public int indexOf(Object obj) {
            return parent.indexOf(obj);
        }
Gets an unmodifiable List view of the keys.
<p/>
The returned list is unmodifiable because changes to the values of
the list (using {@link java.util.ListIterator#set(Object)}) will
effectively remove the value from the list and reinsert that value at
the end of the list, which is an unexpected side effect of changing the
value of a list.  This occurs because changing the key, changes when the
mapping is added to the map and thus where it appears in the list.
<p/>
An alternative to this method is to use {@link #keySet()}.

@return The ordered list of keys.
@see #keySet()
jphp-runtime/src/php/runtime/memory/support/ArrayMemoryMap.java