[concurrency-interest] Handling Null Values in ConcurrentHashMap

Joshua Bloch josh at bloch.us
Fri May 12 15:43:59 EDT 2006


Tim and Bob,

These are interesting/cool patterns, but they're a bit scary, as they
cast something that isn't a T to (T).  You have to be very careful
what you do with the the pseudo-null.  I can see using this inside a
library, but I would think more than twice before putting it in the
JDK.

       Josh

On 5/12/06, Tim Peierls <tim at peierls.net> wrote:
> So how about something like this?
>
>
> public enum Null {
>     VALUE;
>     public static T value() { return (T) VALUE; }
>     public static boolean isNull(T value) { return value == VALUE; }
> }
>
> Use Null.<Foo>value() in Set<Foo>, but don't forget to check
> Null.isNull(foo).
>
> --tim
>
>
> --tim
>
>
> On 5/12/06, Bob Lee <crazybob at crazybob.org> wrote:
> > The nice part about using an enum is that everything works even after
> > you serialize and deserialize your object.
> >
> > Bob
> >
> > On 5/12/06, Tim Peierls <tim at peierls.net> wrote:
> > > On 5/12/06, Bob Lee <crazybob at crazybob.org> wrote:
> > >
> > > > On 5/12/06, Doug Lea <dl at cs.oswego.edu> wrote:
> > > > > Would it be easier to declare somewhere
> > > > >    static final Object NULL = new Object();
> > > > > and replace all use of nulls in uses of maps with NULL?
> > > >
> > > > Enums also work great here:
> > > >
> > >
> http://crazybob.org/2005/12/null-placeholders-in-jdk-15.html
> > >
> > >
> > > And for getting around in a generics-enabled world:
> > >
> > > public class Null<T> {
> > >     private static final Object NULL = new Object();
> > >     public static T null() { return (T) NULL; }
> > >     public static boolean isNull(T x) { return x == NULL; }
> > >     private Null() {}
> > > }
> > >
> > > Get a null value for type Foo with Null.<Foo>null(). Test it with
> > > Null.isNull(foo) or get CCEs in the same way that you would get NPE if
> you
> > > were using primitive null.
> > >
> > > --tim
> >
>
>
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