When to use static method and field?

Question

I know what static is, but just not sure when to use it.

static variable: I only used it for constant fields. Sometimes there are tens of constants in a class, so us

Answer 1

Is there any other typical use cases?

Global Variables

Is it against OOP design?

Not exaclty, the point is that static methods are stateless since you don't need a particular instance of a class. My favorite approach is for utility methods (like Apache Commons). But you may be aware that some methods may be better placed as class members instead of static.

Also static methods can make class testability harder once you can't override these methods or replace by mock implementation.

Performance difference ?

There's a performance Android recommendation from Google that says "prefer static over virtual":

http://developer.android.com/training/articles/perf-tips.html#PreferStatic

I'm not sure it's true for JVM since Android uses a different VM, but it makes sense given the reasons the link points out:

If you don't need to access an object's fields, make your method static. Invocations will be about 15%-20% faster. It's also good practice, because you can tell from the method signature that calling the method can't alter the object's state."