maps of maps of maps of...

[Tim Harsch]

Is there something in the collecitons to represent map of maps?  Something like compound keys in database, or similar to Perl:
$mymap->{KEY1}{KEY2}{KEY3}=1. 

I realize I can create Maps of Maps, but then I have to handle the book keeping of instantiating maps at each level of access and that could be tedious for some keys >= 2.
Map<STRING,Map<STRING,Map<STRING,INTEGER>>>

Also, extending some of the API methods, like put( K, V) to now be put(K1, K2, K3, V) could take some doin'.

And then to make it generic for any number of keys, well hmmm...

Tim

[Dimitris Andreou]

How about something similar to Map<List<K>, V> ? If it fits your case,
it would be a much more compact representation than nested maps
(especially if you are thinking of HashMaps).

Regards,
Dimitris

2009/12/31 Tim Harsch <harsc...@gmail.com>:

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[Jared Levy]

http://code.google.com/p/google-collections/issues/detail?id=173 describe my solution when there are two key fields:

I created a Table interface that maps a two-part key (a row and a column) to a value. 


It includes methods like

  V put(R rowKey, C columnKey, V value) 
  V get(Object rowKey, Object columnKey) 

Hopefully, we'll be able to release that some time in the next few months.

Jared

--

[Pat Farrell]

  V put(R rowKey, C columnKey, V value) 
  V get(Object rowKey, Object columnKey) 

Hopefully, we'll be able to release that some time in the next few months.

And we all are hopeful that its more like

 

  V get(R rowKey, C columnKey) 

[Tim Harsch]

I blogged the code to the Triplet class I mentioned:
http://harschware.blogspot.com/2009/12/triplet-class.html

[Dimitris Andreou]

Given a Pair type, is there a non-syntactic difference between
Table<R,C,V> and a Map<Pair<C, R>, V> ? Can you describe the main
motivation for it?

Regards,
Dimitris

2009/12/31 Jared Levy <jared....@gmail.com>:

[Craig P. Motlin]

Get should take Object, not generic types.  This is one of areas where Sun actually got it right.

http://java.sun.com/javase/6/docs/api/java/util/Map.html#get(java.lang.Object)

[Jared Levy]

The Table interface has several additional methods, such as

Set<R> rowSet();
Map<C, V> column(R row);

In addition, Table's get() and set() are more convenient than creating a pair every time you want to access the data.

The main motivation is that there are often situations where you want a two-part key. A Table simplifies the resulting code, just as a Multimap simplifies a one-to-many mapping.

[Dimitris Andreou]

I see. I was curious about you doing all this work for just 2d
keys/tables (this doesn't scale to 3d for example, unless someone
repeats the whole exercise) but I guess it's a quite a sweet spot
anyway (tables in UIs, in databases, and in-between).

I'm eager to see in the future your answer for dense tables and
whether relational operators are introduced (and which).

Regards,
Dimitris

2010/1/1 Jared Levy <jared....@gmail.com>: