(answered encapsulation earlier). A Null pointer is a pointer that either inadvertently or by design is pointing to a block of memory that is invalid. Inadvertent NULL pointers are generally due to a lack of initializing them or bypassing an assignment of the pointer to a valid block of memory. Designed NULL pointers are generally a result of a subroutine call in which the subroutine wishes to signal that there is no valid data available.