The Java Language Specification, Third Edition

CLASSES Inheritance, Overriding, and Hiding 8.4.8
It is a compile time error if a type declaration T has a member method m1 and
there exists a method m2 declared in T or a supertype of T such that all of the following
conditions hold:
• m1 and m2 have the same name.
• m2 is accessible from T.
• The signature of m1 is not a subsignature (§8.4.2) of the signature of m2. • m1 or some method m1 overrides (directly or indirectly) has the same erasure
as m2 or some method m2 overrides (directly or indirectly).
DISCUSSION
These restrictions are necessary because generics are implemented via erasure. The rule
above implies that methods declared in the same class with the same name must have different
erasures. It also implies that a type declaration cannot implement or extend two distinct
invocations of the same generic interface. Here are some further examples.
A class cannot have two member methods with the same name and type erasure.
class C { T id (T x) {...} }
class D extends C {
Object id(Object x) {...}
}
This is illegal since D.id(Object) is a member of D, C.id(String) is
declared in a supertype of D and:
• The two methods have the same name, id
• C.id(String) is accessible to D
DRAFT
• The signature of D.id(Object) is not a subsignature of that of
C.id(String)
• The two methods have the same erasure
DISCUSSION
Two different methods of a class may not override methods with the same erasure.
class C { T id (T x) {...} }
interface I { Tid(T x); }
class D extends C implements I {
227