CODEWITHSUNDEEP
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JeffBr
JeffBr

Reputation: 25

How to remove a bunch of objects in a list

No mather what i try i cannot seem to remove all the objects in my list. I always end up removing half of the items, while the other half remains for some reason.

Any input greatly appreciated.

private List<Connection> listCon = new ArrayList<Connection>();

public void addConnection()
{    
        listCon.add(new Connection(isConnected)); 
}


public void inActivate() 
{
    for(int i=0; i < listCon.size(); i++)
    {
        removeConnection(listCon.get(i));
    }
}

public void removeConnection(Connection c)
{
    listCon.remove(c);
}

Upvotes: 2

Views: 257

Answers (6)

Terry Bell
Terry Bell

Reputation: 11

why not just use, listCon.clear() ?

Failing that, reverse your loop:

for(int i = listCon.size -1; i >=0; i--) { listCon.remove(i); }

or as Will Kru suggests, always remove the 0th element

Finally, use an iterator and iterator.remove: for(Iterator<Connection> i = listCon.iterator(); i.hasNext();) { Connection c = i.next(); i.remove(); }

Upvotes: 1

EboMike
EboMike

Reputation: 77722

The classical problem of deleting objects from a list while you're iterating through it.

for(int i=0; i < listCon.size(); i++)
{
    removeConnection(listCon.get(i));
}

You

  • Delete object 0
  • What used to be object 1 is now object 0, what used to be object 2 is now object 1
  • Delete object 1 (which is what was object 2 just earlier)
  • What used to be object 2 is now object 1, what used to be object 3 is now object 2

Many solutions to this. One common one is to delete element 0 until the size is 0.

EDIT: If your goal is really just to clear the array with no other operations, then listCon.clear() is by far the better approach. I'm assuming though that removeConnection does more than just removing the element from the list, you just didn't show that to keep your example simple.

Upvotes: 7

Peter Lawrey
Peter Lawrey

Reputation: 533492

You are removing every second item because you are altering the list while you loop. i.e. you remove the 0th element, in which case the 1th element takes it place and you move onto the new 1th element etc. If you used a debugger you would see this happening.

The simplest thing to do is to remove the elements in reverse order as this doesn't result in elements being shifted around.

for(int i=listCon.size()-1; i>=0; i--)
    removeConnection(listCon.get(i));

Upvotes: 3

Will Kru
Will Kru

Reputation: 5212

You are changing the array structure while looping over it, which is a really bad idea as you experienced. Because the array elements get shifted once you remove an element, you will skip elements.

The best way to do this is using a while loop:

while(listCon.size() > 0){
  listCont.remove(0);
}

@ebomike beat me to it

Upvotes: 2

trojanfoe
trojanfoe

Reputation: 122381

You are changing what is referenced by 'i' after removing an object from the list in this method:

public void inActivate() 
{
    for(int i=0; i < listCon.size(); i++)
    {
        removeConnection(listCon.get(i));
    }
}

Instead, use an iterator:

Iterator it = listCon.iterator(); 
while(it.hasNext())
{
    it.remove();
}

or in this case, just clear the list:

listCon.clear();

Upvotes: 5

Thomas
Thomas

Reputation: 759

What about using the lists .clear() method?

And if you really need to remove step by step while iterating you should use the lists iterator and its remove() method.

Upvotes: 2

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