Answers to Questions and Exercises: Annotations

Questions

Question 1: What is wrong with the following interface:

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

Answer 1: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();
}

Question 2: Compile this program:

interface Closable {
   void close();
}

class File implements Closable {
   @Override
   public void close() {
       //... close this file...
   }
}

What happens? Can you explain why?

Answer 2: The compiler generates an error complaining that File.close doesn't override any method from its superclass. This is because it is not overriding Closable.close, it is implementing it!

Question 3: 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 complains that open has been deprecated (in the interface). What can you do to get rid of that warning?

Answer 3: 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() {}
}