references to the same object and removing one would not remove the other one, hope it makes sense.

can you tell us what are the two references being referred to here? The x variable is obviously one of the references, but what is the second one that you are mentioning?

Henry

[Jason Attin]

OK I think I got it, like the ball analogy actually :-).

can you tell us what are the two references being referred to here?

Before reading all the comments I would have said x and mc. Now I think I got it, mc ceased to exist when the method exited so we only have x.

[Leoncio Thiago]

Jason Attin wrote:OK I think I got it, like the ball analogy actually :-).

can you tell us what are the two references being referred to here?

Before reading all the comments I would have said x and mc. Now I think I got it, mc ceased to exist when the method exited so we only have x.

Great explanation, thanks for sharing the understanding.

--Thiago

[Henry Wong]

Leoncio Thiago wrote:
Great explanation, thanks for sharing the understanding.

Agreed. Considering the number of topics on the ranch, where the OP never posts back after the initial post, it is nice to have a follow up with full explanations. Have a cow !!

Henry