Re: query

Daniel Pitts <>
Mon, 30 Aug 2010 14:34:41 -0700
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

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.

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