Explanation of the get-put principle

by

Consider a bunch of bananas. This is a Collection<? extends Fruit> in that it’s a collection of a particular kind of fruit – but you don’t know (from that declaration) what kind of fruit it’s a collection of. You can get an item from it and know it will definitely be a fruit, but you can’t add to it – you might be trying to add an apple to a bunch of bananas, which would definitely be wrong. You can add null to it, as that will be a valid value for any kind of fruit.

Now consider a fruitbowl. This is a Collection<? super Banana>, in that it’s a collection of some type “greater than” Banana (for instance, Collection<Fruit> or Collection<TropicalFruit>). You can definitely add a banana to this, but if you fetch an item from the bowl you don’t know what you’ll get – it may well not be a banana. All you know for sure is that it will be a valid (possibly null) Object reference.

(In general, for Java generics questions, the Java Generics FAQ is an excellent resource which contains the answer to almost anything generics-related you’re likely to throw at it.)

More Related Contents:

  1. What causes javac to issue the “uses unchecked or unsafe operations” warning
  2. What’s the reason I can’t create generic array types in Java?
  3. Static method in a generic class?
  4. Combining Raw Types and Generic Methods
  5. Jackson – Deserialize using generic class
  6. Cannot create an array of LinkedLists in Java…?
  7. How can I convert int[] to Integer[] in Java?
  8. Java map with values limited by key’s type parameter
  9. Why is T bounded by Object in the Collections.max() signature?
  10. why does List.toArray() return Object[] and not String[]? how to work around this?
  11. Why doesn’t Java allow generic subclasses of Throwable?
  12. Java Generics: Generic type defined as return type only
  13. How to determine if a List is sorted in Java?
  14. Generics : List
  15. Class object of generic class (java)
  16. Whats the use of saying
  17. Error: Generic Array Creation [duplicate]
  18. Java Generics and adding numbers together
  19. Java Generic List
  20. How to Convert List to List
  21. How can I determine the type of a generic field in Java?
  22. Can you pass an int array to a generic method in java?
  23. How do generics of generics work?
  24. Is this valid Java?
  25. Java casting “.class”-operator used on a generic type, e.g. List, to “Class
  26. Why generic type is not applicable for argument extends super class for both?
  27. What do < and > mean such as implements Comparable?
  28. How to reference a generic return type with multiple bounds
  29. Java generics and array initialization
  30. Generic pair class

Leave a Comment