It has long been recommended that we should scope our variables as narrowly as possible. Why is that so?
- Readability is greatly improved if the scope of a named variable is so small that you can see its declaration only a few lines above its usage.
- Variables leaving scope are quickly reclaimed from the stack or collected by the garbage collector.
- Invalid reuse of locally scoped variables is impossible.
- An assign-once strategy (e.g.,
finallocal variables in Java) is much easier.
- Local variables are not shared state and are therefore automatically thread safe.
But what about methods?
It's commonly discussed that you should manage the visibility of your methods by making only that public what should be exposed by the public interface. Keep overrideable methods for subclasses abstract or empty.
Layout rules can be used to make locality and access more significant: public methods first, followed by private methods; or put a private method near its first usage, just below the public method which uses it.
Why don't we scope methods as well?
Create objects for the public methods and move the private ones along with them. If you had parameter lists — especially long ones — for the private methods you can promote these parameters to instance variables of the method object.
You then have your original class declare its dependencies on these fragments and orchestrate their invocation. This keeps your original class in a coordinating role, freed from the detail of private methods. The lifetime of your method objects depends on their intended use. Mostly I create them within the scope just before being called and let them die immediately after. You may also choose to give them more significant status and have them passed in from outside the object or created by a factory.
These method objects give the newly created method scope a name and a location. They stay very narrowly focused and at a consistent level of abstraction. Often they become home for more functionality working on the state the took with them (e.g., promoted parameters or instance variables used by just these methods).
If you have private methods that are quite often reused within different other methods it's perhaps time to accept their importance and promote them to public methods in a method object.
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