Re: (Probably) easy question about inheritance

The most important thing to think about when you're designing
inheritance (IS-A) relationships is a little guideline called the
Liskov Substitution Principle. Start by assuming some code that knows
ONLY about the base class. If you pass an instance of the derived
class into this code, will its assumptions about the base class hold?
So, in your example, will code that manipulates a Matrix behave
correctly (make the correct assumptions, etc.) if a Vector is passed
in?

That's the philosophical question.

It strikes me: if a Vector has 3 dimensions, can it really be said to
extend a *generic* Matrix if a Matrix can have any number of
dimensions? A good example of where mathematical IS-A-ness differs
from programmatic IS-A-ness....

For your particular question, you can't do what you want with fields,
but if you have a Matrix accessor getElement(int i), you can provide
an a Vector accessor getX() that simply delegates to getElement(0):

class Matrix {
  private int array[];
  public int getElement(int i) { return array[i]; }
}

class Vector extends Matrix {
  public int getX() { return array[0]; }
}

But again, this strikes me as not being the right fit.

Fred