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Using a New Scope to Avoid Type Capture with Java Generics

From: "andrew cooke" <andrew@...>

Date: Tue, 9 Sep 2008 08:25:18 -0400 (CLT)

This is a little trick for Java Generics that's worth knowing.  It's
mentioned very briefly in Wadler's book (p27-8), but you could easily miss
it.

The problem happens when you are dealing with type parameters that you
know are consistent, but which are not explicit in your code.  This is
common if you are processing a not-completely-typed collection.

To make things clearer, I'll use the following example:  Consider a system
that transfers various values from one place to another (perhaps it is a
scheduler sending messages of different types within a system).

We might have an interface that can both provide and receive certain
message types:

public interface Connection<Type>
{
  Type read();
  void write(Type value);
}

And our scheduler might look something like this:

class Scheduler
{
  public void process(Collection<Connection<?>> cnxs)
  {
    for (Connection<?> cns: cnxs) {
      cnx.write(cnx.read);
    }
  }
}

(Note that <?> is shorthand for <? extends Object> and we use it here
because the cnxs collection contains a Connections with a variety of
different type parameters).

Unfortunately that won't compile!  The error given be Eclipse with Java
1.6 is "The method write(capture#2-of ?) in the type
Connection<capture#2-of ?> is not applicable for the arguments
(capture#3-of ?)".

The reason this won't compile is that the type parameter for the value
being returned by the Connection and the type parameter for the value it
will receive are being treated separately.  Each is being treated as
"capture-of ?" which means "some subclass of Object".  And the compiler is
then (understandably) saying "I can't send 'subclass X of Object' to a
method that expects 'subclass Y of Object' because I don't know if they
are the same subclass".

To make this work we need to introduce the common type parameter
explicitly.  Unfortunately the following code, or something like it,
doesn't work (as far as I can tell).  There is no way to introduce a type
parameter in the middle of a block of code (what we really want here is
better support for polymorphism):

class Scheduler
{
  public void process(Collection<Connection<?>> cnxs)
  {
    // syntax error!
    for (<E> Connection<E> cns: cnxs) {
      E value = cnx.read();
      cnx.write(value);
    }
  }
}

But what we can do is add a helper method that introduces a new type
parameter:

class Scheduler
{
  public void process(Collection<Connection<?>> cnxs)
  {
    for (Connection<?> cnx: cnxs) {
      helper(cnx);
    }
  }
  private <E> void helper(Connection<E> cnx)
  {
    E value = cnx.read();
    cnx.write(value);
  }
}

This does what we want!  The code validates, compiles, and runs.

In summary then:  Sometimes you can "lose" an explicit generic type
parameter (often because you are dealing with a collection of different
types).  You can re-introduce that type parameter by adding an extra
helper method.

Andrew

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