Item 37: Use marker interfaces to define types

1.  A marker interface is an interface that contains no method declarations, but merely designates (or “marks”) a class that implements the interface as having some property.

 

2.  Marker interfaces have two advantages over marker annotations. First, marker interfaces define a type that is implemented by instances of the marked class; marker annotations do not. The existence of this type allows you to catch errors at compile time that you couldn’t catch until runtime if you used a marker annotation. Another advantage of marker interfaces over marker annotations is that they can be targeted more precisely. If an annotation type is declared with target ElementType.TYPE, it can be applied to any class or interface. Suppose you have a marker that is applicable only to implementations of a particular interface. If you define it as a marker interface, you can have it extend the sole interface to which it is applicable, guaranteeing that all marked types are also subtypes of the sole interface to which it is applicable.

 

3.  The chief advantage of marker annotations over marker interfaces is that it is possible to add more information to an annotation type after it is already in use, by adding one or more annotation elements with defaults.

 

4.  Another advantage of marker annotations is that they are part of the larger annotation facility. Therefore, marker annotations allow for consistency in frameworks that permit annotation of a variety of program elements.

 

5.  If the marker applies only to classes and interfaces, ask yourself the question, Might I want to write one or more methods that accept only objects that have this marking? If so, you should use a marker interface in preference to an annotation. This will make it possible for you to use the interface as a parameter type for the methods in question, which will result in the very real benefit of compile-time type checking. If you answered no to the first question, ask yourself one more: Do I want to limit the use of this marker to elements of a particular interface, forever? If so, it makes sense to define the marker as a subinterface of that interface. If you answered no to both questions, you should probably use a marker annotation.