When should methods be made private? [closed]

Answer 1

The only rule I follow is to make as little as possible public.

Look at it this way. You can always make something public later on - it won't break any existing code. Trying to make something private that was public could well end up breaking a lot of existing code.

If someone wants more functionality from your class then they can make the request and you can expose what they need. Chances are that they'll want something that's subtly different to what you already have anyway so you'll need a new interface.

Answer 2

I am working on a system made up of mainly two things : importing data from various sources, and generating reports. The problem is that the whole thing was designed by someone lacking basic OO design skills. In the same 'class' they would have 'private' methods to read data from a database, calling 'private' methods to validate said data, which is also being called by another 'private' 500-line function duplicated more or less in the whole application which simply formats said data.

You should move away private avgSurveyTime() private fetchVendors() private fetchSendCounts() from the actual class which handles page layout.

Answer 3

Public and private methods are very different beasts. Use caution before making a method public.

• Public methods must validate all of their parameters.
• They must be properly documented, including any exceptions that they might throw.
• All edge cases must be analyzed and delt with (in code or in documentation).
• Any requirements involving the order in which public methods are called must be documented or, preferably, removed.
• Object state requirements must also be documented and validated.
• Public methods must not change their signature or behavior in any way that may break an application when moving from one version to the next.
• Public methods may need to be designed with marshalling requirements. (Such as .Net's CLS restrictions.)

Private methods have none of these limitations.

Answer 4

A helpful guidance technique to follow is to write an interface for your class before you start the implementation. Then when implementing, tell yourself that a method not in the interface should be private, or not in that class at all.

If you didn't know a method needed to be there when you were designing the contract of the class, it probably shouldn't be part of its public interface.

Answer 5

If it's not needed outside the class it is in at that very moment, make it private. If later it is, you can make it protected or public as needed.

When in doubt - make it private.

Answer 6

Generally you should expose as little as possible and make everything private that is possible.

If you make a mistake and hide something you should be exposing, no problem, just make it public. However, if you make something public and then later decide it should be private, then you may have problems because now the public method may be used by many other classes.

You are free to change implementation of your private methods without any external effects. If you have all public classes, that may not be possible, because those methods could be used by somethings outside your classes.