HashMap vs Hashtable vs HashSet

Posted on 24. Aug, 2009 by Tahir Akram in Development

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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");

Tags: Development, Interview Questions, Java

8 Responses to “HashMap vs Hashtable vs HashSet”

Patrick

25. Aug, 2009

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…
Tahir Akram

25. Aug, 2009

Thanks Patrick for your comments. I will definitely update it.
Trent

26. Aug, 2009

You also forgot LinkedHashSet, which is useful when you want to iterate in the order that items were inserted.
geo

26. Aug, 2009

There’s also a LinkedHashMap, which you can use to iterate over the keys in insertion order.
teto

27. Aug, 2009

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.
Nils Breunese

29. Aug, 2009

And don’t forget ConcurrentHashMap in the java.util.concurrent packages: http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html
Zia Ur Rahman Baig

25. Mar, 2010

Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t.
java hunter

07. May, 2010

its better understanding for java beginners… thanks lot..