K&S CHAPTER 3

[akash azal]

2. Given:
class CardBoard {
Short story = 5;
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;
// do Stuff
} }
When // doStuff is reached, how many objects are eligible for GC?
A. 0
B. 1
C. 3
D.compilation fails

[Deepak Bala]

What do you think the answer is ?

[akash azal]

B c1 is eligible for GC

[Deepak Bala]

akash azal wrote:B c1 is eligible for GC

That would be correct

[akash azal]

but in K&S answer is c

[Sunny Mattas]

but in K&S answer is c

hi
There is a mistake with this question in K&S.
Book says second object garbage collected is

Short story = 5;

But this will not garbage collected because value of short is under 127 which means it is in cache memory.
If value is changed to

Short story = 230;

then two objects will be garbage collected.

Regards
Sunny Mattas
SCJP5

[akash azal]

thanks for clearing doubt

[Aakash Goel]

But this will not garbage collected because value of short is under 127 which means it is in cache memory.

What is the logic behind this?

I am completely unaware of the logic used here. Can somebody explain?

[Byju Joy]

What is the logic behind this?

I believe it's 10% of logic + 20% of heuristic + 30% of assumption + 40% of confusion behind this.

Statistically speaking, according Pareto's principle, 80% of numeric variables are using only 20% of possible values. 80% of Short variables take values in the range of 0-127 only. So they are 'cached' or 'shared'.

Later when economy improves, and memory cards becomes cheaper this decision could be revised to cache all Short values.

[Henry Wong]

Aakash Goel wrote:
What is the logic behind this?

I am completely unaware of the logic used here. Can somebody explain?

The "cache" being referred to here, is the cached in the Short class -- basically, the class will pre-instantiate a certain number of Short objects in a range... Later, when the valueOf() method is called, and the value is within the range, it will return the object from the cache, instead of creating a new one. And since autoboxing uses the valueOf() method, it uses the cache.

In relation to this question, the short is within the range, hence, it can't be GCed, because the object returned is still in the cache. In the errata, the short value is changed so that it is not in the cache.

Henry

[Henry Wong]

Byju Joy wrote:
Later when economy improves, and memory cards becomes cheaper this decision could be revised to cache all Short values.

Interestingly, this can be done at any time, without the need to change the specification. The specification defines what range must be cached. It doesn't say what should be done with values outside of the range. Hence, a version of the JVM can be released, which caches everything, and it will still be compliant with the Java specification.

Henry