Question

In the Android class SparseArray, a static final variable DELETED is defined as a simple Object. Later in the class its reference is used as an identifier for deleted entities that were added to the container. Why are deleted entities not just nulled out? What is the purpose of differentiating between a null slot and a deleted slot?

Note: while directly asking about the SparseArray class, the question is in general.

Answer 1

It’s because null is a valid value to store in the array, so you need to be able to distinguish between null and DELETED.

As an example of why you need to distinguish them, consider get(key, default):

 @SuppressWarnings("unchecked")
 public E get(int key, E valueIfKeyNotFound) {
     int i = binarySearch(mKeys, 0, mSize, key);

     if (i < 0 || mValues[i] == DELETED) {
         return valueIfKeyNotFound;
     } else {
         return (E) mValues[i];
     }
 }

Now if you do something like:

array.put(1, null);
array.get(1, someDefault);

You will (correctly) get null.

If you replace DELETED with null, you would get someDefault instead.