ArrayList objects

Question

Need some inputs:

Lets say i have N ArrayList and in each i am adding foo() object.

Foo foo = new Foo()

A.add(foo);

B.add(foo);

N.add(foo);

Now modification done on any one foo() object will reflect in all the other arraylist?

If YES WHY? and
whether this behaviour can also be achieved using any other collection like Vector etc...?
IF i make foo as null will it reflect in all arraylist?

Michael Borgwardt · Accepted Answer · 2012-02-21 12:59:59Z

8

Yes, because all lists only contain a reference to the same objects
Yes, all collections work like that
No, because you can only set a reference to null, and each list has a copy of the reference.

answered Feb 21, 2012 at 12:59

Michael Borgwardt

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5 Comments

aviad Over a year ago

could you elaborate on 'No' in 3?

Michael Borgwardt Over a year ago

@aviad: A B and N each contain a copy of the reference foo, and all of them refer to the same object. Setting foo to null will not have any effect on the copies.

aviad Over a year ago

I thought that the object itself will be nullified (foo=null) then all the collections refer to null. do I miss something?

Michael Borgwardt Over a year ago

@aviad: Yes, you simply thought wrong. foo = null has no effect whatsoever on the object itself (Objects cannot be null), it only removes the reference and if there is no other reference the object will eventually be picked up by the garbage collector.

Ravibhushan Over a year ago

Thanks Michael, MJM and aviad for clarification :)

Sam · Accepted Answer · 2012-02-21 13:29:28Z

Any implementation of Collection API such as ArrayList or Vector hold reference to an object in heap memory so when you would get an index of a List by get(index) method, you achive reference to object so:

Yes, if get an index of list by get(index) method and then change the stat of the achieved object, changes stay in memory.
Yes,All Collection API have this behavior.
No,When achieve to a index of list, act is: "You achieve a copy of reference to object" and when set it to null, list instance don't any change.

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