How to remove duplicate values from a HashMap

Question

I don't know how can best describe my problem but here it is, I'm trying to remove the same names(values) from HashMap<String, String> map = new HashMap<String, String>();

for example if this map contain names like

    map.put("Vivaldi","Antonio");
    map.put("Belucci", "Monica");
    map.put("Gudini", "Harry");
    map.put("Verdo", "Dhuzeppe");
    map.put("Maracci", "Bruno");
    map.put("Carleone", "Vito");
    map.put("Bracco", "Luka");
    map.put("Stradivari", "Antonio");

I want to remove all entries with the value "Antonio" from it by using method removeTheFirstNameDuplicates, I looked in Google for a couple of days and all examples are close to what I want but not really what I need.

My thoughts are, I need something that will check a map and if it contains the same values in it then remove the duplicate. But how can I do this?

Your map should probably be reversed, and be a Map<String, List<String>> where the keys are the first names, and the values are lists of names. What do you use this map for? Are you sure you shouldn't have a List<Musician> instead? — JB Nizet
– JB Nizet, Commented Mar 8, 2014 at 12:28
@JBNizet I think Stradivari was a luthier rather than a musician. ;) — Boris the Spider
– Boris the Spider, Commented Mar 8, 2014 at 12:39

Richard Miskin · Accepted Answer · 2014-03-08 15:57:01Z

You can do it with the following method which only iterates over the map once:

private static void removeTheFirstNameDuplicates(final Map<String, String> map) {
    final Iterator<Entry<String, String>> iter = map.entrySet().iterator();
    final HashSet<String> valueSet = new HashSet<String>();
    while (iter.hasNext()) {
        final Entry<String, String> next = iter.next();
        if (!valueSet.add(next.getValue())) {
            iter.remove();
        }
    }
}

The add() method on HashSet will return false if a value has already been added to the set. The method above uses this to detect that a duplicate has been found and then removes the duplicate from the HashMap by using the remove() method on the iterator.

It is worth noting that, depending on the Map implementation you use, the iteration order may not be guaranteed so which duplicate you remove is also not guaranteed.

If you were to use a TreeMap rather than a HashMap you would be certain to iterate over the map alphabetically by key e.g. Berluccio, Bracco, Carleone … Verdo. You would then always keep Stradivari and remove Vivaldi.

Richard Miskin · Accepted Answer · 2014-03-08 15:45:18Z

0

Try this

    ArrayList<String> values = new ArrayList<String>();
    ArrayList<String> keys = new ArrayList<String>();

    java.util.Iterator<Entry<String, String>> iterate = map.entrySet()
            .iterator();
    while (iterate.hasNext()) {
        Entry mapEntry = iterate.next();
        String key = (String) mapEntry.getKey();
        String value = (String) mapEntry.getValue();

        values.add(value);
        keys.add(key);
    }

    for (int i = 0; i < values.size(); i++) {
        if (Collections.frequency(values, values.get(i)) > 1) {
            map.remove(keys.get(i));
        }
    }
    System.out.println(map.toString());

edited Mar 8, 2014 at 15:45

Richard Miskin

1,2607 silver badges12 bronze badges

answered Mar 8, 2014 at 13:49

Abiel Paltao

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2 Comments

Richard Miskin Over a year ago

Is each call to Collections.frequency() going to iterate over the entire list?

Abiel Paltao Over a year ago

Yup. Its on the fly thinking. There could be a better/shorter solution.

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