When to use static methods

One rule-of-thumb: ask yourself “Does it make sense to call this method, even if no object has been constructed yet?” If so, it should definitely be static.

So in a class Car you might have a method:

double convertMpgToKpl(double mpg)

…which would be static, because one might want to know what 35mpg converts to, even if nobody has ever built a Car. But this method (which sets the efficiency of one particular Car):

void setMileage(double mpg)

…can’t be static since it’s inconceivable to call the method before any Car has been constructed.

(By the way, the converse isn’t always true: you might sometimes have a method which involves two Car objects, and still want it to be static. E.g.:

Car theMoreEfficientOf(Car c1, Car c2)

Although this could be converted to a non-static version, some would argue that since there isn’t a “privileged” choice of which Car is more important, you shouldn’t force a caller to choose one Car as the object you’ll invoke the method on. This situation accounts for a fairly small fraction of all static methods, though.

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