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Setting a HashMap to a Collection and vice versa

Posted on 2004-09-13
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Last Modified: 2008-01-09
Hi,

I have a HashMap called userSelection that I want to convert to a java Collection and then after it is passed into a new class, I want to change the collection back to a HashMap.  I work with collections and hashmaps all the time and have all the imports setup correctly, all I need is a few lines of code as an example to show how to convert these things back and forth.

Thanks in advance!! :)
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Question by:gdlp2004
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LVL 92

Expert Comment

by:objects
ID: 12049835
HashMap is a Collection already.
If you want the keys then use userSelection.keySet()
Or to get the values use userSelection.values()
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LVL 92

Expert Comment

by:objects
ID: 12049908
but as a HashMap is a subclass of Collection you can just say:

HashMap hm = new HasMap();
Collection c = hm;

Depends though exactly what it is you need to do.
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LVL 15

Expert Comment

by:Javatm
ID: 12049950
Yap thats correct just wanna add another comment what objects said with HasMap and Collections are true so the answers
that you need will already be on what objects said.

This tutorial can also add help :
http://java.sun.com/docs/books/tutorial/collections/index.html
http://java.sun.com/docs/books/tutorial/collections/interfaces/index.html

Hope that helps . . .
Javatm
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LVL 1

Expert Comment

by:Dave_Toland
ID: 12049985
don't forget you will need to cast back to a hashmap

HashMap hm = (HashMap)c;
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Accepted Solution

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MogalManic earned 2000 total points
ID: 12051148
HashMap is NOT a collection.  It is a MAP.  
A hasmap CONTAINS 3 related Sets (implements collection):
  -KeySet() - Set of keys
  -Values() - Set of values
  -entrySet() - Set of keys AND values (Map.Entry instances)

The only way to switch back and forth between collection and map is to use the entrySet() method:

Collection entries=m.entrySet();
...
HashMap map=new HashMap(entries.size())
for(Iterator it=entries.Iterator();it.hasNext();) {
  Map.Entry each=(Map.Entry) it.next();
  map.put(each.getKey(), each.getValue);
}
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LVL 92

Expert Comment

by:objects
ID: 12051301
> HashMap is NOT a collection.  It is a MAP.  

I stand corrected :)
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Author Comment

by:gdlp2004
ID: 12054884
Before I test this out, I need to know how to set a HashMap to a collection...is it as easy as everyone originally posted, or is it more complex like MogalManic said?

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Author Comment

by:gdlp2004
ID: 12054952
Oh nevermind, I see he did show the two separate ways...I will test them both out right now...
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Author Comment

by:gdlp2004
ID: 12172396
Mogal Manic was right on the money...many thanks!
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Expert Comment

by:MogalManic
ID: 12174167
no problem
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