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Index › Programming Questions & Help › Syntax Questions › same object concurrently in multiple arraylists

oompa_l Full Member Offline Posts: 212	same object concurrently in multiple arraylists? Jan 30th, 2009, 9:25pm   can the same object be in different arraylists at the same time? I am getting a nullpointerexception trying to add an object to different arraylists - is that a problem? thanks

PhiLho  YaBB Moderator Offline Posts: 4190 Near Paris (France)	Re: same object concurrently in multiple arraylist Reply #1 - Jan 30th, 2009, 9:33pm   No, not a problem at all. You don't put objects but references to them.

oompa_l Full Member Offline Posts: 212	Re: same object concurrently in multiple arraylist Reply #2 - Jan 30th, 2009, 9:42pm   that's what i thought. thanks so much for the reply...

oompa_l Full Member Offline Posts: 212	Re: same object concurrently in multiple arraylist Reply #3 - Jan 30th, 2009, 11:31pm   might be asking too much, but here's the applet: http://www.upsdn.ca/processing/animal_particles_06/applet/ can't figure out what's wrong. And just my luck, the applet doesnt load properly in my browser...but there's access to the code. thanks!

PhiLho  YaBB Moderator Offline Posts: 4190 Near Paris (France)

Re: same object concurrently in multiple arraylist Reply #4 - Jan 31st, 2009, 9:55am   Ah, I see. The line: ArrayList wetAnimals, dryAnimals, mixedAnimals, hotAnimals, coldAnimals, temperateAnimals = new ArrayList(); doesn't work like you expect. Only temperateAnimals with be initialized, the other lists will be null. Write: ArrayList wetAnimals = new ArrayList(); ArrayList dryAnimals = new ArrayList(); and so on. Or: ArrayList wetAnimals, dryAnimals, mixedAnimals, hotAnimals, coldAnimals, temperateAnimals; wetAnimals = new ArrayList(); dryAnimals = new ArrayList();

oompa_l Full Member Offline Posts: 212	Re: same object concurrently in multiple arraylist Reply #5 - Feb 1st, 2009, 4:56am   thanks pHihLO! that's something that I never would have guessed. is there a clue that would have led me to intuiting that? thanks again for all your help...i would be pretty lost without it

PhiLho  YaBB Moderator Offline Posts: 4190 Near Paris (France)

Re: same object concurrently in multiple arraylist Reply #6 - Feb 1st, 2009, 10:05am   No, sometime I would be tempted to use the same shortcut... For your information, you can use a similar shortcut: int x, y, z; x = y = z = 1.0; It must be done in two steps. And that's not good for your case, because the array lists would be the same (ie. you assign the references to the newly created object, not a new object to each).