Remove an Element From HashTable?

Question

I want to remove elements from HashTable, I use hashTable.remove() for this but not getting

Hashtable<Integer,String> players = new Hashtable<Integer,String>();

        players.put(1, "Sachin Tendulkar");
        players.put(2, "Rahul Dravid");
        players.put(3, "Virat Kohli");
        players.put(4, "Rohit Sharma");

        Enumeration<String> enumration = players.elements();

        while(enumration.hasMoreElements()){
            String elmnt = enumration.nextElement();
            System.out.println(elmnt);
            if(elmnt.startsWith("R")){
                players.remove(elmnt);
                System.out.println(elmnt+"::Contains R");
            }

        }

        System.out.println(players);

The output that i get is:

Rohit Sharma
Rohit Sharma::Contains R
Virat Kohli
Rahul Dravid
Rahul Dravid::Contains R
Sachin Tendulkar
{4=Rohit Sharma, 3=Virat Kohli, 2=Rahul Dravid, 1=Sachin Tendulkar}

Solution 1

That's because you're trying to remove the value, but the remove() method expects a key.

When you're calling player.elements() you get an enumeration of the values, not the keys. The remove() method works on a key, not a value.

From HashTable remove() documentation

public V remove(Object key): Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.

Also, from the same documentation, here's a brief explanation on why it is not a good idea to use Enumeration to remove elements:

Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast.

What I would suggest you to do is to iterate over the entries, not the values.

for (Entry<Integer, String> entry: players.entrySet()){
        String value = entry.getValue();
        System.out.println(value);
        if(value.startsWith("R")){
            players.remove(entry.getKey());
            System.out.println(value+"::Contains R");
        }
}

This way you have both the key and the value in each iteration. You can examine the value (which is the player's name) and then use the key in order to manipulate your table.

Solution 2

    for(Iterator<Map.Entry<Integer, String>> iter =  players.entrySet().iterator(); iter.hasNext(); ) {
        Map.Entry<Integer, String> e = iter.next();
        if(e.getValue().startsWith("R")) {
            iter.remove();
        }
    }

Solution 3

please add key to remove() method rather than value.

    Hashtable<String, Integer> h = new Hashtable<String, Integer>();
    h.put("n1", 30);
    h.put("n2", 30);
    h.remove("n1"); //here we are giving "n1" which is key of the hashTable 

Solution 4

You need to pass key as argument in hashTable.remove() to remove from HashTable.

Reference : Remove from HashTable

Best way to remove an entry from a hash table. This may be helpful to you, have a look at it.

Solution 5

Don't remove from enumerations.

Javadocs:

"Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and elements methods are not fail-fast."

You should using iterator:

import java.util.Hashtable;
import java.util.Iterator;


public class Test {
    public static void main(String[] args) {
        Hashtable<Integer,String> players = new Hashtable<Integer,String>();

        players.put(1, "Sachin Tendulkar");
        players.put(2, "Rahul Dravid");
        players.put(3, "Virat Kohli");
        players.put(4, "Rohit Sharma");

       Iterator<Integer> iterators = players.keySet().iterator();
       while(iterators.hasNext()) {
           int key = iterators.next();
           if(players.get(key).startsWith("R")) {
               iterators.remove();
           }
       }

        System.out.println(players);
    }
}

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