When should I write Static Methods?

Question

So I understand what a static method or field is, I am just wondering when to use them. That is, when writing code what design lends itself to using static methods and field

Answer 1

It gives a better idea of the intent when you use a static factory -- it also lets you have different factories that take the same argument types but have a different meaning. For example, imagine if Bitmap had LoadFromResource(string) -- it would not be possible to have two constructors that both took string.

EDIT: From stevemegson in the comments

A static factory can also return null, and can more easily return an instance that it got from cache. Many of my classes have a static FromId(int) to get an instance from a primary key, returning an existing cached instance if we have one.

Answer 2

Here are some examples of when you might want to use static methods:

1) When the function doesn't make use of any member variables. You don't have to use a static method here, but it usually helps if you do.

2) When using factory methods to create objects. They are particularly necessary if you don't know the type to be created in advance: e.g.

class AbstractClass {
    static createObject(int i) {
        if (i==1) {
           return new ConcreteClass1();
        } else if (i==2) {
           return new ConcreteClass2();
        }
     }
}

3) When you are controlling, or otherwise keeping track of, the number of instantiations of the class. The Singleton is the most used example of this.

4) When declaring constants.

5) Operations such as sorts or comparisons that operate on multiple objects of a class and are not tied to any particular instance.

6) When special handling has to be done before the first instantiation of an object.