How to ensure that the toString () method is overridden in a class?

Question

Suppose I have an interface and many classes that implement this interface. I want to provide an override of the default implementation of toString() in each of these classes (that is, if some classes do not override it, this should lead to a compilation error).

Can this be achieved? Declaration public abstract String toString(); with or without @Override annotation in the interface enclosure is legal, but has no effect.

Answer 1

Write an annotation and annotation handler and use it at compile time.

Your annotation will look like this:

 public @ interface MustOverrideToString { } 

and your annotation handler will look for any class that

• extends class using MustOverrideToString annotation
• does not override toString

Answer 2

Yup, sort of.

 protected abstract String internToString(); 

and then

 @Override public String toString() { return internToString(); } 

in the base class.

Answer 3

I think you do not need to declare anything other than implementing the toString() method in a specific class.

Answer 4

If you create an abstract class, you do not need another method:

 public abstract class Base{ public abstract String toString(); } ... public class Sub extends Base{} //will not compile