Preventing a Java class from being instantiated and inherited

Question

I've been asked to modify a simple class so it would not be able to be instantiated and inherited. I got confused on how to use abstract for not being instantiated and final for not being inherited.

How should I write such a class ?

Thanks in advance.

Answer 1

A class can't be both final and abstract. You can prevent instantiation by hiding the constructor, and raising an exception to prevent instantiation via reflection.

That looks like this:

final class MyClass {
    private MyClass() {
        throw new IllegalStateException("Cannot be instantiated"); //some exception
    }
}

Answer 2

// final can not be inherited
// only static methods will work on this class
final class FrozenClass {
  private FrozenClass() {
    // can not be instantiated
  }
}

Answer 3

For this reason you have to make your class final and add a private constructor: look at this example:

final class  uninstantiable {
    private uninstantiable() {
    }
}

so now whenever you try to instantiate new object it will err classname has private access in packagename.