Answers to Questions and Exercises: Annotations

Questions

1. Question: What is wrong with the following interface:

   public interface House {
       @Deprecated
       public void open();
       public void openFrontDoor();
       public void openBackDoor();
   }
   

   Answer The documentation should reflect why open is deprecated and what to use instead. For example:

   public interface House { 
       /**
        * @deprecated use of open 
        * is discouraged, use
        * openFrontDoor or 
        * openBackDoor instead.
        */
       @Deprecated
       public void open(); 
       public void openFrontDoor();
       public void openBackDoor();
   }
   

2. Question: Consider this implementation of the House interface, shown in Question 1.

   public class MyHouse implements House {
       public void open() {}
       public void openFrontDoor() {}
       public void openBackDoor() {}
   }
   

   If you compile this program, the compiler produces an error because open was deprecated (in the interface). What can you do to get rid of that warning?

   Answer: You can deprecate the implementation of open:

   public class MyHouse implements House { 
       // The documentation is 
       // inherited from the interface.
       @Deprecated
       public void open() {} 
       public void openFrontDoor() {}
       public void openBackDoor() {}
   }
   

   Alternatively, you can suppress the warning:

   public class MyHouse implements House { 
       @SuppressWarnings("deprecation")
       public void open() {} 
       public void openFrontDoor() {}
       public void openBackDoor() {}
   }
   

3. Will the following code compile without error? Why or why not?

   public @interface Meal { ... }
   
   @Meal("breakfast", mainDish="cereal")
   @Meal("lunch", mainDish="pizza")
   @Meal("dinner", mainDish="salad")
   public void evaluateDiet() { ... }
   

   Answer: The code fails to compile. Before JDK 8, repeatable annotations are not supported. As of JDK 8, the code fails to compile because the Meal annotation type was not defined to be repeatable. It can be fixed by adding the @Repeatable meta-annotation and defining a container annotation type:

   @Repeatable(MealContainer.class)
   public @interface Meal { ... }
   
   public @interface MealContainer {
       Meal[] value;
   }
   

Exercises

1. Exercise: Define an annotation type for an enhancement request with elements id, synopsis, engineer, and date. Specify the default value as unassigned for engineer and unknown for date.

   Answer:

   /**
    * Describes the Request-for-Enhancement (RFE) annotation type.
    */
   public @interface RequestForEnhancement {
       int id();
       String synopsis();
       String engineer() default "[unassigned]";
       String date();    default "[unknown]";
   }