Re: "Filtering" a Map

From:

Eric Sosman <esosman@acm-dot-org.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 18 Oct 2006 09:08:44 -0400

Message-ID:

<ZZydnTLZVJ3CuqvYnZ2dnUVZ_sadnZ2d@comcast.com>

Mize-ze wrote:

Hi All,
I have a HashMap object that contains Strings as both Key and Value;
Some of the keys (String) start with "rhs:"
I Need to filter the entries in the map to get all the "rhs:" entries.
I found no other way then to create a new Map and conditionally insert
entries into it.

[code]

   public Map<String,String> getRhs()
    {
        Map<String,String> ret = new HashMap<String,String>();

        for(Iterator<Map.Entry<String,String>> iter =
r.entrySet().iterator();iter.hasNext(); )
        {
            Map.Entry mapEntry = iter.next();
            if (((String)mapEntry.getKey()).indexOf("rhs:")==0)
            {

ret.put((String)mapEntry.getKey(),(String)mapEntry.getValue());
            }
        }
    }
[/code]

If there any more efficient way to do this? how bad is it? it looks
ugly.

What do you want to do with the "filtered" Map afterwards?
Can you just leave the original Map as it stands, and write a
method or two to do what you need with the rhs: entries only?

Alternatively, you could write your own Map implementation
that's backed by two ordinary Maps: one for the rhs: entries
and one for everything else. put() and get() and so on would
check the supplied key and delegate to put() and get() on the
appropriate ordinary Map, and whenever you needed to get hold
of the rhs:-only Map it would be instantly available.

--
Eric Sosman
esosman@acm-dot-org.invalid