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how do we convert an ArrayList into a hashMap ??

On 8/30/2010 7:10 AM, parag mohite wrote:
> how do we convert an ArrayList into a hashMap ??

The same way you convert a roomful of single men into
married couples: You add a suitable amount of the missing
ingredient.

A HashMap -- any kind of Map -- contains *pairs* of keys
and values. An ArrayList -- any kind of List -- contains
individual items. You can't make a sensible key/value pair
out of just one item from a List, so you need to add some
additional information from somewhere else. Well, you *could*
make a sort of degenerate Map in which the key and value in
each pair are the same object

List<Thing> theList = ...;
Map<Thing,Thing> theMap = new HashMap<Thing,Thing>();
for (Thing thing : theList)
theMap.put(thing, thing);

.... but that's sort of like making all those single men "married"
by having them go off individually and do something in private.

Perhaps you meant a HashSet instead of a HashMap? That
would make more sense, because a HashSet -- any Set -- is a
collection of individual objects, not a collection of pairwise
associations between objects. If so, a simple way is

List<Thing> theList = ...;
Set<Thing> theSet = new HashSet<Thing>(theList);

--
Eric Sosman
lid

parag mohite wrote:
>> how do we convert an ArrayList into a hashMap [sic] ?? [sic]

Eric Sosman wrote:
> The same way you convert a roomful of single men into
> married couples: You add a suitable amount of the missing
> ingredient.
>
> A HashMap -- any kind of Map -- contains *pairs* of keys
> and values. An ArrayList -- any kind of List -- contains
> individual items. You can't make a sensible key/value pair
> out of just one item from a List, so you need to add some
> additional information from somewhere else. Well, you *could*
> make a sort of degenerate Map in which the key and value in
> each pair are the same object
>
> List<Thing> theList = ...;
> Map<Thing,Thing> theMap = new HashMap<Thing,Thing>();
> for (Thing thing : theList)
> theMap.put(thing, thing);
>
> ... but that's sort of like making all those single men "married"
> by having them go off individually and do something in private.

There's another degenerate Map that fits the original question as stated:

Map <Object, List <Thing>> theMap
= new HashMap <Object, List <Thing>> ();
theMap.put( "Key", theList );

or perhaps the List is the key and the other object is the value.

In this business the "obvious" interpretation of a vaguely-stated requirement
might not be the correct one. It's best to get the requirements nailed down
unambiguously.

--
Lew

On 8/30/2010 4:44 AM, Lew wrote:
> parag mohite wrote:
>>> how do we convert an ArrayList into a hashMap [sic] ?? [sic]
>
> Eric Sosman wrote:
>> The same way you convert a roomful of single men into
>> married couples: You add a suitable amount of the missing
>> ingredient.
>>
>> A HashMap -- any kind of Map -- contains *pairs* of keys
>> and values. An ArrayList -- any kind of List -- contains
>> individual items. You can't make a sensible key/value pair
>> out of just one item from a List, so you need to add some
>> additional information from somewhere else. Well, you *could*
>> make a sort of degenerate Map in which the key and value in
>> each pair are the same object
>>
>> List<Thing> theList = ...;
>> Map<Thing,Thing> theMap = new HashMap<Thing,Thing>();
>> for (Thing thing : theList)
>> theMap.put(thing, thing);
>>
>> ... but that's sort of like making all those single men "married"
>> by having them go off individually and do something in private.
>
> There's another degenerate Map that fits the original question as stated:
>
> Map <Object, List <Thing>> theMap
> = new HashMap <Object, List <Thing>> ();
> theMap.put( "Key", theList );
>
> or perhaps the List is the key and the other object is the value.
>
> In this business the "obvious" interpretation of a vaguely-stated
> requirement might not be the correct one. It's best to get the
> requirements nailed down unambiguously.
>
The third common option is to have a map where the key is a component of
the value:

for (Thing thing: theList) {
thingsByName.put(thing.getName(), thing);
}

I use this frequently for building indexes.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

>> parag mohite wrote:
>>>> how do we convert an ArrayList into a hashMap [sic] ?? [sic]


I think another possibility is that the OP wants a HashSet instead of a
HashMap...

Set hash = new HashSet( arrayList );

(Generics should be used here, of course.) But we'll never get
resolution without the OP's clarification.

On Mon, 30 Aug 2010, Eric Sosman wrote:

> On 8/30/2010 7:10 AM, parag mohite wrote:
>> how do we convert an ArrayList into a hashMap ??
>
> The same way you convert a roomful of single men into
> married couples: You add a suitable amount of the missing
> ingredient.

Civil unions?

tom

--
It's odd to discover your quips in other people's .sig files. --
Benjamin Rosenbaum

parag mohite wrote:
>>> how do we convert an ArrayList into a hashMap ??
>

Eric Sosman wrote:
>> * *The same way you convert a roomful of single men into
>> married couples: You add a suitable amount of the missing
>> ingredient.
>

Tom Anderson wrote:
> Civil unions?
>

As opposed to marriages, which often are not very civil.

--
Lew
Very happily married,
by a civil ceremony, as it happens.

On 8/31/2010 8:57 AM, Tom Anderson wrote:
> On Mon, 30 Aug 2010, Eric Sosman wrote:
>
>> On 8/30/2010 7:10 AM, parag mohite wrote:
>>> how do we convert an ArrayList into a hashMap ??
>>
>> The same way you convert a roomful of single men into
>> married couples: You add a suitable amount of the missing
>> ingredient.
>
> Civil unions?

Yes, I'm from Massachusetts, and yes, I realized that the metaphor
was fraying around the edges. But it was too good to pass up: Just as
in figure skating, artistic merit outpoints technical accuracy.

--
Eric Sosman
lid

On Mon, 30 Aug 2010 04:10:27 -0700 (PDT), parag mohite
<> wrote, quoted or indirectly quoted someone
who said :

>how do we convert an ArrayList into a hashMap ??

see http://mindprod.com/jgloss/hashmap.html#INITIALISING
http://mindprod.com/jgloss/arraylist.html


You have to extract your objects from the ArrayList and feed them
object and key one at a time to the HashMap constructor.
--
Roedy Green Canadian Mind Products
http://mindprod.com

You encapsulate not just to save typing, but more importantly, to make it easy and safe to change the code later, since you then need change the logic in only one place. Without it, you might fail to change the logic in all the places it occurs.

On Tue, 31 Aug 2010, Eric Sosman wrote:

> On 8/31/2010 8:57 AM, Tom Anderson wrote:
>> On Mon, 30 Aug 2010, Eric Sosman wrote:
>>
>>> On 8/30/2010 7:10 AM, parag mohite wrote:
>>>> how do we convert an ArrayList into a hashMap ??
>>>
>>> The same way you convert a roomful of single men into
>>> married couples: You add a suitable amount of the missing
>>> ingredient.
>>
>> Civil unions?
>
> Yes, I'm from Massachusetts, and yes, I realized that the metaphor
> was fraying around the edges. But it was too good to pass up: Just as
> in figure skating, artistic merit outpoints technical accuracy.

I'll have to remember that one for my next code review.

tom

--
MADSKILLZ!