In Java, is there any disadvantage to static methods on a class?

The main disadvantage is that you cannot swap, override or choose method implementations at runtime.

In my opinion, there are four reasons to avoid static methods in Java. This is not to say that static methods are never applicable, only to say that they should generally be avoided.

1. As others have pointed out, static methods cannot be mocked out in a unit test. If a class is depending on, say, DatabaseUtils.createConnection(), then that dependent class, and any classes that depend on it, will be almost impossible to test without actually having a database or some sort of "testing" flag in DatabaseUtils. In the latter case, it sounds like you actually have two implementations of a DatabaseConnectionProvider interface -- see the next point.

2. If you have a static method, its behavior applies to all classes, everywhere. The only way to alter its behavior conditionally is to pass in a flag as a parameter to the method or set a static flag somewhere. The problem with the first approach is that it changes the signature for every caller, and quickly becomes cumbersome as more and more flags are added. The problem with the second approach is that you end up with code like this all over the place:

   boolean oldFlag = MyUtils.getFlag();MyUtils.someMethod();MyUtils.setFlag( oldFlag );

   One example of a common library that has run into this problem is Apache Commons Lang: see StringUtilsBean and so forth.

3. Objects are loaded once per ClassLoader, which means that you could actually have multiple copies of your static methods and static variables around unwittingly, which can cause problems. This usually doesn't matter as much with instance methods, because the objects are ephemeral.

4. If you have static methods that reference static variables, those stay around for the life of the classloader and never get garbage collected. If these accumulate information (e.g. caches) and you are not careful, you can run into "memory leaks" in your application. If you use instance methods instead, the objects tend to be shorter-lived and so are garbage-collected after a while. Of course, you can still get into memory leaks with instance methods too! But it's less of a problem.

Hope that helps!

The performance advantage is likely negligible. Use static methods for anything that's not state dependent. This clarifies the code, as you can immediately see with a static method call that there's no instance state involved.