I want to learn about null values or null references.

For example I have a class called Apple and I created an instance of it.

Apple myApple = new Apple("yummy"); // The data is stored in memory


Then I ate that apple and now it needs to be null, so I set it as null.

myApple = null;


After this call, I forgot that I ate it and now want to check.

bool isEaten = (myApple == null);


With this call, where is myApple referencing? Is null a special pointer value? If so, if I have 1000 null objects, do they occupy 1000 object memory space or 1000 int memory space if we think a pointer type as int?

In your example myApple has the special value null (typically all zero bits), and so is referencing nothing. The object that it originally referred to is now lost on the heap. There is no way to retrieve its location. This is known as a memory leak on systems without garbage collection.

If you originally set 1000 references to null, then you have space for just 1000 references, typically 1000 * 4 bytes (on a 32-bit system, twice that on 64). If those 1000 references originally pointed to real objects, then you allocated 1000 times the size of each object, plus space for the 1000 references.

In some languages (like C and C++), pointers always point to something, even when "uninitialized". The issue is whether the address they hold is legal for your program to access. The special address zero (aka null) is deliberately not mapped into your address space, so a segmentation fault is generated by the memory management unit (MMU) when it is accessed and your program crashes. But since address zero is deliberately not mapped in, it becomes an ideal value to use to indicate that a pointer is not pointing to anything, hence its role as null. To complete the story, as you allocate memory with new or malloc(), the operating system configures the MMU to map pages of RAM into your address space and they become usable. There are still typically vast ranges of address space that are not mapped in, and so lead to segmentation faults, too.
