= Covariance

[[covariance-inJava5]]
== Covariance in Java 5

Java 5 (and hence AspectJ 5) allows you to narrow the return type in an
overriding method. For example:

[source, java]
....
class A {
  public A whoAreYou() {...}
}

class B extends A {
  // override A.whoAreYou *and* narrow the return type.
  public B whoAreYou() {...}
}
....

[[covariance-and-join-point-matching]]
== Covariant methods and Join Point matching

The join point matching rules for `call` and `execution` pointcut
designators are extended to match against covariant methods.

Given the classes `A` and `B` as defined in the previous section, and
the program fragment

[source, java]
....
A a = new A();
B b = new B();
a.whoAreYou();
b.whoAreYou();
....

The signatures for the call join point `a.whoAreYou()` are simply:

[source, java]
....
A A.whoAreYou()
....

The signatures for the call join point `b.whoAreYou()` are:

[source, java]
....
A A.whoAreYou()
B B.whoAreYou()
....

Following the join point matching rules given in xref:joinpointsignatures.adoc#jpsigs[Join Point Signatures].

`call(* whoAreYou())`::
  Matches both calls, (since each call join point has at least one
  matching signature).
`call(* A.whoAreYou())`::
  Matches both calls, (since each call join point has at least one
  matching signature).
`call(A whoAreYou())`::
  Matches both calls, (since each call join point has at least one
  matching signature).
`call(A B.whoAreYou())`::
  Does not match anything - neither of the call join points has a
  signature matched by this pattern. A lint warning is given for the
  call `a.whoAreYou()` ("does not match because declaring type is `A`, if
  match required use ``target(B)``").
`call(A+ B.whoAreYou())`::
  Matches the call to `b.whoAreYou()` since the signature pattern
  matches the signature `B B.whoAreYou()`. A lint warning is given for
  the call `a.whoAreYou()` ("does not match because declaring type is `A`,
  if match required use ``target(B)``").
`call(B A.whoAreYou())`::
  Does not match anything since neither join point has a signature
  matched by this pattern.
`call(B whoAreYou())`::
  Matches the call to `b.whoAreYou()` only.
`call(B B.whoAreYou())`::
  Matches the call to `b.whoAreYou()` only.

The rule for signature matching at call and execution join points is
unchanged from AspectJ 1.2: a call or execution pointcut matches if the
signature pattern matches at least one of the signatures of the join
point, and if the modifiers of the method or constructor are matched by
any modifier pattern or annotation pattern that may be present.
