Question about Garbage Collection for my OCA Exam

user_15178339 0
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class CardBoard {
    Short story = 200;

    CardBoard go(CardBoard cb){
        cb = null;
        return cb;
    }

    public static void main(String[] args){
        CardBoard c1 = new CardBoard();
        CardBoard c2 = new CardBoard();
        CardBoard c3 = c1.go(c2);
        c1 = null;
// When this line is reached, what is eligible for the GC.
    }
}

Can somebody please explain what happens here? I understand that c1 and the Short are eligible for the Garbage Collector. But what happens with c2 and why isn't c3 also available for it? I might oversee something here but I would be glad for every input that helps me get around this topic. My OCA exam is next week and I still have trouble with this kind of stuff.

Edit: The comment of what is eligible for the GC.

garbage-collection java
2 Answers

Considering Java is pass-by-value, we know that Java isn't passing the object, it's passing a reference to the object.

Because of this, when you did CardBoard c3 = c1.go(c2);, the method go(Cardboard cb) just copied the reference value of c2 to cb and therefore c2 is unharmed by the cb = null;. So, c2 is not eligible for GC.

c3 never gets to be constructed as an object, it is just a variable of type CardBoard initialized to null. Thus, it cannot be garbage collected because it is not even created on the heap.

user_152666680

Perhaps visualizing the objects involved will help. After the first statement in main, we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

After the second statement in main, we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c2 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

(theoretically, it's also possible that the two CardBoard share the Short object, the Javadoc of Short.valueOf says:

This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.

However, the JDK implementation only caches in the range -128 to 127, so we are going to assume that going forward)

Upon entering the go() method, we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c2 ----> |-----------|  /     |-------|
     --> |  story    | /      |  200  |
    /    +-----------+        +-------+
   / 
  /
cb

and before leaving go(), we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c2 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+
    
  
cb --x

and after the 3rd statement in main, we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c2 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+
    

c3 --x

And before leaving main(), we have:

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c1 -x    |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+

         +-----------+        +-------+
         | CardBoard |   ---> | Short |
c2 ----> |-----------|  /     |-------|
         |  story    | /      |  200  |
         +-----------+        +-------+
    

c3 -x

As we can see, of the 4 objects allocated, 2 remain reachable, and 2 are eligible for collection.

user_1834060
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