Immutable class interview questions

In this post, I am going to share java interview questions which are generally asked on immutable class. Immutable class is important topic as it deals with creation of thread safe class.
Here I am providing some important immutable class interview questions with answers.

1. What is immutable class?

Answer : Immutable objects are those objects whose state can not be changed once created. Class whose objects possess this characteristic can be termed as immutable class.
For example : String , Integer.

2. What are steps for creating an immutable class?

  • Make your class final : 
    If you make your class final, no class will be able to extend it, hence will not be able override methods of this class.
  • Declare all instance variable with private and final : 
    If you make instance variable private, no outside class will be able to access instance variables and if you make them final, you can not change it.
  • Say no to setter methods :
    Don’t create setter method for any instance variables, hence there will be no explicit way to change state of instance variables.
  • Initialize all variables in constructor :
    You can initialize variables in constructor. You need to take special care while working with mutable object. You need to do deep copy in case of immutable objects.
  • Perform cloning of mutable objects while returning from getter method:
    If you return clone of object from getter method, it won’t return original object, so your original object will remain intact.
    Read : Why String is immutable in java

3. Can you answer if below Employee class is immutable or not? If not what will you do to make it immutable?


Employee class is not an immutable class because you can still perform employee.getAddresses().add(“New address”) and it will be added to employee’s addresses list.
You need to return clone of addresses list from getAddresses method ,so employee.getAddresses().add(“New address”)  won’t impact employee’s addresses list.
If you change above getAddresses() method to below method, Employee class will become immutable.

4. Can you provide example of immutable class?

Answer  : Yes. I have created  Country class which is immutable.

5. Do immutable classes thread safe? If yes then how?

Answer : 
Immutable classes are thread safe because you can not change state of immutable objects, so even if two thread access immutable object in parallel, it won’t create any issue.

6. What precautions you need to take if you are working with mutable instance variables?

If you are working with mutable instance variable(addresses list in above Employee class) then do following:

  • Do deep copy mutable variable in constructor.
  • Return clone of instance variable from getter of that instance variable rather than actual variable.
You can read more about it at how to create immutable class in java.

7. Why immutable objects are considered to be good keys for HashMap?

Answer : Immutable object’s hashcode won’t change, so it makes key retrieval faster as you can cache different hashcode for different keys. In case of mutable object, hashcode may be dependent on mutable fields, if any value for these field changes, it might change hashcode, so you might not able to find your key in HashMap as hashcode is different now.

8. Can you give some good examples of immutable classes?

String, Integer, Long, Double, Character, Boolean etc and much more. Date is not an immutable class.

Related Posts

  • 22 May

    Java interview questions

    I have written a lot of java interview questions and this is index posts for all java interview questions. I will keep adding more interview questions to this post. Core Java Core java interview questions This list includes top 50 core java interview questions. Whether you are fresher or experienced programmer, this interview questions will […]

  • 09 November

    SOLID Principles in Java

    In this post, we will see 5 SOLID Principles in Java. Robert C. Martin gave five objected oriented design principles, and the acronym S.O.L.I.D is used for it. Each letter of this acronym talks about principles in Java. When you use all the principles of S.O.L.I.D in a combined manner, it becomes easier for you […]

  • 08 November

    Java interview questions for 2 years experience

    In this post, we are going to see Java interview questions for 2 to 3 years experienced. These are the most asked interview questions for freshers(0-3 years experienced). This question list will help you to crack java interview. I have already shared detailed answer over here before, you might find it helpful as well. I […]

  • 08 November

    Java interview questions for 5 years experience

    In this post, we will see Java interview questions for 5 to 6 years experience. When you have 5 years of experience as java developer, you need to have a good understanding of collections, multithreading concepts. If you are looking for the below queries then this post will help you as well. Java interview questions […]

  • 11 September

    Java technical interview questions

    In this post, we will see top Java technical interview asked in the Core Java interview.I have covered a lot of interview question on Java programming. Here is list of other java interview question you might find useful. Core java interview questions Java Collections interview questions Java String interview questions OOPs interview questions in java […]

  • 06 August

    Top 15 Spring Boot Interview questions

    In this post, we will see top 15 Spring Boot interview questions with answers.If you want to read more about Spring boot, you can go through Spring boot tutorial. If you are looking for below queries then this post will help you as well. Spring boot interview questions for 3 years experience Spring boot interview […]


  1. Hi Arpit, great article. Should you use also clone() for deep copy of arraylist ? For this line:
    tempList.add(listOfStates.get(i))? Or you are assuming that list is a list of Strings? Thanks

    1. Hi Goran,
      I am assuming that list is a list of Strings. As String is immutable in java, you don’t need to use deep copy for ArrayList.
      Thank you!!

Leave a Reply

Your email address will not be published. Required fields are marked *

Subscribe to our newletter

Get quality tutorials to your inbox. Subscribe now.