Why can you not inherit from a class whose constructor is private?

Question

Why does Java disallow inheritance from a class whose constructor is private?

Answer 1

Because a class must call its super class constructor always. If the super class constructor can't be accessed, then the sub class can't be initialized.

More info: JLS 8.8.10. Preventing Instantiation of a Class

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Regarding Brian Roach's comments:

The call [to the parent class constructor] is only implicit if you don't do it explicitly and the parent has a public or protected no-arg constructor (or hasn't defined any in which case there's a default no-arg). It's required because ... that's how the language works. Children [classes] must call [their] parent's constructor.

Note that when you instantiate any class in Java, there's always a implicit call to Object constructor since it is the super class of all classes. It will execute its default constructor:

public Object() {
}

Note from the JLS link:

It is a compile-time error if a default constructor is implicitly declared but the superclass does not have an accessible constructor (§6.6) that takes no arguments and has no throws clause.

Answer 2

Java doesn't prevent sub-classing of class with private constructors.

public class Main {
    static class A {
        private A() {
            System.out.println("Subclassed A in "+getClass().getName());
        }
    }

    static class B extends A {
        public B() {

        }
    }

    public static void main(String... ignored) {
        new B();
    }
}

prints

Subclassed A in Main$B

What it prevents is sub-classes which cannot access any constructors of its super class. This means a private constructor cannot be used in another class file, and a package local constructor cannot be used in another package.

In this situation, the only option you have is delegation. You need to call a factory method to create an instance of the "super" class and wrap it.