Java – Does a Null Object ID Violate Encapsulation?

How will encapsulation help when making changes in code and from its rippling effects. For a data member, if I change its type from int to float, (even if I am exposing this using property) I will need to change variable type where I am using already using this code.

The benefit of encapsulation is that it lets you change the internal implementation without breaking client code. It doesn't protect you if you decide that you need to change the interface to your code, but that's a different matter.

Example: Say you have a value representing the price per unit of some commodity. The price is expressed in cents, and because you don't deal in fractional cents you decided to make the property an integer (I'll use C here because I'm not very familiar with C#):

int _price

int pricePerUnit(void) {
    return _price;
}

int priceForUnits(int units) {
    return units * _price;
}

That all works out fine until one day when somebody notices that your firm is losing a lot of money due to rounding errors. Many of the commodities that you track are bought and sold in lots of many thousands of units, so you need to start tracking the price to an accuracy of at least 0.001 cent. Because you were smart enough to encapsulate the price instead of letting clients access it directly, you can make that change pretty quickly:

double _dprice

int pricePerUnit(void) {
    return (int)_dprice;
}

int priceForUnits(int units) {
    return (int)(units * _dprice);
}

The interface that clients use to obtain prices stays the same, but the data they get back is now more accurate. If the price per unit is $1.001, priceForUnits(1000000) will now return a price that's $1000 greater than before. That happens even though you haven't changed the interface to your system at all, and you therefore haven't broken any client code.

Now, that may not always be all that you need to do. Sometimes you'll need to change or augment your interface so that you can report the price more accurately to clients, too:

double pricePerUnit() {
    return _dprice;
}

A change like that will break client code, so you might instead keep the old interface and provide a newer, better routine:

int pricePerUnit() {
    return (int)_dprice;
}

double accuratePricePerUnit() {
    return _dprice;
}

You and the rest of your team can then embark on the process of converting all the clients of your system to use the newer, better accuratePricePerUnit(). The client code will get more accurate as you make progress on that task, but even the old stuff should continue to work as well as it did in the past.

Anyway, the point is that encapsulation lets you change the way the internals work while presenting a consistent interface, and that helps you make useful changes without breaking other code. It doesn't always protect you from having to update other code, but it can at least help you do that in a controlled manner.

The disadvantages are the inefficiency of the extra indirection, as you pointed out, and the fact that the compiler doesn't enforce it. All it takes is your worst programmer using one unencapsulated reference to destroy the benefits.

Also, the right way to solve a null pointer problem isn't to replace it with a non-null default value with essentially the same characteristics. The problem with null pointer dereferences isn't that they cause a segfault. That's just a symptom. The problem is that the programmer might not always handle an unexpected default/uninitialized value. That problem still must be handled separately with your self-encapsulation pattern.

The right way to solve a null pointer problem is to not create the object until a semantically valid non-null value can be put into the attribute, and to destroy the object before it is necessary to set any of its attributes to null. If there is never the possibility for a pointer to be null, there is never a need to check it.

Usually when people think an attribute must be null, they are trying to do too much in one class. It often makes the code much cleaner to split it into two classes. You can also split functions to avoid null assignments. Here's an example from another question where I refactored a function to avoid a problematic null assignment.