Question

Sometimes it’s very difficult to find an error in an application because of static data. Is it a good approach to use static data in OOP or is it better to avoid it?
Thanks in advance.

UPDATE:

How often are you using static classes and for what purposes?

Answer 1

Generally speaking static methods leads to code that is hard to test since

• we cannot know if any other class have altered a possible state,
• we cannot easily mock or switch out a static implementation.

There are a few valid use cases though.

• To implement a Singleton pattern
• To provide costructional logic (e.g., named factory methods), such as
  • Lists.newArrayList()
  • Lists.newArrayListWithExpectedSize()
    An example of the latter could be

I would normally refrain from using static methods and let the default principle be that the existence of any static content needs to be motivated.

As pointed out by @Louis, static variables are useful to provide single instance constants. Typically this can also apply to member variables holding objects such as a Logger.