Saturday, 7 January 2017

Top Java Interview Questions 2018 for experienced candidates

I have prepared this list from my personal experience of recent interviews. Topics are in the order of recent interview trend.

Singleton Pattern

This is one of the most commonly asked question in interviews. Interviewer starts this question asking about design patterns. Candidate generally answers singleton and factory patterns. Then interviewer asks to write a singleton class. You should learn how to create a singleton class with private constructor, static getInstance method, double checked locking. You should also learn enum based singleton class. Follow up questions in singleton pattern are as follows:
  • How to make singleton class thread safe?
  • How to prevent deserialization to create new object of singleton class? 
  • How to prevent cloning and reflexion to create new object of singleton class?
You can find the singleton pattern implementation here.


This is also most commonly asked question in collections. Interviewer start this question asking about collections and what are the data structure/collection you have used in development. Candidate generally answers HashMap, LinkedList, ArrayList, HashSet.  Then interviewer check your knowledge of equals, hashcode and hashing algorithm. You should be aware of HashMap class internal implementation. Follow up questions in HashMap are as follows: 
  • How to use HashMap in multi threading environment ? You should know that HashTable is thread safe. You can make HashMap thread safe by using Collections.synchronizedMap(Map)  
  • What is concurrent hashmap ? How concurrent hashmap is better then thread safe hashmap in multi threading environment ?


LinkedList is something where interviewer can judge your knowledge of collections and algorithm at the same time. 
  1. Collections: What is the difference in ArrayList and LinkedList ? 
  2. Algorithm: How to find a loop in LinkedList ? If you answer it then follow up question may asked:
    • How to find the starting point of loop in LinkedList ?
    • How to find the length of the loop in LinkedList ? 


Interviewer can ask you any of following questions:

1. How to run 5 threads sequentially.
2. Print number 1 to 10 using two threads where thread 1 prints even number and thread 2 prints odd number.
3. Producer consumer implementation using wait notify.
4. What is deadlock ? How to identify deadlock in java application ? How to prevent deadlock situations in application development ?
5. CountDownLatch vs CyclicBarrier ?
6. Future Object.
7. How to name a thread in executor service ?
8. How to interrupt a thread explicitly ?


1. What is immutable object ? How can you write Immutable Class ? If you know the internal implementation of String class and know how String is immutable, you can answer it easily.
2. Difference between String object created using new and literal.
3. Difference between StringBuilder and StringBuffer.
4. Why character array is used over String to store password ? 

Theory Java

1. S.O.L.I.D. (5 class design principle) in Java namely Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion.
2. Give example of 5-6 design patterns being used in Java classes. 

Factory: java.util.Calendar#getInstance()
Abstract Factory: javax.xml.parsers.DocumentBuilderFactory#newInstance(), javax.xml.transform.TransformerFactory#newInstance(), javax.xml.xpath.XPathFactory#newInstance()
Builder: java.lang.StringBuilder#append(), java.lang.StringBuffer#append()
Decorator: All subclasses of, OutputStream, Reader and Writer.
Chain of responsibility: javax.servlet.Filter#doFilter()
Iterator: java.util.Iterator
Observer: JMS message listener
Singleton: java.lang.Runtime#getRuntime()
Adapter: java.util.Arrays#asList(), (returns a Reader), (returns a Writer)

3. Anti patterns in Java. What is god class ?

Advance Java

1. How many ways you can create an object in java ?
        Answer: New Keyword, Cloning, Deserialization and Reflection.
2. Explain Java Memory Model with Heap structure. Explain here three parts of heap i.e. Young, Old and Permanent Generation. Also Explain Minor and Major GC
3. How to do JVM performance tuning ? Explain here parameters 
             a) Heap Memory: -Xms, -Xmx, -Xmn,
             b) Permanent Generation Memory: -XX:PermSize, -XX:MaxPermSize
             c) Garbage Collection i.e. 
                 -XX:+UseParellelGC (-XX:ParellelGCThreads=<N>), 
                 -XX:+UseConcMarkSweepGC (-XX:ParellelCMSThreads=<N>), 

 4. What is classloader in java ? How to write custom class Loader ? What is linkage error ?
 5. Do you know about serialization ? When should we use that ? What is the role of Serialization id ? serializable vs externalizable ?


These are legacy interview questions which are not asked frequently now a days but they are good to know.

1. Difference between ArrayList and LinkedList.
2. Difference between ArrayList and Vector.


1. Http vs https
2. Rest vs soap