Which access specifier is preferred for constant variables?

Answer 1

There's no such thing as a constant variable (other than as an oxymoron). A named value is either declared constant or it is declared variable, it cannot be both a constant and a variable.

You declare a constant much as you would a variable, except you use the final keyword modifier and name the constant using ALL CAPS and underscores for spaces.

final int HOURS_IN_DAY = 24;

final int DAYS_IN_WEEK;

DAYS_IN_WEEK = 7;

Note that we don't need to assign a constant value at the point of instantiation. However, once assigned we cannot then change the value. It wouldn't be a constant if we could!

The preferred access specifier depends on how useful the constant is. If only one method makes use of the constant, then it should simply be declared within that method as a local constant. If several methods of the same class make use of the constant then declare it a private member of the class. Both methods are used in the following example:

public class MyClass {

private static int HOURS_IN_DAY = 24;

public int hoursInDays (int days) {

return days * HOURS_IN_DAY;

}

public int hoursInWeeks (int weeks) {

final int DAYS_IN_WEEK = 7;

return weeks * DAYS_IN_WEEK * HOURS_IN_DAY;

}

}

Note the use of the static keyword in the constant declaration. It doesn't make sense for every object of a class to store independent copies of the same constant value, thus we must declare class constants as static members so that all objects of the class can share the same instance of the constant and thus share the same value.

If we want the constant to be accessible to our subclasses, then we can declare the constant protected. If we want to make the constant available outside of the class then we can declare it public.

Keep in mind that although constants are not variable, we should only expose what needs to be exposed outside of a class, no more and no less. A constant that provides a class implementation detail has no business being accessible outside of the class in which it is declared.

You should also be aware that the rules governing constants are different for primitive data types and objects. With primitive data types, the value is constant as one would expect. But with objects, only the reference is constant (we cannot refer to a different object) but if the referenced object allows mutations then it is not truly constant. Unfortunately, the only way to create a truly constant object from a mutable class is to subclass the mutable class and render its setters private.