Can not remove an object into a ArrayList

Question

I don't know how to remove an object from an ArrayList. I know there is remove but in my case, its more complicate.

I have a ArrayList (listsBlocksRemaining), which contains object that I put into another ArrayList (listBlocksIntoRibbon) when there are print. Then, what I want, its that the object that are print should be removed from listBlocksRemaining, there are print so no more remaining.

This is my code :

public void algoGlouton(Graphics g) {

    int offsetX = 13;
    int offsetY = 600;
    int widthAvailable= 0;
    int smallestHeight= 300;
    for(Bloc b : listsBlocksRemaining) {
        if(widthRemaining+b.getWidth () < ribbon.getWidth ()) {
            widthAvailable+= b.getWidth();
             g.setColor(b.getColor());
             g.fillRect(offsetX, offsetY-b.getHeight(), b.getWidth(), b.getHauteur());
             listBlocksIntoRibbon.add(b);
             b.setX(offsetX);
             b.setY(offsetY-b.getHauteur());
             offsetX += b.getLargeur();
        } else {
            for(Bloc b1 : listBlocksIntoRibbon) {
                if(b1.getHauteur() < smallestHeight) {
                    smallestHeight= b1.getHeight();
                }
            }
        }
    }
}

I tried this :

for(int i = 0; i<listsBlocksRemaining.size();i++) {
        for(int j = 0; j<listBlocksIntoRibbon.size();j++) {
            if(listsBlocksRemaining.get(i) == listBlocksIntoRibbon.get(j)) {
                listsBlocksRemaining.remove(i);
            }
        }
}

and this :

for(int i = 0; i<listsBlocksRemaining.size();i++) {
        Bloc b = listsBlocksRemaining.get(i);
        if(listBlocksIntoRibbon.equals(b)) {
            listsBlocksRemaining.remove(b);
        }
}

Cordially

I prefer to override equals() and hashCode() methods in Bloc and then just call listsBlocksRemaining.remove(b) — istovatis
– istovatis, Commented Dec 13, 2018 at 17:28
If I understand correctly, you want to remove all the elements of listBlocksIntoRibbon from listsBlocksRemaining, right? listsBlocksRemaining.removeAll(listBlocksIntoRibbon) does that. docs.oracle.com/javase/8/docs/api/java/util/… — JB Nizet
– JB Nizet, Commented Dec 13, 2018 at 17:28
First of all thanks for your help, but what I want is that when a block has been placed into the listBlocksIntoRibbon I instantly delete it from listBlocksRemaining, with what you said, it'll cause an Exception. — A.Arthur
– A.Arthur, Commented Dec 13, 2018 at 17:40

Dave Drake · Accepted Answer · 2018-12-13 17:28:50Z

1

You don't need to find the object in listBlocksRemaining in order to remove it. You can just remove the object from the list as:

for( Bloc bloc : listBlocksIntoRibbon )
  listBlocksRemaining.remove(bloc);

or even:

listBlocksRemaining.removeAll( listBlocksIntoRibbon );

answered Dec 13, 2018 at 17:28

Dave Drake

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5 Comments

A.Arthur Over a year ago

First of all thanks for your help, but what I want is that when a block has been placed into the listBlocksIntoRibbon I instantly delete it from listBlocksRemaining, with what you said, it'll cause an Exception.

Justin Over a year ago

Then right after you do listBlocksIntoRibbon.add(b);, just do listBlocksRemaining.remove(b);, but you should loop through listsBlocksRemaining with an iterator and not a for loop.

Dave Drake Over a year ago

If you want to instantly remove it from the list you are iterating over, you need to use an actual Iterator, and call remove() on the iterator after you add it to listBlocksIntoRibbon.

A.Arthur Over a year ago

Yes, I've just see someone put an anwser with a Iterator, but there is still a problem. Thanks :)

A.Arthur Over a year ago

Yes, its what I did, but i've still an error.. could I give you my project (eclipse) and check why its not doint it correctly please ?

Justin · Accepted Answer · 2018-12-13 17:43:24Z

0

To safely remove from a collection while iterating over it you should use an Iterator.

Try this:

 Iterator<Bloc> itr = listsBlocksRemaining.iterator();
 while(itr.hasNext()){
      Bloc b = itr.next();
      ...
       listBlocksIntoRibbon.add(b);
       itr.remove();
       ...
 }

edited Dec 13, 2018 at 17:43

answered Dec 13, 2018 at 17:29

Justin

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1 Comment

A.Arthur Over a year ago

My bad, I import the wrong thing.. thanks, i'll try now

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