Re: [android-developers] Framework Cloning of Referenced Objects?

None of the methods you are using will copy an object.  Generally you must explicitly call clone() to do this.

On Sat, Nov 5, 2011 at 6:25 AM, Jay <johnabloodworth3@gmail.com> wrote:

I assume this is an Android question, though it may just betray my
ignorance of Java memory management.

I have an activity designed to randomly pick a person from a list, and
to not repeat anyone until everyone has been chosen. The core logic of
the pick method looks like this:

       ArrayList<Person> peopleCopy = new ArrayList<Person>(mPeople);
       peopleCopy.removeAll(mRecent);
       Person person = peopleCopy.get(mRNG.nextInt(peopleCopy.size()));
       mRecent.add(person);
       // Display chosen person in UI

Both mPeople and mRecent are referenced by adapters that feed into
ListViews.

The bug I was finding is that the same person was being chosen more
than once in a cycle, despite the code above. I had a hell of a time
tracking it down; I couldn't duplicate it on the emulator or on the
actual device when connected to the debugger. I finally managed to
"catch" the bug in progrss on the device and plug it in to the
debugger. Based on the object id fields, it appears that the problem
is that the Person objects referenced my mRecent were different from
the ones in peopleCopy, which is why the removeAll method was not
subtracting them correctly. I implemented appropriate equals() and
hashcode() methods on Person and that seems to have fixed the problem.

My question is where/when/why were the Persons in mRecent and/or
mPeople being replaced with clones? It wasn't happening all the time;
based on observations with the debugger, it appears that usually the
lists referenced the same objects as I expected them to. Does the
framework sometimes clone objects referenced by adapters or does Java
or am I misreading this whole situation?

Thanks,
Jay

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Dianne Hackborn
Android framework engineer
hackbod@android.com

Note: please don't send private questions to me, as I don't have time to provide private support, and so won't reply to such e-mails.  All such questions should be posted on public forums, where I and others can see and answer them.

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