Re: "Filtering" a Map

Thomas Hawtin <>
Wed, 18 Oct 2006 17:16:50 +0100
Mize-ze wrote:

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.

   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 =;
            if (((String)mapEntry.getKey()).indexOf("rhs:")==0)

                    ^^^^^^^^Why the casts?
     indexOf==0 can be replaced with startsWith^^^^^^^^^^^^^^^^^^



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

As Thomas Weidenfeller points out, you can use a SortedSet (or
NavigableSet in 1.6). That will make general usage, say, twice as slow,
but will improve this case.

If you don't need the original map, you can use remove of the entrySet

Using two maps would be the obvious solution. Either one map with
startsWith "rhs:" and and other map without, or have the second map
contain all entries.

Another solution would be to write a custom map, that delegates to the
original map but filters out non-"rhs:" keys (size might have a slow

Going further out, you could have a linked map implementation the adds
"rhs:" on one end and non-"rhs:" on the other.

The best solution depends on how it is going to be used, how much effort
you want to put into it, etc.

Tom Hawtin

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