CODEWITHSUNDEEP
java
TopChef
TopChef

Reputation: 45513

Instance Field Declaration in Java

I'm just getting the hang of OOP and have been playing around with Java a lot. One of the troubles I have is deciding whether I need a private instance field for a particular class. Is there a rule of thumb I should be using in terms of whether I need to make something a private instance field of not?

Thanks.

Upvotes: 0

Views: 2386

Answers (4)

tommyo
tommyo

Reputation: 551

If it´s a natural part of the object or something the object needs to perform some task on a regular basis then by all means make it an attribute. If it is a constant then you should make it a public class variable (or rather a constant :P). That is, declare it "public static final w/e"

Public instance variables are not used as often because it often leads to messier code. Think as previously stated of the instance variables (or attributes) as the objects state. It´s usualy clearer to change the objects state by performing operations on it rather than juggle publics around. Good luck.

Upvotes: 1

dmn
dmn

Reputation: 985

When learning object-oriented programming, think of it as a way to model real-world concepts as code. Nouns become objects; and the actions done to, on or by the nouns become methods. Instance variables are the properties of those nouns. For instance if you have a class representing a Car, slamOnTheBreaks() would be a method the Driver would call to slam on the breaks, and a Car has some number of seats inside, right? So an instance variable would be int numberOfSeats;.

Think of instance variables on a need-to-know, need-to-change basis. By making numberOfSeats public, that would allow the Driver to change the number of seats in the car, which doesn't make any sense. They only need to know how many seats the car has, which they can find out when they get in the car, or rather, calling the method public int getNumberOfSeats().

As Danny mentioned, the story is different for constants. If a value is a constant, it will remain constant for the entire duration of the program's execution, so for example if you want Driver Bob to be the only Driver for all those Car and Truck objects you create, Bob had better be a.) accessible, a.k.a. public, so he can be in every Car and Truck (assuming no inheritance between Car and Truck), and b.) unable to change.

Upvotes: 0

Danny
Danny

Reputation: 7518

"Avoid public fields except for constants. (Many of the examples in the tutorial use public fields. This may help to illustrate some points concisely, but is not recommended for production code.) Public fields tend to link you to a particular implementation and limit your flexibility in changing your code." Controlling Access to Members of a Class

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1503529

Well, ask yourself whether it's logically part of the state of an instance of the object. Is it something about the object which is valid for the whole lifetime of the object, or is it something which only applies during the course of a single method (in which case it should be a local variable)? Or is it actually applicable to the class itself (in which case it should be static)?

If you could give some examples where you aren't quite sure, that would help.

(Note that I've assumed that the choice here is the kind of variable - static, instance or local. Instance variables should pretty much always be private :)

Upvotes: 2

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