你能编写一个需要Java中的无参数构造函数或工厂的接口吗？

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

In Java, is there a way to enforce a constraint that any Class that implements an interface has a no-arg constructor?

Not at compile-time, no.

If not can you enforce that it has a factory that returns an instance of the class?

Not at compile-time, no.

Unit tests can check for both of these, of course, if you can work out the types to check.

Answer 2

No, to both. In general, though, you should neither want or need to do that.

Answer 3

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.