HashMap vs Hashtable vs HashSet

I was reading about collection framework of Java. And was studying Hashtable, HashMap and HashSet. Its quite interesting to know the differences between them. In this post I will discuss these three with examples.

Hashtable

Hashtable is basically a datastructure to retain values of key-value pair.

  • It didn’t allow null for both key and value. You will get NullPointerException if you add null value.
  • It is synchronized. So it comes with its cost. Only one thread can access in one time
Hashtable<Integer,String>; cityTable = new Hashtable<Integer,String>();
cityTable.put(1, "Lahore");
cityTable.put(2, "Karachi");
cityTable.put(3, null); /* NullPointerEcxeption at runtime*/

System.out.println(cityTable.get(1));
System.out.println(cityTable.get(2));
System.out.println(cityTable.get(3));

HashMap

Like Hashtable it also accepts key value pair.

  • It allows null for both key and value
  • It is unsynchronized. So come up with better performance
HashMap<Integer,String> productMap = new HashMap<Integer,String>();
productMap.put(1, "Keys");
productMap.put(2, null);

HashSet

HashSet does not allow duplicate values. It provides add method rather put method. You also use its contain method to check whether the object is already available in HashSet. HashSet can be used where you want to maintain a unique list.

HashSet<String> stateSet = new HashSet<String>();
stateSet.add ("CA");
stateSet.add ("WI");
stateSet.add ("NY");

if (stateSet.contains("PB")) /* if CA, it will not add but shows following message*/
     System.out.println("Already found");
else
    stateSet.add("PB");
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37 thoughts on “HashMap vs Hashtable vs HashSet

  1. You should mention that Hashtable and HashMap both implement Map but HashSet implements Set !!! Map and Set are not meant for the same use. The Hash* is just an indication on how these Maps and Set are implemented…

  2. There is also a EnumMap and EnumSet, that are optimized versions to be used with enums, the underlying implementation uses an array to hold the elements and thus the performance is better. Take a look at the javadocs, it could be interesting.

  3. HashSet Allows Duplicate , Please remove that ,if you want to check please run this program.

    HashSet set = new HashSet( true );
    set.add( new Integer( 3 ) );
    set.add( new Integer( 3 ) );

    Now tell me if it break or not.

    • This is just wrong. Execute fallowing Unit Test and you will recognize, that these Integer-Objects are not the same. Well HashSet may uses the HashCode and/or the equals Method which checks for value, but not for the real object.
      @Test public void it(){ Assert.assertTrue(new Integer(3) == new Integer(3)); }

  4. “HashMap
    Like Hashtable it also accepts key value pair.
    It allows null for both key and value It is unsynchronized. So come up with better performance”It allows on one null key.

  5. Pingback: HASHMAP VS HASHTABLE VS HASHSET | yizhaoee

  6. Is there any difference between the access time of them? I mean searching in which one is faster when you have unique entry?

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