The Java Language Specification, Third Edition

CLASSES Class Body and Member Declarations 8.1.6
public ??? getNumberOfScales() { return 91; }
}
It is impossible to declare a method named getNumberOfScales whose signature
and return type are compatible with those of both the methods declared in interface
Fish and in interface StringBass, because a class cannot have multiple
methods with the same signature and different primitive return types (§8.4).
Therefore, it is impossible for a single class to implement both interface Fish and
interface StringBass (§8.4.8).
A class may not at the same time be a subtype of two interface types which
are different invocations of the same generic interface (§9.1.2), or an invocation of
a generic interface and a raw type naming that same generic interface.
DISCUSSION
Here is an example of an illegal multiple inheritance of an interface:
class B implements I
class C extends B implements I
This requirement was introduced in order to support translation by type erasure (§4.6).
8.1.6 Class Body and Member Declarations
A class body may contain declarations of members of the class, that is, fields
(§8.3), classes (§8.5), interfaces (§8.5) and methods (§8.4). A class body may also
contain instance initializers (§8.6), static initializers (§8.7), and declarations of
constructors (§8.8) for the class.
ClassBody:
{ ClassBodyDeclarationsopt }
ClassBodyDeclarations:
ClassBodyDeclaration
ClassBodyDeclarations ClassBodyDeclaration
ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
DRAFT
189