Difference between HashMap and HashTable? Can we make hashmap synchronized?

Java Interview Questions site discussing core java IT technical interview questions in detail. These are some of the java job interview questions me and my friends have faced regularly in campus interviews and off campuses. I have consolidated all of them from different people and communities into a vast resource of core java interview questions all in one place. So I am giving you a chance to prepare well by going through each of the java interview faqs organized by java topics, before attending a technical interview on java.





Java Collections Interview Questions
What is HashMap and Map?

Map is Interface and Hashmap is class that implements this interface.

What is the significance of ListIterator?

Or

What is the difference b/w Iterator and ListIterator?

Iterator : Enables you to cycle through a collection in the forward direction only, for obtaining or removing elements

ListIterator : It extends Iterator, allow bidirectional traversal of list and the modification of elements

Difference between HashMap and HashTable? Can we make hashmap synchronized?

1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.

Note on Some Important Terms
1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.

HashMap can be synchronized by

Map m = Collections.synchronizeMap(hashMap);