How can I make a class not extendable without using the final keyword?

Question

How can I achieve this without the final keyword? What must I change to the constructors?

 public final class testName {
     testName() {
        //do something
     }
 }

Answer 1

If you make all your Constructors private, then the class will also no longer be extendable.

public class TestName {

    private TestName(){do something}

}

To see why, check section 3.4.4.1, 'The default constructor'. By declaring your private default constructor, the last sentence of the paragraph holds:

Such a [private] constructor can never be invoked from outside of the class, but it prevents the automatic insertion of the default constructor.

So effectively by declaring a constructor in the superclass that is not accessible, there is no (other) constructor that your subclass could call and thus Java prevents compilation.