Popular Java interview questions 2

Popular Java interview questions 2

1. Difference between Array and ArrayList in Java 

	Array	ArrayList
Resizable	No	Yes
Primitives	Yes	No
Iterating values	for, for each	Iterator , for each
Length	length variable	size method
Performance	Fast	Slow in comparision
Multidimensional	Yes	No
Add Elements	Assignment operator	add method

2.  What is BlockingQueue in Java Collections Framework? 

BlockingQueue implements the java.util.Queue interface . BlockingQueue supports  operations that wait for the queue to become non-empty when retrieving an element , and wait  for space to become available in the queue when storing an element .
It does not accept null elements.
Blocking queues are primarily designed for the producer-consumer problems.
BlockingQueue implementations are thread-safe and can also be used in inter-thread communications.
This concurrent Collection class was added in jdk 1.5

3. Explain the importance of hashCode() and equals() method ? Explain the contract also ?

Contract of hashCode() and equals() method

a.  If  object1.equals(object2) , then  object1.hashCode() == object2.hashCode() should always be true.

b. If object1.hashCode() == object2.hashCode() is true does not guarantee object1.equals(object2)

4.  Difference between ArrayList and LinkedList in Java 

	ArrayList	LinkedList
Implementation	Resizable Array	Douby-LinkedList
ReverseIterator	No	Yes , descendingIterator()
Initial Capacity	10	Constructs empty list
get(int) operation	Fast	Slow in comparision
add(int) operation	Slow in comparision	Fast
Memory Overhead	No	Yes

5. How ConcurrentHashMap gives better performance as compared to Synchronized HashMap?

ConcurrentHashMap maintains  a list of 16 locks by default ( number of locks equal to the initial capacity , which is by default  16) each of which is used to lock on a single bucket of the Map.This indicates that 16 threads (number of threads equal to the concurrency level , which is by  default 16) can modify the collection at the same time , given ,each thread works on different bucket. So unlike hashtable, we perform any sort of operation ( update ,delete ,read ,create) without locking on entire map in ConcurrentHashMap.

6. Difference between HashMap and Hashtable in Java 

	HashMap	Hashtable
Synchronized	No	Yes
Thread-Safe	No	Yes
Null Keys and Null values	One null key ,Any null values	Not permit null keys and values
Iterator type	Fail fast iterator	Fail safe iterator
Performance	Fast	Slow in comparison
Superclass and Legacy	AbstractMap , No	Dictionary , Yes