How can you limit inheritance for a class?

One of the questions you might get at an interview or a certification is: How can you limit inheritance for a class? The commonly accepted answers to this question are:

Commenting with // or /* */
By making all constructors private.
Declaring the class final.
Declaring all methods final.

Number 1: Commenting with // or /* */ Few people provide this answer, yet you probably use this solution often. It just doesn’t seem like an answer to an interview. If it makes you more comfortable, you can give it as the last answer; for me, that’s a bonus.

Number 2: Making all constructors private If all constructors are private, inheritance does not work. Try compiling these two classes:

public class AClass {
   private AClass(){
   }
}
public class BClass extends AClass {
}

and you will receive the following compilation error:

AClass() has private access in AClass
public class BClass extends AClass {

Number 3: Making the class final Number 3 is using the final keyword; try compiling the two classes modified as follows:

public final class AClass {
   public AClass(){
   }
}
public class BClass extends AClass {
}

you will receive the following error:

error: cannot inherit from final AClass

Number 4: Making all methods final: Observe the following class redefinitions and try compiling:

public class AClass {
    public AClass() {
    }
    public final void method() {
    }
}
public class BClass extends AClass {
    public BClass() {
        super();
    }
    @Override
    public void method() {
    }
}

You will get the following error:

method() in BClass cannot override method() in AClass
    public void method() {
  overridden method is final

Be careful, the question is “How can I limit” not “How can I prevent (completely)” inheritance. The solution of adding final in front of class completely blocks any possibility of inheriting from the class.

Addendum Number 1: Here are two interpretations:

public class BClass /*extends AClass*/ {
    public BClass() {
        super();
    }
    //@Override
    public void method() {
    }
}

and

public abstract class AClass {
    public AClass() {
    }
    /*public final void method() {
    }*/
}

A little borderline, I admit, but shall we bet that it is the most used?

And that’s it for today. As usual, if you have questions or a topic you would like me to cover, feel free to write to me.

How can you limit inheritance for a class?

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