The Java Language Specification, Third Edition

8.4.8 Inheritance, Overriding, and Hiding CLASSES
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DISCUSSION
The rules above allow for covariant return types - refining the return type of a method when
overriding it.
For example, the following declarations are legal although they were illegal in prior versions
of the Java programming language:
class C implements Cloneable {
C copy() { return (C)clone(); }
}
class D extends C implements Cloneable {
D copy() { return (D)clone(); }
}
The relaxed rule for overriding also allows one to relax the conditions on abstract
classes implementing interfaces.
DISCUSSION
Consider
class StringSorter {
// takes a collection of strings and converts it to a sortedlist
List toList(Collection c) {...}
}
and assume that someone subclasses StringCollector
class Overrider extends StringSorter{
List toList(Collection c) {...}
}
Now, at some point the author of StringSorter decides to generify the code
class StringSorter {
// takes a collection of strings and converts it to a list
List toList(Collection c) {...}
}
An unchecked warning would be given when compiling Overrider against the new
definition of StringSorter because the return type of Overrider.toList() is List,
which is not a subtype of the return type of the overridden method, List