How can you limit inheritance for a class?

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

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

Number 1:Commenting with // or /* */ Number 1. Few people give it. Yet you probably also use this solution often. It just doesn’t seem like an answer to an interview. Wrong. If it makes you more comfortable you can give it as the last answer, for me that’s a bonus. Think about it, spoiler: you’ll find more at the end of the post.

Number 2:Making all builders private Number 2. is about the fact that if all constructors are private the inheritance game doesn’t work, try compiling these two classes

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

and you will get this 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 seen above modified as follows

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

you will get

error: cannot inherit from final AClass

Number 4: Making all methods final The fourth and final one is making all methods final, observe the following redefinition of the classes seen above 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 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 with editing 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 well. As usual if you have a question about this post, want a follow-up or have a topic you would like me to cover in future posts you can write to me.

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