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Differences between HashMap and Hashtable in Java

What is HashMap and Hashtable in Java what are the main differences between them?

1 Answer
Arushi

Hashtable was part of the original java.util and is a concrete implementation of a Dictionary. However, Java 2 re-engineered Hashtable so that it also implements the Map interface. Thus, Hashtable is now integrated into the collections framework. It is similar to HashMap, but is synchronized.

Like HashMap, Hashtable stores key/value pairs in a hash table. When using a Hashtable, you specify an object that is used as a key, and the value that you want to be linked to that key. The key is then hashed, and the resulting hash code is used as the index at which the value is stored within the table.

The HashMap class is the Hash table based implementation of the Map interface.

  1. This class makes no guarantees as to the iteration order of the map; in particular, it does not guarantee that the order will remain constant over time.
  2. This class permits null values and the null key.

Example

Live Demo

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class HashMap_HashTable {
   public static void main(String args[]) {
      // Create a hash map
      HashMap hm = new HashMap();
     
      // Put elements to the map
      hm.put("Zara", new Double(3434.34));
      hm.put("Mahnaz", new Double(123.22));
      hm.put("Ayan", new Double(1378.00));
      hm.put("Daisy", new Double(99.22));
      hm.put("Qadir", new Double(-19.08));
      // Get a set of the entries
      Set set = hm.entrySet();
      // Get an iterator
      Iterator i = set.iterator();
     
      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();
      // Deposit 1000 into Zara's account
      double balance = ((Double)hm.get("Zara")).doubleValue();
      hm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " + hm.get("Zara"));
      HashMap< String, String> hMap = new HashMap< String, String>();
      hMap.put("1", "1st");
      hMap.put("2", "2nd");
      hMap.put("3", "3rd");
      Collection cl = hMap.values();
      Iterator itr = cl.iterator();
      while (itr.hasNext()) {
         System.out.println(itr.next());
      }
   }
}

Output

Daisy: 99.22
Ayan: 1378.0
Zara: 3434.34
Qadir: -19.08
Mahnaz: 123.22
Zara's new balance: 4434.34
1st
2nd
3rd

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