Incompatible return type for method allowed when overriding method in interface of abstract superclass

Issue #107 resolved
Jesper Öqvist created an issue

JastAddJ 7.1.1-305-gb3a081f Java SE 7

When overriding an interface method it is possible to use an incompatible return type for the overriding method if a method with the same name is present in an abstract superclass. The test case below illustrates the problem:

// Overriding method from interface of abstract superclass with incompatible return type
// .result=COMPILE_FAIL
interface I {
    int m();

abstract class S implements I {
    public int m() {
        return 0;

public class Test extends S {
    public byte m() {
        return 0;

Comments (3)

  1. Jesper Öqvist reporter

    The part of JastAddJ that should check for incompatible return types in overriding methods uses the interfacesMethodsIterator attribute to find interface methods to check, however it does not work if the interface method is "shadowed" by a method in an abstract superclass. We might be able to fix the problem by altering the behaviour of interfacesMethodsIterator, but this may also cause other parts of the analysis to behave incorrectly.

  2. Jesper Öqvist reporter

    It seems like the only thing that needs to be changed to fix this issue is the mayOverride attribute. In the Java 5 module the attribute looked like this:

    refine TypeHierarchyCheck eq MethodDecl.mayOverride(MethodDecl m) {
      return type().subtype(m.type());

    This does not work as it should when both types are primitive, because it allows widening/narrowing primitive conversion when primitive return types should really be identical for overriding.

  3. Log in to comment