Re: How to use java.util.Map in a more Perl like way.

From:
Thomas Hawtin <usenet@tackline.plus.com>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 03 Aug 2007 20:07:56 +0100
Message-ID:
<46b37ad7$0$1620$ed2619ec@ptn-nntp-reader02.plus.net>
robertjparks@gmail.com wrote:

Hi, I make extensive use of N-dimensional Maps in my code and would
like to find out if there is a way to manipulate them in a more
Perlish fashion. For example, say I have 2D map and I want to write
all the way through to the end. My code else up looking like this:

Map<String, Map<String,String>> map = new HashMap<String,
Map<String,String>>();
String key1= "key1";
String key2="key2";
String val="val";
// write to the structure building it up as you go
if(!map.containsKey(key1)) map.put(key1,new HashMap<String,String>());
if(!map.get(key1).containsKey(key2)) map.get(key1).put(key2,val);

In perl, you don't have to build up and walk through the structure in
order to write to it. For example, this would suffice:
my %map=();
my $key1= "key1";
my $key2="key2";
my $val="val";
# write to the structure in 1 shot
map{$key1}{$key2}=$val

To avoid having to "walk through and build up the structure" every
time I write to it, I wrote a static MapUtils to do the it. You can
say I am lazy here but the walking code blows out really fast when you
have an 5 level deep Map and I like to keep things short and neat.

public MapUtils{
    public static put(Map<String,Map<String,String>> map, String key1,
String, key2,String, val){
        if(!map.containsKey(key1)) map.put(key1,new
HashMap<String,String>());
        if(!map.get(key1).containsKey(key2)) map.get(key1).put(key2,val);
    }
}

Now I can just say:

MapUtils.put(map, key1, key2, val);

Which make my code much more readable.

Now here is where I need help!

How do I write MapUtils.put() so that it can take a Map<?,?> of any
number of dimensions and types and a list of N-keys and 1 value of any
type? I tried messing around with generics and wildcards but didn't
get too far. Maybe what I want to do is not possible. If this is the
case, I would like to hear why.


Instead of your put, you could write a static get method that creates if
necessary.

import static collection.HashMaps.get;
....
         Map<String,Map<String,String>> map2;
         Map<String,Map<String,Map<String,String>>> map3;
         ...
         get(map2, key1).put(key2, value);
         get(get(map3, key1), key2).put(key3, value);
....

package collection;

public final class HashMaps {
     private Maps() {
         throw new Error();
     }
     public static <K, MK, MV> Map<MK, MV> get(
         Map<K, Map<MK, MV>> map, K key
     ) {
         Map<MK, MV> nested = map.get(key);
         if (nested == null) {
             nested = new java.util.HashMap<MK, MV>();
             map.put(key, nested);
         }
         return nested;
     }
}

Perhaps better would be to write your own Map-like types to create on
demand.

Another approach is to use a single map with composite key. That also
may be faster and more memory efficient.

Tom Hawtin

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