secrets of list class part 2: xAll strikes back

I continue my series of entries about less known features of the List class and today I'm going to explain the uses of addAll(), retainAll(), removeAll() methods.
Assume that you have two Lists and you want to merge them somehow.

/* list1 consists of [1, 2, 3, 4, 5] and list2 consists of [6, 7, 8, 9]*/
/* I want to merge both lists*/
/* I can use addAll and add list2 at the end of list1*/
list1.addAll(list2);
/* if we take a look at list1 we will see*/
System.out.println(list1);
/* output is [1, 2, 3, 4, 5, 6, 7, 8, 9] */

If you take a look at the illustration above you'll see that red, green part of the sets and two blue part of the sets (elements of the intersection are used 2 times each) are obtained after addAll().


If we don't want elements of the second list to be added to the end of the first list then we can use addAll() with an index argument.

/* list1 consists of [1, 2, 3, 4, 5] and list2 consists of [6, 7, 8, 9] (again)*/
/* I want to merge both lists*/
/* but I want that list2's elements come first*/
list1.addAll(0, list2);
System.out.println(list1);
/* output is [6, 7, 8, 9, 1, 2, 3, 4, 5] */

Notice that

 list1.addAll(list2) 

and

 list1.addAll(list1.size(), list2) 

are the same thing and adding the list2 to the list1 does not effect the content of list2.

Now, it's the removeAll()'s turn. You can remove all the elements of a Collection from a List using this method.

/* list1 consists of [1, 2, 3, 4, 5] and list2 consists of [6, 7, 8, 9] (again)*/
/* I want to remove elements of list2 from list1*/
list1.removeAll(list2);
System.out.println(list1);
/* output is [1, 2, 3, 4, 5]. list1 is intact because there are no elements */
/* of list2 in list1*/

/* list1 consists of [1, 2, 3, 4, 5] and list2 consists of [3, 8, 9, 4]*/
/* I want to remove elements of list2 from list1*/
list1.removeAll(list2);
System.out.println(list1);
/* output is [1, 2, 5]. 3 and 4 which are elements of both list1 and list2 */
/* are removed from list1*/

If you look at the picture above you'll see that we took only the red part of list1 and removed the intersection from it.

The last method of my entry will be retainAll(). retainAll() retains only shared elements of list1 and list2.

/* list1 consists of [1, 2, 3, 4, 5] and list2 consists of [3, 8, 9, 4]*/
/* I want to retain only shared elements of both lists */
list1.retainAll(list2);
System.out.println(list1);
/* output is [3, 4] */

If you take a look at the picture above you'll see that we retain only the intersection which is the blue part of list1. That's all for part 2 secrets of List class part 2.

concurrentmodificationexception or: how I learned to stop worrying and loved for-each loop as it is

Few days ago, I got a ConcurrentModificationException out of nowhere while I was iterating through a list and removing elements from it. I remember doing this thing and not getting any exception. I delved a little bit and realized that the exception is thrown because of my misuse of for-each loop (introduced in jdk 5). Its developers noted:

Consider, for example, the expurgate method. The program needs access to the iterator in order to remove the current element. The for-each loop hides the iterator, so you cannot call remove. Therefore, the for-each loop is not usable for filtering. Similarly it is not usable for loops where you need to replace elements in a list or array as you traverse it. Finally, it is not usable for loops that must iterate over multiple collections in parallel.

For-each loop creates and hides the iterator, so when you try to remove an element using the list itself you'll get ConcurrentModificationException because you have to do the removing and adding through the iterator. So the code below will throw ConcurrentModificationException.

/*
integerList is an ArrayList 
with Integer objects as elements
*/

for(Integer integer : integerList){
 if(integer.intValue()%2 == 0) // remove even elements
  integerList.remove(integer);
}

You can fix your error by not using for-each loop. For instance, you can define your iterator and work with it or you can iterate through the array list and do the add/remove directly from the list.

First one's code:

Iterator iterator = integerList.iterator(); // create the iterator
while(iterator.hasNext()){
 Integer integer = iterator.next(); // take the element
 if(integer.intValue()%2 == 0) 
  iterator.remove(); // remove using the iterator
}

Second approach's code:

for(int i=0 ; i < integerList.size(); i++){
 Integer integer = integerList.get(i); // take the element
 if(integer.intValue() %2 == 0){
        // remove from the list
  integerList.remove(i);
/*
decrease the index by one, so
that we won't skip any elements
after the element left-shifting which
occurs after each removal.
*/

  i--;
 }
}