Do uninitialized primitive instance variables use memory?

Question

In Java, does it cost memory to declare a class level instance variable without initializing it?
For example: Does int i; use any memory if I don't initialize it with i = 5;?

Details:

I have a huge super-class that many different (not different enough to have their own super classes) sub-classes extend. Some sub-classes don't use every single primitive declared by the super-class. Can I simply keep such primitives as uninitialized and only initialize them in necessary sub-classes to save memory?

Answer 1

All members defined in your classes have default values, even if you don't initialize them explicitly, so they do use memory.

For example, every int will be initialized by default to 0, and will occupy 4 bytes.

For class members :

int i; 

is the same as :

int i = 0; 

Here's what the JLS says about instance variables :

If a class T has a field a that is an instance variable, then a new instance variable a is created and initialized to a default value (§4.12.5) as part of each newly created object of class T or of any class that is a subclass of T (§8.1.4). The instance variable effectively ceases to exist when the object of which it is a field is no longer referenced, after any necessary finalization of the object (§12.6) has been completed.

Answer 2

Yes, memory allocates though you are not assigning any value to it.

int i; 

That takes 32 bit memory (allocation). No matter you are using it or not.

Some sub-classes don't use every single primitive declared by the super-Class. Can I simply keep such primitives as uninitialized and only initialize them in necessary sub-classes to save memory?

Again, no matter where you initialized, the memory allocates.

Only thing you need to take care is, just find the unused primitives and remove them.

Edit: Adding one more point that unlike primitive's references by default value is null, which carries a a memory of

 4 bytes(32-bit)   8 bytes on (64-bit)