Java interfaces and return types

by

If the return type must be the type of the class that implements the interface, then what you want is called an F-bounded type:

public interface A<T extends A<T>>{ public T b(); }

public class C implements A<C>{
  public C b() { ... }
}

public class D implements A<D>{
  public D b() { ... }
}

In words, A is declaring a type parameter T that will take on the value of each concrete type that implements A. This is typically used to declare things like clone() or copy() methods that are well-typed. As another example, it’s used by java.lang.Enum to declare that each enum’s inherited compareTo(E) method applies only to other enums of that particular type.

If you use this pattern often enough, you’ll run into scenarios where you need this to be of type T. At first glance it might seem obvious that it is1, but you’ll actually need to declare an abstract T getThis() method which implementers will have to trivially implement as return this.

[1] As commenters have pointed out, it is possible to do something sneaky like X implements A<Y> if X and Y cooperate properly. The presence of a T getThis() method makes it even clearer that X is circumventing the intentions of the author of the A interface.

More Related Contents:

  1. Is there a way to refer to the current type with a type variable?
  2. How to implement enum with generics?
  3. How to make a Java class that implements one interface with two generic types?
  4. Java Raw Type and generics interaction
  5. Java generics – Make Generic to extends 2 interfaces
  6. Reflection for Class of generic parameter in Java?
  7. How can I determine the type of a generic field in Java?
  8. Is it possible to use a primitive type (int) in as a generic type in Java?
  9. List is a raw type. References to generic type List should be parameterized
  10. How to reference a generic return type with multiple bounds
  11. Code after if block inside a for loop doesn’t work
  12. What are the reasons why Map.get(Object key) is not (fully) generic
  13. What is an interface in Java?
  14. Why to use Interfaces, Multiple Inheritance vs Interfaces, Benefits of Interfaces?
  15. What’s the difference between primitive and reference types?
  16. How to call javascript from Android?
  17. Java Generics: Cannot cast List to List? [duplicate]
  18. Comparing the values of two generic Numbers
  19. What is the difference between and ?
  20. Comparable and Comparator contract with regards to null
  21. Call method not defined in interface from implemented interface in Java
  22. static initialization in interface
  23. LRU cache in Java with Generics and O(1) operations
  24. Using a generic class to perform basic arithmetic operations
  25. Meaning of “Cannot reduce the visibility of the inherited method” with an interface
  26. Java Interface Usage Guidelines — Are getters and setters in an interface bad?
  27. Unbounded wildcards in Java
  28. Java casting “.class”-operator used on a generic type, e.g. List, to “Class
  29. Why generic type is not applicable for argument extends super class for both?
  30. Consumer mapped Class in HashMap

Leave a Comment