Java What are the potential difficulties to consider while implementing a marker interface in Java?

[Hilton D]

For example, let's say I have a class hierarchy of animals, and I want to mark some of them as endangered. I can create a marker interface called "Endangered" that I saw in a similar article by scaler like this:

public interface Endangered {
  // empty interface, no methods declared
}

Then I can have several classes that implement the Endangered interface:

public class Tiger implements Endangered {
  // class implementation
}

public class Panda implements Endangered {
  // class implementation
}

I can now use the Endangered interface to declare instances of the Tiger and Panda classes as endangered:

Tiger tiger = new Tiger();
if (tiger instanceof Endangered) {
  // tiger is endangered
}

Panda panda = new Panda();
if (panda instanceof Endangered) {
  // panda is endangered
}

However, there are certain possible concerns to consider while employing marker interfaces:

Because Java does not enable multiple inheritances, a class can only implement one marker interface at a time. This may restrict the design's flexibility and extension.

 

[JavaJames]

That’s a misunderstanding. A class can only extend from one parent, but it CAN implement any number of interfaces.

See the Java Language Spec.

Chapter 8. Classes

or try

interface A{}
interface B{}
public class MyClass implements A,B {
    public static void main(String... args) {
        var x = new MyClass();
        System.out.println(x instanceof A);
        System.out.println(x instanceof B);
    }
}

 

[Deleted member 2829]

I don't really see how an interface is the easiest or proper way to implement an 'endangered' class property. This would require a recompile when a species becomes (or ceases to be) endangered. Why not have a static class member that can be set or unset programmatically ? Am I missing something ?

 

[JavaJames]

I agree. Bring endangered is subject to change, so needs must be implemented by a variable updatable at run time.
Marker interfaces are useful for class attributes that are fixed at compile time (eg Serialisable) and have the big advantage that the compiler can then check for any errors related to their use.