HashMap Duplicate Values - Identify the duplicates

Question

I understand that HashMap doesn't allow insertion of duplicate values and it replaces the last duplicate value with the latest entry. Is there a way to print the duplicates which were found during the put method?

I have the following code snippet:

for( int i = 0; i <= elements.length - 1; i++) {
    nodeDBList = (NodeList) xPath.compile(elements[i]).evaluate(dbDocument, XPathConstants.NODESET);
    for (int j = 0; j < nodeDBList.getLength(); j++) {
        if(nodeDBList.item(j).getFirstChild() != null)
            dbList.put(nodeDBList.item(j).getFirstChild().getNodeValue().toLowerCase().trim(), 
                       nodeDBList.item(j).getNodeName().toLowerCase().trim());

    }
}

by values, you mean keys?

codeMan
– codeMan

2014-11-25 13:43:26 +00:00
Commented Nov 25, 2014 at 13:43 — codeMan
– codeMan, Commented Nov 25, 2014 at 13:43
override the HashMap

Luca Rasconi
– Luca Rasconi

2014-11-25 13:45:56 +00:00
Commented Nov 25, 2014 at 13:45 — Luca Rasconi
– Luca Rasconi, Commented Nov 25, 2014 at 13:45

Mena · Accepted Answer · 2014-11-25 13:47:12Z

5

Wrong. HashMap does not support duplicate keys, which are hashed.

Duplicate values are totally acceptable for different keys.

You can search for existing values by iterating them through the values() method and using the equals method.

Edit

There seems to be a confusion between keys and values here.

According to the HashMap implementation of Map's public V put(K key, V value);, the method put will return the original value for a given key if any, or null.

Quote from the API

@return the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

edited Nov 25, 2014 at 13:47

answered Nov 25, 2014 at 13:40

Mena

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2 Comments

user2967948 Over a year ago

I believe, when we try to insert a duplicate value, hashmap internally checks if thats a duplicate and allows the latest value. I need to print all those duplicate values encountered during the .put method. Is there a way to print them?

user2967948 Over a year ago

I actually have a key (which is the xml node) and multiple values associated with the key. I can see that any duplicate values are currently ignored and the last one(from the duplicate values) gets added to the map.

Jaap Coomans · Accepted Answer · 2014-11-25 13:43:17Z

3

Well, the answer can be found in the API description of HashMap: The put method returns the value that was previously associated with the key.

Returns: the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

answered Nov 25, 2014 at 13:43

Jaap Coomans

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Comments

Eran · Accepted Answer · 2014-11-25 13:41:51Z

1

The old value of the key is returned by the put method, so you can output it.

Assuming the value of your HashMap is of type String :

for( int i = 0; i <= elements.length - 1; i++)
{
    nodeDBList = (NodeList) xPath.compile(elements[i]).evaluate(dbDocument, XPathConstants.NODESET);
    for (int j = 0; j < nodeDBList.getLength(); j++) {
        if(nodeDBList.item(j).getFirstChild() != null) {
            String oldVal = dbList.put(nodeDBList.item(j).getFirstChild().getNodeValue().toLowerCase().trim(), nodeDBList.item(j).getNodeName().toLowerCase().trim());
            if (oldVal != null) {
                System.out.println(oldVal);
            }
        }
    }
}

answered Nov 25, 2014 at 13:41

Eran

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1 Comment

user2967948 Over a year ago

Thanks, the above code snippet serves my purpose. Thanks a million Eran.

Luca Rasconi · Accepted Answer · 2014-11-25 13:46:01Z

Override the HashMap

this is an example

public class MyMap<K, V> extends HashMap<K,V> {

    private static final long serialVersionUID = -1006394139781809796L;

    @SuppressWarnings({ "unchecked" })
    @Override
    public V put(K key, V value) {
        if (value == null) {
            return super.put(key, value);
        }
        if (value.getClass() == Timestamp.class) {
            DateFormat dateTimeFormatter;
            dateTimeFormatter = DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.MEDIUM, getLocale());
            super.put((K) (key + "_f"), (V) dateTimeFormatter.format(new Date(((Timestamp) value).getTime())));

            DateFormat dateFormatter;
            dateFormatter = DateFormat.getDateInstance(DateFormat.SHORT, getLocale());
            super.put((K) (key + "_f_date"), (V) dateFormatter.format(new Date(((Timestamp) value).getTime())));

        } 
        if (value.getClass() == java.sql.Date.class) {
            DateFormat dateFormatter;
            dateFormatter = DateFormat.getDateInstance(DateFormat.SHORT, getLocale());
            super.put((K) (key + "_f"), (V) dateFormatter.format(new Date(((java.sql.Date) value).getTime())));
        } 
        return super.put(key, value);
    }
}

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