Why is T bounded by Object in the Collections.max() signature?

by

The two have the same bounds but there is a subtle difference.

 <T extends Object & Comparable<? super T>>

This will cause T to become an Object under erasure.

 <T extends Comparable<? super T>>

This will cause T to become Comparable under erasure.

In this case it is done because .max predates Java 5. We can see in this link Joachim kindly provided that the signature of .max in Java 1.4.2 is:

public static Object max(Collection coll)

Had we used <T extends Comparable<? super T>> as a bound, our signature would be

public static Comparable max(Collection coll)

Which would break the APIs. I’ve managed to find this page that discusses converting old APIs to generic ones and it gives .max as a specific example.

Here they explain why max is defined this way:

You also need to ensure that the revised API retains binary compatibility with old clients. This implies that the erasure of the API must be the same as the original, ungenerified API. In most cases, this falls out naturally, but there are some subtle cases. We’ll examine one of the subtlest cases we’ve encountered, the method Collections.max(). As we saw in section More Fun with Wildcards, a plausible signature for max() is:

public static <T extends Comparable<? super T>> T max(Collection<T> coll) This is fine, except that the erasure of this signature is: public static Comparable max(Collection coll) which is different than the original signature of max(): public static Object max(Collection coll)

One could certainly have specified this signature for max(), but it was not done, and all the old binary class files that call Collections.max() depend on a signature that returns Object.

More Related Contents:

  1. What causes javac to issue the “uses unchecked or unsafe operations” warning
  2. Instantiating a generic class in Java [duplicate]
  3. Deserialize a List object with Gson?
  4. Can’t add value to the Java collection with wildcard generic type
  5. Static method in a generic class?
  6. Combining Raw Types and Generic Methods
  7. Jackson – Deserialize using generic class
  8. Generic array creation error
  9. Cannot create an array of LinkedLists in Java…?
  10. How can I convert int[] to Integer[] in Java?
  11. Generic type inference not working with method chaining?
  12. What is the difference between ? and Object in Java generics?
  13. why does List.toArray() return Object[] and not String[]? how to work around this?
  14. How to make a Java class that implements one interface with two generic types?
  15. Java Generics: Generic type defined as return type only
  16. How to determine if a List is sorted in Java?
  17. Difference between an unbound wildcard and a raw type
  18. Where are generic types stored in java class files?
  19. Error: Generic Array Creation [duplicate]
  20. Java Generics and adding numbers together
  21. Can I use generics over reference types only?
  22. Java Generic List
  23. String gets assigned to a List without a compilation error [duplicate]
  24. How to Convert List to List
  25. How can I determine the type of a generic field in Java?
  26. Java generic methods in generics classes
  27. How do generics of generics work?
  28. Collections.emptyList() returns a List?
  29. Cannot use Java 8 method with lambda arguments without specifying type arguments
  30. Java generic type

Leave a Comment