For-each loop with null value in ArrayList

Question

Consider this simple example :

List<Integer> integerArrayList = new ArrayList<>();
        integerArrayList.add(5);
        integerArrayList.add(6);
        integerArrayList.add(7);
        integerArrayList.add(null);
  

 for(int i=0;i<integerArrayList.size();i++){
         System.out.println(integerArrayList.get(i));
     }

  

This code will print [5,6,7,null] but, if I use for-each loop like this :

  for(int el : integerArrayList)
       System.out.println(el);
    

Then this code will throw NullPointerException, and I don't understand why.

As far as I know, for-each loop above is equivalent to this :

for (Iterator<Integer> i = integerArrayList.iterator(); i.hasNext();) {
            Integer item = i.next();
            System.out.println(item);
        }

But this loop doesn't throw NullPointerException!

I would like to know what is the explanation for this.

Thanks in advance.

Answer 1

If you do for(int el : integerArrayList), or e.g. int x = integerArrayList.get(k), then you "unbox" the Integer in the list to an int. And while an Integer may be null, and int must have a proper int value (even if this value is 0), but not null.

To fix it, just use Integer instead of int, i.e.

for(Integer el : integerArrayList)