Properly removing an Integer from a List

Question

Here\'s a nice pitfall I just encountered. Consider a list of integers:

List list = new ArrayList();
list.add(5);
list.add(6);         


        

Answer 1

Well here is the trick.

Let's take two examples here:

public class ArrayListExample {

public static void main(String[] args) {
    Collection collection = new ArrayList<>();
    List arrayList = new ArrayList<>();

    collection.add(1);
    collection.add(2);
    collection.add(3);
    collection.add(null);
    collection.add(4);
    collection.add(null);
    System.out.println("Collection" + collection);

    arrayList.add(1);
    arrayList.add(2);
    arrayList.add(3);
    arrayList.add(null);
    arrayList.add(4);
    arrayList.add(null);
    System.out.println("ArrayList" + arrayList);

    collection.remove(3);
    arrayList.remove(3);
    System.out.println("");
    System.out.println("After Removal of '3' :");
    System.out.println("Collection" + collection);
    System.out.println("ArrayList" + arrayList);

    collection.remove(null);
    arrayList.remove(null);
    System.out.println("");
    System.out.println("After Removal of 'null': ");
    System.out.println("Collection" + collection);
    System.out.println("ArrayList" + arrayList);

  }

}

Now let's have a look at the output:

Collection[1, 2, 3, null, 4, null]
ArrayList[1, 2, 3, null, 4, null]

After Removal of '3' :
Collection[1, 2, null, 4, null]
ArrayList[1, 2, 3, 4, null]

After Removal of 'null': 
Collection[1, 2, 4, null]
ArrayList[1, 2, 3, 4]

Now let's analyze the output:

1. When 3 is removed from the collection it calls the remove() method of the collection which takes Object o as parameter. Hence it removes the object 3. But in arrayList object it is overridden by index 3 and hence the 4th element is removed.

2. By the same logic of Object removal null is removed in both cases in the second output.

So to remove the number 3 which is an object we will explicitly need to pass 3 as an object.

And that can be done by casting or wrapping using the wrapper class Integer.

Eg:

Integer removeIndex = Integer.valueOf("3");
collection.remove(removeIndex);