Couple of Java OOP questions..

techguy

Hi,

I'm doing a bit of revision for my exam and there are a couple of small questions I can't seem to figure out. I think I have them correct i'm just looking for some confirmation..

1)

How does an instance data value differ from a class data value?

An instance data value is a value that is stored in relation to the object(instance of a class) - many objects can be created from one class all having different values for instance data values. A class data value only stores 1 value and is available to all objects of that class. If one object changes the class value it is relected to all objects of that class.

2)

Which of the following would be the best choice to make a class variable (as opposed to an
instance variable) of the class Person? Why?

a) averageLifeExpectancy
b) age
c) gender
d) typeOfCarDriven

I'm thinking any of the first 3. I say this because any of those attributes are attributes of a person..not all people drive cars. I don#t remember the answer that the lecturer gave but I do remember not understanding the reason.

3)

To import a package named tools located in directory, ./packages/ what code would one have
to write?

I think that this is either a typo (the '.' shouldn't be there) or else it's a trick question. The command to answer the question would be

import ..packages.tools.*;

but this obviosuly throws an error about floats or something. Since a fullstop is a valid directory name in windows anyway maybe there is a way to use it in java?

Any opinions on my answer esp the last one..thanks.

Creamy Goodness

for question two, the average life expectancy would be the best.

for question three, it is not a typo, ./ just means from the current directory. so in the current directory there's a folder called packages and inside that a folder called tools.

import packages.tools.*;

is the line you would want.

techguy

Creamy Goodness wrote: »

for question two, the average life expectancy would be the best.

for question three, it is not a typo, ./ just means from the current directory. so in the current directory there's a folder called packages and inside that a folder called tools.

import packages.tools.*;

is the line you would want.

Thanks.. why would you choose average life expectancy?

I'm assuming I was spot on with question 1 then?

Creamy Goodness

I'd choose the life expectancy variable because the rest can be tied to a single object.

life expectancy is somethings that doesn't really tie to a person object as much as the others. i'm sure there are better ways to explain it...

question one is spot on, perhaps mention that the keyword static is usually used for class variables.

Thomas_S_Hunterson

techguy wrote: »

Thanks.. why would you choose average life expectancy?

Average life expenctancy is the same for all people.

Age and gender is not.
/edit:too slow

fasty

Average life expectancy is something that's going to be the same for everyone. It's not instance data.

techguy

Now I remember that was the reason..

My response was the averageLifeExpextancy is not the same for all people, technically speaking.

Doesn't that figure vary for different parts of the world?
< http://en.wikipedia.org/wiki/Life_expectancy#Variation_in_the_world_today >

techguy

fasty wrote: »

Average life expectancy is something that's going to be the same for everyone. It's not instance data.

I suppose, for a person from whatever region, aking my view into account.

Thomas_S_Hunterson

techguy wrote: »

I suppose, for a person from whatever region, aking my view into account.

No, technically your view is irrelevant:P

The average life expectancy is simply that. The average age of death (/edit: or at least an estimate of) of every male and female on the planet.

If you want to talk about the life expectancy of a specific country or race you can do that seperately.

techguy

Sean_K wrote: »

No, technically your view is irrelevant:P

The average life expectancy is simply that. The average age of death (/edit: or at least an estimate of) of every male and female on the planet.

If you want to talk about the life expectancy of a specific country or race you can do that seperately.

Agreed, its the average!

ronivek

It depends on how fine-grained you might ever want that value to be; but in the interest of reducing data duplication it should probably never be an instance variable at any rate.