Why can't a Java class be both abstract and final

Question

Suppose I've a utility class which contains only static methods and variables. e.g:

public abstract final class StringUtils
{
    public static final String NEW_LINE = System.getProperty("line.separator");

    public static boolean isNotNullOrSpace(final String string)
    {
        return !(string == null || string.length() < 1 || string.trim().length() < 1);
    }
}

In this scenario, it makes sense to make the class both abstract and final. Abstract because making an object of this class will be of no use as all methods are accessible statically. Final because the derived class cannot inherit anything from this class as it does not have any non-static member.

C# allows static modifier for such classes. Why doesn't Java support this?

Answer 1

It is not possible because the Java language specification states that:

It is a compile-time error to declare an abstract class type such that it is not possible to create a subclass that implements all of its abstract methods [1]

Other than this, there is no inherent reason why an abstract final class would be illogical - the word abstract is generally used in contrast to concrete to mean that no direct instances of a type may exist.

This is the same reason why abstract methods cannot have access modifier private.

Answer 2

A final class can't be extended, an abstract class needs to be extended in order to be instantiated. Therefore, a final abstract class would be a logical contradiction.

If your class just have static methods, maybe you should just hide its constructor, by defining it as private.-

private StringUtils() {
}