= Generics

[[generics-inJava5]]
== Generics in Java 5

This section provides the essential information about generics in Java 5
needed to understand how generics are treated in AspectJ 5. For a full
introduction to generics in Java, please see the documentation for the
Java 5 SDK.

=== Declaring Generic Types

A generic type is declared with one or more type parameters following
the type name. By convention formal type parameters are named using a
single letter, though this is not required. A simple generic list type
(that can contain elements of any type `E`) could be declared:

[source, java]
....
interface List<E> {
   Iterator<E> iterator();
   void add(E anItem);
   E remove(E anItem);
}
....

It is important to understand that unlike template mechanisms there will
only be one type, and one class file, corresponding to the `List`
interface, regardless of how many different instantiations of the `List`
interface a program has (each potentially providing a different value
for the type parameter `E`). A consequence of this is that you cannot
refer to the type parameters of a type declaration in a static method or
initializer, or in the declaration or initializer of a static variable.

A _parameterized type_ is an invocation of a generic type with concrete
values supplied for all of its type parameters (for example,
`List<String>` or `List<Food>`).

A generic type may be declared with multiple type parameters. In
addition to simple type parameter names, type parameter declarations can
also constrain the set of types allowed by using the `extends` keyword.
Some examples follow:

`class Foo<T> {...}`::
  A class `Foo` with one type parameter, `T`.
`class Foo<T,S> {...}`::
  A class `Foo` with two type parameters, `T` and `S`.
`class Foo<T extends Number> {...}`::
  A class `Foo` with one type parameter `T`, where `T` must be
  instantiated as the type `Number` or a subtype of `Number`.
`class Foo<T, S extends T> {...}`::
  A class `Foo` with two type parameters, `T` and `S`. `Foo` must be
  instantiated with a type `S` that is a subtype of the type specified
  for parameter `T`.
`class Foo<T extends Number & Comparable> {...}`::
  A class `Foo` with one type parameter, `T`. `Foo` must be instantiated
  with a type that is a subtype of `Number` and that implements
  `Comparable`.

=== Using Generic and Parameterized Types

You declare a variable (or a method/constructor argument) of a
parameterized type by specifying a concrete type specfication for each
type parameter in the generic type. The following example declares a
list of strings and a list of numbers:

[source, java]
....
List<String> strings;
List<Number> numbers;
....

It is also possible to declare a variable of a generic type without
specifying any values for the type parameters (a _raw_ type). For
example, `List strings`. In this case, unchecked warnings may be issued
by the compiler when the referenced object is passed as a parameter to a
method expecting a parameterized type such as a `List<String>`. New code
written in the Java 5 language would not be expected to use raw types.

Parameterized types are instantiated by specifying type parameter values
in the constructor call expression as in the following examples:

[source, java]
....
List<String> strings = new MyListImpl<String>();
List<Number> numbers = new MyListImpl<Number>();
....

When declaring parameterized types, the `?` wildcard may be used, which
stands for "some type". The `extends` and `super` keywords may be used
in conjunction with the wildcard to provide upper and lower bounds on
the types that may satisfy the type constraints. For example:

`List<?>`::
  A list containing elements of some type, the type of the elements in
  the list is unknown.
`List<? extends Number>`::
  A list containing elements of some type that extends Number, the exact
  type of the elements in the list is unknown.
`List<? super Double>`::
  A list containing elements of some type that is a super-type of
  Double, the exact type of the elements in the list is unknown.

A generic type may be extended as any other type. Given a generic type
`Foo<T>` then a subtype `Goo` may be declared in one of the following
ways:

`class Goo extends Foo`::
  Here `Foo` is used as a raw type, and the appropriate warning messages
  will be issued by the compiler on attempting to invoke methods in
  `Foo`.
`class Goo<E> extends Foo`::
  `Goo` is a generic type, but the super-type `Foo` is used as a raw
  type and the appropriate warning messages will be issued by the
  compiler on attempting to invoke methods defined by `Foo`.
`class Goo<E> extends Foo<E>`::
  This is the most usual form. `Goo` is a generic type with one
  parameter that extends the generic type `Foo` with that same
  parameter. So `Goo<String<` is a subclass of `Foo<String>`.
`class Goo<E,F> extends Foo<E>`::
  `Goo` is a generic type with two parameters that extends the generic
  type `Foo` with the first type parameter of `Goo` being used to
  parameterize `Foo`. So `Goo<String,Integer<` is a subclass of
  `Foo<String>`.
`class Goo extends Foo<String>`::
  `Goo` is a type that extends the parameterized type `Foo<String>`.

A generic type may implement one or more generic interfaces, following
the type binding rules given above. A type may also implement one or
more parameterized interfaces (for example,
`class X implements List<String>`, however a type may not at the same
time be a subtype of two interface types which are different
parameterizations of the same interface.

=== Subtypes, Supertypes, and Assignability

The supertype of a generic type `C` is the type given in the extends
clause of `C`, or `Object` if no extends clause is present. Given the
type declaration

[source, java]
....
public interface List<E> extends Collection<E> {... }
....

then the supertype of `List<E>` is `Collection<E>`.

The supertype of a parameterized type `P` is the type given in the
extends clause of `P`, or `Object` if no extends clause is present. Any
type parameters in the supertype are substituted in accordance with the
parameterization of `P`. An example will make this much clearer: Given
the type `List<Double>` and the definition of the `List` given above,
the direct supertype is `Collection<Double>`. `List<Double>` is _not_
considered to be a subtype of `List<Number>`.

An instance of a parameterized type `P<T1,T2,...Tn>`may be assigned to a
variable of the same type or a supertype without casting. In addition it
may be assigned to a variable `R<S1,S2,...Sm>` where `R` is a supertype
of `P` (the supertype relationship is reflexive), `m <= n`, and for all
type parameters `S1..m`, `Tm` equals `Sm` _or_ `Sm` is a wildcard type
specification and `Tm` falls within the bounds of the wildcard. For
example, `List<String>` can be assigned to a variable of type
`Collection<?>`, and `List<Double>` can be assigned to a variable of
type `List<? extends Number>`.

=== Generic Methods and Constructors

A static method may be declared with one or more type parameters as in
the following declaration:

[source, java]
....
static <T> T first(List<T> ts) { ... }
....

Such a definition can appear in any type, the type parameter `T` does
not need to be declared as a type parameter of the enclosing type.

Non-static methods may also be declared with one or more type parameters
in a similar fashion:

[source, java]
....
<T extends Number> T max(T t1, T t2) { ... }
....

The same technique can be used to declare a generic constructor.

=== Erasure

Generics in Java are implemented using a technique called _erasure_. All
type parameter information is erased from the run-time type system.
Asking an object of a parameterized type for its class will return the
class object for the raw type (eg. `List` for an object declared to be
of type `List<String>`. A consequence of this is that you cannot at
runtime ask if an object is an `instanceof` a parameterized type.

[[generics-inAspectJ5]]
== Generics in AspectJ 5

AspectJ 5 provides full support for all of the Java 5 language features,
including generics. Any legal Java 5 program is a legal AspectJ 5
progam. In addition, AspectJ 5 provides support for generic and
parameterized types in pointcuts, inter-type declarations, and declare
statements. Parameterized types may freely be used within aspect
members, and support is also provided for generic _abstract_ aspects.

=== Matching generic and parameterized types in pointcut expressions

The simplest way to work with generic and parameterized types in
pointcut expressions and type patterns is simply to use the raw type
name. For example, the type pattern `List` will match the generic type
`List<E>` and any parameterization of that type
(`List<String>, List<?>, List<? extends Number>` and so on. This ensures
that pointcuts written in existing code that is not generics-aware will
continue to work as expected in AspectJ 5. It is also the recommended
way to match against generic and parameterized types in AspectJ 5 unless
you explicitly wish to narrow matches to certain parameterizations of a
generic type.

Generic methods and constructors, and members defined in generic types,
may use type variables as part of their signature. For example:

[source, java]
....
public class Utils {

  /** static generic method */
  static <T> T first(List<T> ts) { ... }

  /** instance generic method */
  <T extends Number> T max(T t1, T t2) { ... }

}

public class G<T> {

   // field with parameterized type
   T myData;

   // method with parameterized return type
   public List<T> getAllDataItems() {...}

}
....

AspectJ 5 does not allow the use of type variables in pointcut
expressions and type patterns. Instead, members that use type parameters
as part of their signature are matched by their _erasure_. Java 5
defines the rules for determing the erasure of a type as follows.

Let `|T|` represent the erasure of some type `T`. Then:

* The erasure of a parameterized type `T<T1,...,Tn>` is `|T|`.
  For example, the erasure of `List<String>` is `List`.

* The erasure of a nested type `T.C` is `|T|.C`.
  For example, the erasure of the nested type `Foo<T>.Bar` is `Foo.Bar`.

* The erasure of an array type `T[]` is `|T|[]`.
  For example, the erasure of `List<String>[]` is `List[]`.

* The erasure of a type variable is its leftmost bound.
  For example, the erasure of a type variable `P` is `Object`,
  and the erasure of a type variable `N extends Number` is `Number`.

* The erasure of every other type is the type itself.

Applying these rules to the earlier examples, we find that the methods
defined in `Utils` can be matched by a signature pattern matching
`static Object Utils.first(List)` and `Number Utils.max(Number, Number)`
respectively. The members of the generic type `G` can be matched by a
signature pattern matching `Object G.myData` and
`public List G.getAllDataItems()` respectively.

==== Restricting matching using parameterized types

Pointcut matching can be further restricted to match only given
parameterizations of parameter types (methods and constructors), return
types (methods) and field types (fields). This is achieved by specifying
a parameterized type pattern at the appropriate point in the signature
pattern. For example, given the class `Foo`:

[source, java]
....
public class Foo {

  List<String> myStrings;
  List<Float>  myFloats;

  public List<String> getStrings() { return myStrings; }
  public List<Float> getFloats() { return myFloats; }

  public void addStrings(List<String> evenMoreStrings) {
     myStrings.addAll(evenMoreStrings);
  }

}
....

Then a `get` join point for the field `myStrings` can be matched by the
pointcut `get(List Foo.myStrings)` and by the pointcut
`get(List<String> Foo.myStrings)`, but _not_ by the pointcut
`get(List<Number> *)`.

A `get` join point for the field `myFloats` can be matched by the
pointcut `get(List Foo.myFloats)`, the pointcut `get(List<Float> *)`,
and the pointcut `get(List<Number+> *)`. This last example shows how
AspectJ type patterns can be used to match type parameters types just
like any other type. The pointcut `get(List<Double> *)` does _not_
match.

The execution of the methods `getStrings` and `getFloats` can be matched
by the pointcut expression `execution(List get*(..))`, and the pointcut
expression `execution(List<*> get*(..))`, but only `getStrings` is
matched by `execution(List<String> get*(..))` and only `getFloats` is
matched by `execution(List<Number+> get*(..))`

A call to the method `addStrings` can be matched by the pointcut
expression `call(* addStrings(List))` and by the expression
`call(* addStrings(List<String>))`, but _not_ by the expression
`call(* addStrings(List<Number>))`.

Remember that any type variable reference in a generic member is
_always_ matched by its erasure. Thus given the following example:

[source, java]
....
class G<T> {
    List<T> foo(List<String> ls) { return null; }
}
....

The execution of `foo` can be matched by `execution(List foo(List))`,
`execution(List foo(List<String>>))`, and
`execution(* foo(List<String<))`but _not_ by
`execution(List<Object> foo(List<String>>)` since the erasure of
`List<T>` is `List` and not `List<Object>`.

==== Generic wildcards and signature matching

When it comes to signature matching, a type parameterized using a
generic wildcard is a distinct type. For example, `List<?>` is a very
different type to `List<String>`, even though a variable of type
`List<String>` can be assigned to a variable of type `List<?>`. Given
the methods:

[source, java]
....
class C {
  public void foo(List<? extends Number> listOfSomeNumberType) {}
  public void bar(List<?> listOfSomeType) {}
  public void goo(List<Double> listOfDoubles) {}
}
....

`execution(* C.*(List))`::
  Matches an execution join point for any of the three methods.
`execution(* C.*(List<? extends Number>))`::
  matches only the execution of `foo`, and _not_ the execution of `goo`
  since `List<? extends Number>` and `List<Double>` are distinct types.
`execution(* C.*(List<?>))`::
  matches only the execution of `bar`.
`execution(* C.*(List<? extends Object+>))`::
  matches both the execution of `foo` and the execution of `bar` since
  the upper bound of `List<?>` is implicitly `Object`.

==== Treatment of bridge methods

Under certain circumstances a Java 5 compiler is required to create
_bridge methods_ that support the compilation of programs using raw
types. Consider the types

[source, java]
....
class Generic<T> {
  public T foo(T someObject) {
    return someObject;
  }
}

class SubGeneric<N extends Number> extends Generic<N> {
  public N foo(N someNumber) {
    return someNumber;
  }
}
....

The class `SubGeneric` extends `Generic` and overrides the method `foo`.
Since the upper bound of the type variable `N` in `SubGeneric` is
different to the upper bound of the type variable `T` in `Generic`, the
method `foo` in `SubGeneric` has a different erasure to the method `foo`
in `Generic`. This is an example of a case where a Java 5 compiler will
create a _bridge method_ in `SubGeneric`. Although you never see it, the
bridge method will look something like this:

[source, java]
....
public Object foo(Object arg) {
  Number n = (Number) arg; // "bridge" to the signature defined in this type
return foo(n);
}
....

Bridge methods are synthetic artefacts generated as a result of a
particular compilation strategy and have no execution join points in
AspectJ 5. So the pointcut `execution(Object SubGeneric.foo(Object))`
does not match anything. (The pointcut
`execution(Object Generic.foo(Object))` matches the execution of `foo`
in both `Generic` and `SubGeneric` since both are implementations of
`Generic.foo`).

It _is_ possible to _call_ a bridge method as the following short code
snippet demonstrates. Such a call _does_ result in a call join point for
the call to the method.

[source, java]
....
SubGeneric rawType = new SubGeneric();
rawType.foo("hi");  // call to bridge method (will result in a runtime failure in this case)
Object n = new Integer(5);
rawType.foo(n);     // call to bridge method that would succeed at runtime
....

==== Runtime type matching with this(), target() and args()

The `this()`, `target()`, and `args()` pointcut expressions all match
based on the runtime type of their arguments. Because Java 5 implements
generics using erasure, it is not possible to ask at runtime whether an
object is an instance of a given parameterization of a type (only
whether or not it is an instance of the erasure of that parameterized
type). Therefore AspectJ 5 does not support the use of parameterized
types with the `this()` and `target()` pointcuts. Parameterized types
may however be used in conjunction with `args()`. Consider the following
class

[source, java]
....
public class C {
  public void foo(List<String> listOfStrings) {}

  public void bar(List<Double> listOfDoubles) {}

  public void goo(List<? extends Number> listOfSomeNumberType) {}
}
....

`args(List)`::
will match an execution or call join point for any of these methods

`args(List<String>)`::
will match an execution or call join point for `foo`.

`args(List<Double>)`::
matches an execution or call join point for `bar`, and _may_ match at
an execution or call join point for `goo` since it is legitimate to
pass an object of type `List<Double>` to a method expecting a
`List<? extends Number>`.
+
In this situation, a runtime test would normally be applied to
ascertain whether or not the argument was indeed an instance of the
required type. However, in the case of parameterized types such a test
is not possible and therefore AspectJ 5 considers this a match, but
issues an _unchecked_ warning. For example, compiling the aspect `A`
below with the class `C` produces the compilation warning: `unchecked
match of List<Double> with List<? extends Number> when argument is an
instance of List at join point method-execution(void C.goo(List<?
extends Number>)) [Xlint:uncheckedArgument]`;

[source, java]
....
public aspect A {
   before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
      for (Double d : listOfDoubles) {
         // do something
      }
   }
}
....

Like all Lint messages, the `uncheckedArgument` warning can be
configured in severity from the default warning level to error or even
ignore if preferred. In addition, AspectJ 5 offers the annotation
`@SuppressAjWarnings` which is the AspectJ equivalent of Java's
`@SuppressWarnings` annotation. If the advice is annotated with
`@SuppressWarnings` then _all_ lint warnings issued during matching of
pointcut associated with the advice will be suppressed. To suppress just
an `uncheckedArgument` warning, use the annotation
`@SuppressWarnings("uncheckedArgument")` as in the following examples:

[source, java]
....
import org.aspectj.lang.annotation.SuppressAjWarnings
public aspect A {
   @SuppressAjWarnings   // will not see *any* lint warnings for this advice
   before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
      for (Double d : listOfDoubles) {
         // do something
      }
   }

   @SuppressAjWarnings("uncheckedArgument")   // will not see *any* lint warnings for this advice
   before(List<Double> listOfDoubles) : execution(* C.*(..)) && args(listOfDoubles) {
      for (Double d : listOfDoubles) {
         // do something
      }
   }
}
....

The safest way to deal with `uncheckedArgument` warnings however is to
restrict the pointcut to match only at those join points where the
argument is guaranteed to match. This is achieved by combining `args`
with a `call` or `execution` signature matching pointcut. In the
following example the advice will match the execution of `bar` but not
of `goo` since the signature of `goo` is not matched by the execution
pointcut expression.

[source, java]
....
public aspect A {
   before(List<Double> listOfDoubles) : execution(* C.*(List<Double>)) && args(listOfDoubles) {
      for (Double d : listOfDoubles) {
         // do something
      }
   }
}
....

Generic wildcards can be used in args type patterns, and matching
follows regular Java 5 assignability rules. For example, `args(List<?>)`
will match a list argument of any type, and
`args(List<? extends Number>)` will match an argument of type
`List<Number>, List<Double>, List<Float>` and so on. Where a match
cannot be fully statically determined, the compiler will once more issue
an `uncheckedArgument` warning.

Consider the following program:

[source, java]
....
public class C {
   public static void main(String[] args) {
      C c = new C();
      List<String> ls = new ArrayList<String>();
      List<Double> ld = new ArrayList<Double>();
      c.foo("hi");
      c.foo(ls);
      c.foo(ld);
   }

   public void foo(Object anObject) {}
}

aspect A {
    before(List<? extends Number> aListOfSomeNumberType)
      : call(* foo(..)) && args(aListOfSomeNumberType) {
       // process list...
    }
}
....

From the signature of `foo` all we know is that the runtime argument
will be an instance of `Object`.Compiling this program gives the
unchecked argument warning: `unchecked match of List<? extends Number>
with List when argument is an instance of List at join point
method-execution(void C.foo(Object)) [Xlint:uncheckedArgument]`. The
advice will not execute at the call join point for `c.foo("hi")` since
`String` is not an instance of `List`. The advice _will_ execute at the
call join points for `c.foo(ls)` and `c.foo(ld)` since in both cases the
argument is an instance of `List`.

Combine a wildcard argument type with a signature pattern to avoid
unchecked argument matches. In the example below we use the signature
pattern `List<Number+>` to match a call to any method taking a
`List<Number>, List<Double>, List<Float>` and so on. In addition the
signature pattern `List<? extends Number+>` can be used to match a call
to a method declared to take a `List<? extends Number>`,
`List<? extends Double>` and so on. Taken together, these restrict
matching to only those join points at which the argument is guaranteed
to be an instance of `List<? extends Number>`.

[source, java]
....
aspect A {
    before(List<? extends Number> aListOfSomeNumberType)
      : (call(* foo(List<Number+>)) || call(* foo(List<? extends Number+>)))
        && args(aListOfSomeNumberType) {
        // process list...
    }
}
....

==== Binding return values in after returning advice

After returning advice can be used to bind the return value from a
matched join point. AspectJ 5 supports the use of a parameterized type
in the returning clause, with matching following the same rules as
described for args. For example, the following aspect matches the
execution of any method returning a `List`, and makes the returned list
available to the body of the advice.

[source, java]
....
public aspect A {
  pointcut executionOfAnyMethodReturningAList() : execution(List *(..));

  after() returning(List<?> listOfSomeType) : executionOfAnyMethodReturningAList() {
    for (Object element : listOfSomeType) {
       // process element...
    }
  }
}
....

The pointcut uses the raw type pattern `List`, and hence it matches
methods returning any kind of list (`List<String>, List<Double>`, and so
on). We've chosen to bind the returned list as the parameterized type
`List<?>` in the advice since Java's type checking will now ensure that
we only perform safe operations on the list.

Given the class

[source, java]
....
public class C {
  public List<String> foo(List<String> listOfStrings) {...}
  public List<Double> bar(List<Double> listOfDoubles) {...}
  public List<? extends Number> goo(List<? extends Number> listOfSomeNumberType) {...}
}
....

The advice in the aspect below will run after the execution of `bar` and
bind the return value. It will also run after the execution of `goo` and
bind the return value, but gives an `uncheckedArgument` warning during
compilation. It does _not_ run after the execution of `foo`.

[source, java]
....
public aspect Returning {
  after() returning(List<Double> listOfDoubles) : execution(* C.*(..)) {
     for(Double d : listOfDoubles) {
        // process double...
     }
  }
}
....

As with `args` you can guarantee that after returning advice only
executes on lists _statically determinable_ to be of the right type by
specifying a return type pattern in the associated pointcut. The
`@SuppressAjWarnings` annotation can also be used if desired.

==== Declaring pointcuts inside generic types

Pointcuts can be declared in both classes and aspects. A pointcut
declared in a generic type may use the type variables of the type in
which it is declared. All references to a pointcut declared in a generic
type from outside of that type must be via a parameterized type
reference, and not a raw type reference.

Consider the generic type `Generic` with a pointcut `foo`:

[source, java]
....
public class Generic<T> {
   /**
    * matches the execution of any implementation of a method defined for T
    */
   public pointcut foo() : execution(* T.*(..));
}
....

Such a pointcut must be refered to using a parameterized reference as
shown below.

[source, java]
....
public aspect A {
  // runs before the execution of any implementation of a method defined for MyClass
  before() : Generic<MyClass>.foo() {
     // ...
  }

  // runs before the execution of any implementation of a method defined for YourClass
  before() : Generic<YourClass>.foo() {
      // ...
  }

  // results in a compilation error - raw type reference
  before() : Generic.foo() { }
}
....

=== Inter-type Declarations

AspectJ 5 supports the inter-type declaration of generic methods, and of
members on generic types. For generic methods, the syntax is exactly as
for a regular method declaration, with the addition of the target type
specification:

`<T extends Number> T Utils.max(T first, T second) {...}`::
  Declares a generic instance method `max` on the class `Util`. The
  `max` method takes two arguments, `first` and `second` which must both
  be of the same type (and that type must be `Number` or a subtype of
  `Number`) and returns an instance of that type.
`static <E> E Utils.first(List<E> elements) {...}`::
  Declares a static generic method `first` on the class `Util`. The
  `first` method takes a list of elements of some type, and returns an
  instance of that type.
<T> Sorter.new(List<T> elements,Comparator<? super T> comparator) `{...}`::
  Declares a constructor on the class `Sorter`. The constructor takes a
  list of elements of some type, and a comparator that can compare
  instances of the element type.

A generic type may be the target of an inter-type declaration, used
either in its raw form or with type parameters specified. If type
parameters are specified, then the number of type parameters given must
match the number of type parameters in the generic type declaration.
Type parameter _names_ do not have to match. For example, given the
generic type `Foo<T,S extends Number>` then:

`String Foo.getName() {...}`::
  Declares a `getName` method on behalf of the type `Foo`. It is not
  possible to refer to the type parameters of Foo in such a declaration.
`public R Foo<Q, R>.getMagnitude() {...}`::
  Declares a method `getMagnitude` on the generic class `Foo`. The
  method returns an instance of the type substituted for the second type
  parameter in an invocation of `Foo` If `Foo` is declared as
  `Foo<T,N extends Number> {...}` then this inter-type declaration is
  equivalent to the declaration of a method `public N getMagnitude()`
  within the body of `Foo`.
`R Foo<Q, R extends Number>.getMagnitude() {...}`::
  Results in a compilation error since a bounds specification is not
  allowed in this form of an inter-type declaration (the bounds are
  determined from the declaration of the target type).

A parameterized type may not be the target of an inter-type declaration.
This is because there is only one type (the generic type) regardless of
how many different invocations (parameterizations) of that generic type
are made in a program. Therefore it does not make sense to try and
declare a member on behalf of (say) `Bar<String>`, you can only declare
members on the generic type `Bar<T>`.

[[declare-parents-java5]]
=== Declare Parents

Both generic and parameterized types can be used as the parent type in a
`declare parents` statement (as long as the resulting type hierarchy
would be well-formed in accordance with Java's sub-typing rules).
Generic types may also be used as the target type of a `declare parents`
statement.

`declare parents: Foo implements List<String>`::
  The `Foo` type implements the `List<String>` interface. If `Foo`
  already implements some other parameterization of the `List` interface
  (for example, `List<Integer>` then a compilation error will result
  since a type cannot implement multiple parameterizations of the same
  generic interface type.

=== Declare Soft

It is an error to use a generic or parameterized type as the softened
exception type in a declare soft statement. Java 5 does not permit a
generic class to be a direct or indirect subtype of `Throwable` (JLS
8.1.2).

=== Generic Aspects

AspectJ 5 allows an _abstract_ aspect to be declared as a generic type.
Any concrete aspect extending a generic abstract aspect must extend a
parameterized version of the abstract aspect. Wildcards are not
permitted in this parameterization.

Given the aspect declaration:

[source, java]
....
public abstract aspect ParentChildRelationship<P,C> {
    // ...
}
....

then

`public aspect FilesInFolders extends ParentChildRelationship<Folder,File> {...`::
  declares a concrete sub-aspect, `FilesInFolders` which extends the
  parameterized abstract aspect `ParentChildRelationship<Folder,File>`.
`public aspect FilesInFolders extends ParentChildRelationship {...`::
  results in a compilation error since the `ParentChildRelationship`
  aspect must be fully parameterized.
`public aspect ThingsInFolders<T> extends ParentChildRelationship<Folder,T>`::
  results in a compilation error since concrete aspects may not have
  type parameters.
`public abstract aspect ThingsInFolders<T> extends ParentChildRelationship<Folder,T>`::
  declares a sub-aspect of `ParentChildRelationship` in which `Folder`
  plays the role of parent (is bound to the type variable `P`).

The type parameter variables from a generic aspect declaration may be
used in place of a type within any member of the aspect, _except for
within inter-type declarations_. For example, we can declare a
`ParentChildRelationship` aspect to manage the bi-directional
relationship between parent and child nodes as follows:

[source, java]
....
/**
 * a generic aspect, we've used descriptive role names for the type variables
 * (Parent and Child) but you could use anything of course
 */
public abstract aspect ParentChildRelationship<Parent,Child> {

  /** generic interface implemented by parents */
  interface ParentHasChildren<C extends ChildHasParent>{
    List<C> getChildren();
    void addChild(C child);
    void removeChild(C child);
  }

  /** generic interface implemented by children */
  interface ChildHasParent<P extends ParentHasChildren>{
    P getParent();
    void setParent(P parent);
  }

  /** ensure the parent type implements ParentHasChildren<child type> */
  declare parents: Parent implements ParentHasChildren<Child>;

  /** ensure the child type implements ChildHasParent<parent type> */
  declare parents: Child implements ChildHasParent<Parent>;

  // Inter-type declarations made on the *generic* interface types to provide
  // default implementations.

  /** list of children maintained by parent */
  private List<C> ParentHasChildren<C>.children = new ArrayList<C>();

  /** reference to parent maintained by child */
  private P ChildHasParent<P>.parent;

  /** Default implementation of getChildren for the generic type ParentHasChildren */
  public List<C> ParentHasChildren<C>.getChildren() {
        return Collections.unmodifiableList(children);
  }

  /** Default implementation of getParent for the generic type ChildHasParent */
  public P ChildHasParent<P>.getParent() {
       return parent;
  }

  /**
    * Default implementation of addChild, ensures that parent of child is
    * also updated.
    */
  public void ParentHasChildren<C>.addChild(C child) {
       if (child.parent != null) {
         child.parent.removeChild(child);
       }
       children.add(child);
       child.parent = this;
    }

   /**
     * Default implementation of removeChild, ensures that parent of
     * child is also updated.
     */
   public void ParentHasChildren<C>.removeChild(C child) {
       if (children.remove(child)) {
         child.parent = null;
       }
    }

    /**
      * Default implementation of setParent for the generic type ChildHasParent.
      * Ensures that this child is added to the children of the parent too.
      */
    public void ChildHasParent<P>.setParent(P parent) {
       parent.addChild(this);
    }

    /**
      * Matches at an addChild join point for the parent type P and child type C
      */
    public pointcut addingChild(Parent p, Child c) :
      execution(* ParentHasChildren.addChild(ChildHasParent)) && this(p) && args(c);

    /**
      * Matches at a removeChild join point for the parent type P and child type C
      */
    public pointcut removingChild(Parent p, Child c) :
      execution(* ParentHasChildren.removeChild(ChildHasParent)) && this(p) && args(c);

}
....

The example aspect captures the protocol for managing a bi-directional
parent-child relationship between any two types playing the role of
parent and child. In a compiler implementation managing an abstract
syntax tree (AST) in which AST nodes may contain other AST nodes we
could declare the concrete aspect:

[source, java]
....
public aspect ASTNodeContainment extends ParentChildRelationship<ASTNode,ASTNode> {
    before(ASTNode parent, ASTNode child) : addingChild(parent, child) {
      // ...
    }
}
....

As a result of this declaration, `ASTNode` gains members:

* `List<ASTNode> children`
* `ASTNode parent`
* `List<ASTNode>getChildren()`
* `ASTNode getParent()`
* `void addChild(ASTNode child)`
* `void removeChild(ASTNode child)`
* `void setParent(ASTNode parent)`

In a system managing orders, we could declare the concrete aspect:

[source, java]
....
public aspect OrderItemsInOrders extends ParentChildRelationship<Order, OrderItem> {}
....

As a result of this declaration, `Order` gains members:

* `List<OrderItem> children`
* `List<OrderItem> getChildren()`
* `void addChild(OrderItem child)`
* `void removeChild(OrderItem child)`

and `OrderItem` gains members:

* `Order parent`
* `Order getParent()`
* `void setParent(Order parent)`

A second example of an abstract aspect, this time for handling
exceptions in a uniform manner, is shown below:

[source, java]
....
abstract aspect ExceptionHandling<T extends Throwable> {

  /**
   * method to be implemented by sub-aspects to handle thrown exceptions
   */
  protected abstract void onException(T anException);

  /**
   * to be defined by sub-aspects to specify the scope of exception handling
   */
  protected abstract pointcut inExceptionHandlingScope();

  /**
   * soften T within the scope of the aspect
   */
  declare soft: T : inExceptionHandlingScope();

  /**
   * bind an exception thrown in scope and pass it to the handler
   */
  after() throwing (T anException) : inExceptionHandlingScope() {
    onException(anException);
  }

}
....

Notice how the type variable `T extends Throwable` allows the components
of the aspect to be designed to work together in a type-safe manner. The
following concrete sub-aspect shows how the abstract aspect might be
extended to handle `IOExceptions`.

[source, java]
....
public aspect IOExceptionHandling extends ExceptionHandling<IOException>{

  protected pointcut inExceptionHandlingScope() :
    call(* doIO*(..)) && within(org.xyz..*);

  /**
   * called whenever an IOException is thrown in scope.
   */
  protected void onException(IOException ex) {
    System.err.println("handled exception: " + ex.getMessage());
    throw new MyDomainException(ex);
  }
}
....