Eclipse/javac disagree on compiling signature with default method collision; who is right?

Question

Here\'s a simple class that demonstrates the issue:

package com.mimvista.debug;

public class DefaultCollisionTest {
    public static interface Interface1 {         


        

Answer 1

Eclipse is right.

I have not found this javac bug in the Java Bug Database and therefore reported it: JDK-8186643

Better explanation by Stephan Herrmann (see his comment below):

Right, reporting an error against an intersection type should only happen when the intersection type is not well-formed and hence the intersection is empty. But as this answer shows, the intersection is not empty and should thus be legal. Actually, the error message class INT#1 inherits ... makes no sense, because at that point nobody mentioned a class INT#1, we only have the intersection of two interfaces, and that intersection is used only as a bound, not as a type.

A class that implements multiple interfaces of the same method can be compiled with both compilers, even if the method of one interface has a default implementation. The class can be referenced as as long as neither I1 nor I2 has a default implementation for a equally named method. Only if one of the two interfaces has a default implementation javac fails.

In case of ambiguity which implementation should apply, the error should already occur when defining a class, not when the class is referred as (see JLS 4.9. Intersection Types).

See following example which works with and , but fails with and javac:

interface I1 {
    String get();
}

interface I2 {
    String get();
}

interface IDefault {
    default String get() {
        return "default";
    };
}

public class Foo implements I1, I2, IDefault {

    @Override
    public String get() {
        return "foo";
    }

    public static void main(String[] args) {
        System.out.print(getOf(new Foo()));
    }

//  static  String getOf(T t) { // fails with javac
    static  String getOf(T t) { // OK
        return t.get();
    }

}