When will garbage collection do the clean up?

[Frankie Law]

I have a question about non reference objects.  Let say I have the follow code below.
The question is, how long will Integer( 20 ) stays in memory?

ArrayList<Integer> numbers = new ArrayList<Integer>( );

numbers.add( new Integer( 10 ) );
numbers.add( new Integer( 20 ) );
numbers.add( new Integer( 30 ) );

numbers.remove( 1 );

Question 2:
Given the same piece of code, but I inserted the numbers into the ArrayList.
How long will num3 stay in memory?  Will garbage collection come and clean up the memory that isn't being use?
Or am I getting the wrong idea?

ArrayList<Integer> numbers = new ArrayList<Integer>( );

Integer num1 = new Integer( 10 );
Integer num2 = new Integer( 20 );
Integer num3 = new Integer( 30 );

numbers.add( num1 );
numbers.add( num2 );
numbers.add( num3 );

numbers.remove( 1 );

[Dana Ucaed]

You don't know when GC clean memory.

GC have specific algorithms.

[Stefan Evans]

There can be no definite duration based answer to "How long does it stay in memory"
The best answer would probably be:  "Until it is cleaned up by the Garbage Collector"
And a garbage collection runs "when it needs to"

It very much depends on the settings you choose for the garbage collector.
A quick google brought up an article: Understanding Java Garbage Collection which explains it better than I ever could.


In general:
Those "new Integers" you create will be allocated memory in the Eden space.
When a GC runs in the eden space, it uses a "Mark and Sweep" algorithm to copy only those values that are currently "alive" (able to be accessed) to the "Survivor" space

In Question 1 it would retain only those items still in the list.  The one that got removed wouldn't be accessible, so it would not be copied to the "Survivor" space.
In Question 2 it would depend on if the variable num2 was still "alive" or not when the garbage collector ran.  

[Paul Clapham]

All you can say for sure is that if the JVM is about to run out of memory, it will first do garbage collection on all unreferenced objects before giving up and throwing the out-of-memory error.

[Dana Ucaed]

In general:
Those "new Integers" you create will be allocated memory in the Eden space.

What is Eden space?

[Henry Wong]

Anda Cristea wrote:
In general:
Those "new Integers" you create will be allocated memory in the Eden space.

What is Eden space?

It's part of the heap, where objects are created. And of course, this is completely in the realm of implementation detail. There is no mention of it in the specifications, and may vary between Java versions, and platforms.... and of course, completely outside of the realm that someone learning Java should be trying to figure out...

Henry

[Campbell Ritchie]

Henry Wong wrote:. . . may vary between Java versions, and platforms.... . . .

I don't think it was there a few versions ago, and as Henry says, it may disappear from the next version.

[Shiva Shankarraj]

Well, probably the answer would be until memory is cleaned by garbage collector.

In question 1 :

objects would remain in the list.

In question 2:

Here variable num2 will still be alive that is, numbers.remove(1).

Meanwhile you can refer resource below for more information on garbage collection,

http://www.flowerbrackets.com/java-garbage-collection/

[Campbell Ritchie]

The likeliest answer to the original question is, “never”. You cannot expect garbage collection to occur at any particular time; it is only likely to occur at all when the heap space is nearly all occupied. I don't think that blog is particularly helpful because it is too much of an oversimplification.