Question

In Java, is there a way to enforce a constraint that any Class that implements an interface has a no-arg constructor? If not can you enforce that it has a factory that returns an instance of the class?

Answer 1

Not on the interface, but you can write a class with a factory method:

public abstract class Foo {

    private Foo() {}

    public static Bar createBar() {
        return new BarImpl();
    }

}

public interface Bar {}

Bar myBar = Foo.createBar();

That’s how Java exposes their factory methods on classes like Calendar.java.