Is there any point in an abstract class having public constructor instead of protected one?

Question

Since an abstract class cannot be instantiated, and since protected members are always visible to subclasses, it would seem that it makes no difference whether its constructors are public or protected.

Is there any example where a public constructor could make a difference compared to a protected one? I would normally prefer the most restrictive access level which is applicable.

Answer 1

No, there is no good reason to make a public constructor for an abstract class: you cannot instantiate your abstract class without subclassing it first, and the language deals with the relevant corner cases for you.

In particular, if you are to subclass your abstract class anonymously, meaning that you are unable to provide your own constructor in the subclass, the language would provide one for you based on the signature of the protected constructor of your abstract base class:

abstract class Foo {
    protected int x;
    protected Foo(int x) {this.x = x;}
    public abstract void bar();
}
public static void main (String[] args) {
    Foo foo = new Foo(123) { // <<== This works because of "compiler magic"
        public void bar() {System.out.println("hi "+x);}
    };
    foo.bar();
}

In the example above it looks like the protected constructor of an abstract class is invoked, but that is not so: the compiler builds a visible constructor for your anonymous class^*, which is what gets invoked when you write new Foo(123).

Demo.

^* The actual visibility is default. Thanks, Pshemo, for spotting the error and supplying a nice test example.