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Computing Thoughts
Generics and covariant return types
by Bruce Eckel
November 8, 2005
I came across another generic puzzle and hope that someone has some insights.


Suppose you have a class with a method f():

public class HasF {
  public HasF f() {
    return this;

Here's a class that uses HasF as a bound for its generic parameter T. It stores an object of type T, and since the type parameter erases to its first bound, the methods of HasF are available:

class Manipulator4<T extends HasF> {
  private T obj;
  public Manipulator4(T x) { obj = x; }
  public T manipulate() { obj.f(); }

The problem is that, in theory, covariant return types allow a return value to be the specified type or something derived from it. So it would seem that manipulate() would be able to return a HasF or something derived from it, which is exactly what T is bounded to be. So it would seem that the definition of manipulate() is OK, but the compiler gives an error: incompatible types, found : HasF, required: T. This is further confusing because T erases to HasF.

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About the Blogger

Bruce Eckel ( provides development assistance in Python with user interfaces in Flex. He is the author of Thinking in Java (Prentice-Hall, 1998, 2nd Edition, 2000, 3rd Edition, 2003, 4th Edition, 2005), the Hands-On Java Seminar CD ROM (available on the Web site), Thinking in C++ (PH 1995; 2nd edition 2000, Volume 2 with Chuck Allison, 2003), C++ Inside & Out (Osborne/McGraw-Hill 1993), among others. He's given hundreds of presentations throughout the world, published over 150 articles in numerous magazines, was a founding member of the ANSI/ISO C++ committee and speaks regularly at conferences.

This weblog entry is Copyright © 2005 Bruce Eckel. All rights reserved.

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