Compilation error with covariant return types (incompatible types)

Raffael Herzog added a comment - 25/Apr/12 8:24 AM Playing around with the example, I found that really needs to be this situation to reproduce the problem: Using classes instead of interfaces: Works Using Number/Integer instead of Base/Child as return types: Works And, interesting: Returning GChild instead of Child in GChild.groovy: Works As a workaround, I can do the following if returning GChild isn't an option: pkg/AbstractChild.java package pkg; public abstract class AbstractChild implements Child { protected abstract Object doIt(); public Child doSomething() { return (Child)doIt(); } } pkg/GChild.groovy package pkg; class GChild extends AbstractChild { // NOTE: including " implements Child" above will reproduce the problem! Object doIt() { return null } }

Knute Axelson added a comment - 13/Sep/12 5:19 PM - edited I'm having the same problem. See sample code below. If you compile and run with Java it works and prints "this should work". If you compile and run with Groovy (tried with 1.8.6 and 2.0.2), you get The return type of java.util.List doSomething() in BugExample$C is incompatible with java.lang.Iterable doSomething() in BugExample$A . At [13:9] at line: 13, column: 9 BugExample.java/groovy import java.util.List; public class BugExample { static interface A { Iterable doSomething(); } static interface B extends A { List doSomething(); } static class C implements B { public List doSomething() { System .out.println( " this should work" ); return null ; } } public static void main( String [] args) { new C().doSomething(); } }