Should a class with only static methods be abstract?

Question

I have a class that offers a collection of static utility-type methods.

On the one hand, I don't want the class to be able to be instantiated. On the other hand, I don't want to send the signal that the class should be inherited from (not that I think that it's likely).

Should this class be abstract or not?

Answer 1

make the class final and make the default constructor private and do not provide any public constructors. that way no one can subclass it or create an instance of it.

Answer 2

Don't declare it abstract; declare a private constructor, so no one, not even a subclass, can instantiate an instance of your utility class.

You can declare your class final, although if all constructors are private, then no one will be able to subclass it anyway.

To borrow the idea from Pshemo's comment in another answer, throw a RuntimeException in the constructor to prevent reflection's setAccessible method in AccessibleObject from allowing instantiation:

public class MyUtility
{
   private MyUtility()
   {
      throw new RuntimeException("Instantiation of MyUtility is not allowed!");
   }

   public static void utilityMethod()
   {
      // Your utility method here.
   }
}

Answer 3

Although a top-level class can't be declared static, you can make the class non-instantiable (and practically 'static') to other classes by declaring the default constructor private, which forbids instantiation because no constructor is visible.

Answer 4

Another version of @mre's answer

enum MyClass{
    ;//this semicolon indicates that this enum will have no instances

    //now you can put your methods
    public static void myMethod(){
        //...
    }
}

Enum by default is final and its constructor is private. Also you cant create its instance with reflection because it checks in Constructor#newInstance if you are trying to instantiate Enum object.