Get an Enumeration (for the Keys) of a Map (HashMap) in Java?

Question

As far as I understand this, it seems that there is not a direct way of getting an Enumeration directly for the Keys of a HashMap. I can only get a keySet(). From that Set, I can get an Iterator but an Iterator seems to be something different than an Enumeration.

What is the best and most performant way to directly get an Enumeration from the Keys of a HashMap?

Background: I am implementing my own ResourceBundle (=>getKeys() Method), and I have to provide/implement a method that returns the Enumeration of all Keys. But my implementation is based on a HashMap so I need to somehow figure out how to best convert betweens these two "Iterator/Enumerator" techniques.

Is there any reason why you need Enumerations instead of Iterators? Enumeration's Javadoc recommends that you use Iterator instead. — lins314159, Jan 12 '10 at 10:20
well, the Class I am implementing requires an Enumeration: http://java.sun.com/javase/6/docs/api/java/util/ResourceBundle.html#getKeys() (see the getKey() Method there) The only thing what I could do is to implement it completely different and not use HashMap at all. Nevertheless I would be very much interested in learning whats the best and most performant way of getting an Enumeration for the Keys of a Map. thanks! — tim, Jan 12 '10 at 10:26

score 48 · Answer 1 · edited Apr 08 '14 at 07:45

I think you can use the method enumeration from java.util.Collections class to achieve what you want.

The API doc of the method enumerate has this to say:

public static Enumeration enumeration(Collection c)
Returns an enumeration over the specified collection. This provides interoperability with legacy APIs that require an enumeration as input.

For example, the below code snippet gets an instance of Enumeration from the keyset of HashMap

 final Map <String,Integer> test = new HashMap<String,Integer>();
 test.put("one",1);
 test.put("two",2);
 test.put("three",3);
 final Enumeration<String> strEnum = Collections.enumeration(test.keySet());
 while(strEnum.hasMoreElements()) {
     System.out.println(strEnum.nextElement());
 }

and resulting the output is:
one
two
three

+1 This is the way to go to as long as you start with the collection. No need to write your own adapter to enumerate a key set. — Samuel Sjöberg, Jan 12 '10 at 12:07

score 22 · Accepted Answer · edited Sep 02 '14 at 06:11

22

Apache commons-collections have an adapter that makes the Iterator available for use like an Enumeration. Take a look at IteratorEnumeration.

Adapter to make an Iterator instance appear to be an Enumeration instances

So in short you do the following:

Enumeration enumeration = new IteratorEnumeration(hashMap.keySet().iterator());

Alternatively, if you (for some reason) don't want to include commons-collections, you can implement this adapter yourself. It is easy - just make an implementation of Enumeration, pass the Iterator in a constructor, and whenever hasMoreElements() and nextElement() are called, you call the hasNext() and next() on the underlying Iterator.

Use this if you are forced to use Enumeration by some API contract (as I assume the case is). Otherwise use Iterator - it is the recommended option.

edited Sep 02 '14 at 06:11

Ken Y-N

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answered Jan 12 '10 at 10:26

Bozho

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Google Collections also supports this using `Iterators.asEnumerator()`: http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Iterators.html#asEnumeration(java.util.Iterator) – Joachim Sauer Jan 12 '10 at 10:31
hello Bozho. Thanks, especially interesting was your comment how to implement this on my own. I did not realize so far that I could do this on my own. – tim Jan 12 '10 at 10:39

richa_v · Answer 3 · 2014-09-02T06:47:22.463

You can write a adapter to adapt to Enumeration.

    public class MyEnumeration implements Enumeration {

        private Iterator iterator;

        public MyEnumeration(Iterator iterator){
            this.iterator = iterator;
        }


        public MyEnumeration(Map map) {
            iterator = map.keySet().iterator();
        }


        @Override
        public boolean hasMoreElements() {
            return iterator.hasNext();
        }


        @Override
        public Object nextElement() {
            return iterator.next();
        }

    }

And then you can use this custom enumeration :)

Get an Enumeration (for the Keys) of a Map (HashMap) in Java?

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