Inheritance for default and abstract methods from generic interface

3049833

According to https://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.4.1.3 (Inheriting Methods with Override-Equivalent Signatures)

If an interface I inherits a default method whose signature is override-equivalent with another method inherited by I, then a compile-time error occurs. (This is the case whether the other method is abstract or default.)

The following code compiles (javac 1.8.0_60) without errors. Is this a correct behaviour?

interface A {
    void foo(String x);
}

interface B<T> extends A {
    default void foo(T x) {}
}

interface I extends A, B<String> { } // No errors.

Unknown

According to https://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.4.1.3 (Inheriting Methods with Override-Equivalent Signatures)
If an interface I inherits a default method whose signature is override-equivalent with another method inherited by I, then a compile-time error occurs. (This is the case whether the other method is abstract or default.)
The following code compiles (javac 1.8.0_60) without errors. Is this a correct behaviour?
interface A {
    void foo(String x);
}

interface B<T> extends A {
    default void foo(T x) {}
}

interface I extends A, B<String> { } // No errors.

No - I don't believe it is.

The second 'foo' method IS override-equivalent with the first 'foo' method as defined in section 8.4.2 of that same spec.

8.4.2. Method Signature

Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (§8.4.4), and, after adapting the formal parameter types of N to the the type parameters of M, the same formal parameter types. 
The signature of a method m<sub>1</sub> is a subsignature of the signature of a method m<sub>2</sub> if either: 

  m<sub>2</sub> has the same signature as m<sub>1</sub>, or  
  the signature of m<sub>1</sub> is the same as the erasure (§4.6) of the signature of m<sub>2</sub>.  

Two method signatures m<sub>1</sub> and m<sub>2</sub> are override-equivalent iff either m<sub>1</sub> is a subsignature of m<sub>2</sub> or m<sub>2</sub> is a subsignature of m<sub>1</sub>. 
It is a compile-time error to declare two methods with override-equivalent signatures in a class. 

Bullet #2 applies: the first method signature has the same erasure as the signature of the second method and hence is a 'subsignature' of the signature of the second method (obviously a String can be a subset of the T type specifier).

Since one is a 'subsignature' then they are 'override-equivalent'.

If you have a MOS account I suggest you report a bug to Oracle.

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