How to remove specific element on List

Question

How to remove specific element on List?

...
java.util.List<Polygon> triangles = new LinkedList<Polygon>();
Point startDrag, endDrag, midPoint;
Polygon triangle;

....
int[] xs = { startDrag.x, endDrag.x, midPoint.x };
int[] ys = { startDrag.y, startDrag.y, midPoint.y };    

triangles.add( new Polygon(xs, ys,3));   
....

public void mouseClicked(MouseEvent e) {
...
   startDrag = new Point(e.getX(), e.getY());
   for (Polygon p:triangles){
    if (p.contains(startDrag)) //Polygon has a 'contains(Point)' method
           remove (p.contains(startDrag));
   }
....

Answer 1

You will not be able to remove the object from the triangles list if you are currently iterating over it. If you try to do so, you will trigger a ConcurrentModificationException. Instead, what you should do is make a copy of the list and iterate over that, and when you get a hit, remove the item from the original:

public void mouseClicked(MouseEvent e) {
...
   Polygon[] triArray = triangles.toArray(new Polygon[triangles.size()]);
   startDrag = new Point(e.getX(), e.getY());
   for (Polygon p:triArray){
    if (p.contains(startDrag)) //Polygon has a 'contains(Point)' method
           triangles.remove (p);
   }
....

Answer 2

To remove by object, such as removing a specific triangle from a triangle list, use List::remove(Object)

For the above purposes, you might want to use List::listIterator(), though. Something like:

ListIterator<Polygon> it = triangles.listIterator();
while (it.hasNext()) {
  if (it.next().contains(startDrag)) {
    it.remove();
    break;
  }
}

Answer 3

For some reason this code works without concurrent modification:

    ArrayList<Polygon> list = new ArrayList<Polygon>();

 list.add(new Polygon(new int[] { 1, 3, 1 }, new int[] { 2, 3, 1 }, 3));
 list.add(new Polygon(new int[] { 2, 3, 1 }, new int[] { 3, 4, 1 }, 3));
 list.add(new Polygon(new int[] { 4, 5, 1 }, new int[] { 5, 6, 1 }, 3));

 for (Polygon i : list) {
  if (i.contains(new Point(2, 3))) {
   System.out.println("Removing " + i);
   list.remove(i);
  }
 }

 for (Polygon i : list) {
  System.out.println(i);
 }

It removes first polygon.

Answer 4

This is a nasty, I dug it - it's the system thread from the Mouse Clicked being identified as not the creator of the java.util.Thingamabob ( a new data structure to be released tomorrow )

Despite documentation, in alignment with common sense, what the thing does actually is see that the thread from MouseClicked is not the Thread that created

java.util.List<Polygon> triangles

using something on the order of Thread ID or such as naming of threads go and then throws from deep within the JVM an exception ( sets the exception flag on the C side ) then returns leaving you with no functioning exception.getMessage() .....

You can implement copy semantics and the original version will work, does some bit-twisting on your brain but it works.