Top 24 Java Date, Time, and Calendar Interview Questions Answers

The Date and Time API is a significant one for day-to-day Java development work, but many interviewers don't ask enough questions on this topic. Most of the questions are based on either Java Collection framework like  HashMap, ConcurrentHashMap, or ArrayList or multi-threading concepts like volatile, synchronized, and atomic variables. This is not good, and many interviewers have realized that a good understanding of date and time API is also important for a good Java developer. That's the main reason for increasing date, time, and calendar-based Java interview questions in recent times, particularly in the last couple of years.

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Top 24 Date, Time, and Calendar Interview Questions and Answers

I have been sharing interview questions on various Java topics from the last couple of years like multithreading, collections, design patterns, coding problems, etc.. Still, one topic I haven't touched yet was date and time, and that's why I thought to share some of the interesting Date and time-based questions from recent Java interviews. In this article, you will find those.

1. Does SimpleDateFormat be safe to use in the multithreaded program? (answer) 
Unfortunately, DateFormat and all its implementations, including SimpleDateFormat, are not thread-safe; hence, it should not be used in the multi-threaded program until external thread-safety measures are applied e.g. confining SimpleDateFormat object into a ThreadLocal variable. I

f you don't do that, you will get an incorrect result while parsing or formatting dates in Java. Though, for all practical date-time purposes, I highly recommend the joda-time library.

25 Java Date, Time, and Calendar Interview Questions Answers

2. How do you format a date in Java? like in the ddMMyyyy format? (answer
You can either use the SimpleDateFormat class or the joda-time library to format the date in Java. DateFormat class allows you to format date on many popular formats. Please see the answer for code samples to format the Date into different formats like dd-MM-yyyy or ddMMyyyy.

3. Can you tell some differences between the old and new Date Time API of Java 8? 
Even though the new Date and Time API are a completely new APIs, you can deduce the following difference between them:
  • 1) In the old API, Date was mutable, but in Java 8, all date and time classes like LocalDate, LocalTime, or LocalDateTime are Immutable.
  • 2) In the old API, SimpleDateFormat was not thread-safe, but in Java 8, Formatter is thread-safe.
  • 3) In the old Date and Calendar API, the Year starts with 1900, Months start with 0, which is corrected in Java 8; here, the numbers make more sense.
  • 4) Old Date and Calendar API has just one class Date to represent date and time, but Java 8 has separated classes for Date and Time e.g. LocalDate and LocalTime

4. Which date does the following Date instance represent? 

Date aDate = new Date(2015, 12, 25, 20, 40);

This is a tricky question for casual Java developers who have no written date and time-based code because this will not represent Christmas day or 25th December 2015. This code has got two bugs. First, the year starts from 1900 to represent 2015, you need to pass 115 i.e. 2015-1900.

The second issue with this code is that the month is not correct. The month starts from 0, so December would be 11, not 12, interesting right? That's why you should new Date and Time API from Java 8, which solves all these issues. Again, you can check The Complete Java Masterclass to learn more about the new Date and Time API.

Top 10 Java Date, Time and Calendar Interview Questions Answers

5. How do you copy a Date in Java? (answer) 
It's a simple question to answer. The Date class implements the clone() method, so just call the date.clone() to create a copy of the Date object in Java.

6. What is the relationship between java.sql.Date and java.util.Date? (answer) 
This is one of the exciting questions as not many developers know this fine but obvious detail. The java.sql.Date extends java.util.Date and suppress all time-related methods to act as just a date class. This is in direct violation of the Liskov substitution principle.

7. Can you format Calendar in Java? (answer) 
No, the Calendar cannot be formatted in Java. You can only format dates.

8. How do you convert a Calendar to Date and vice-versa? (answer)
Well, you can use the Calendar.setTime() and Calendar.getTime() method to convert the Calendar to Date and vice-versa.

9. What is the equivalent of Date class in Java 8? (answer)
The Instance class is the equivalent to java.util.Date in Java 8 because it also represents a millisecond value or an instance in the timescale. Java 8 has also added conversion method e.g. toInstant() and fromDate() to convert instance to java.util.Date and vice-versa. See Java SE 8 for Really Impatient By Cay S. Horstmann for more details.

Top 10 Java Date, Time and Calendar Interview Questions Answers

10. How do you convert a millisecond to a Date in Java? (answer) 
One of the simplest questions to answer is the getTime() method, which returns the millisecond from the Epoch.

12. How do you get a month and year from a Date object in Java? (answer) 
You can convert a Date to a Calendar and then use its get() method with various fields to get the month, year, day of the week, and other date particulars e.g.
int day = calendar.get(Calendar.DATE); // returns the day
int month = calendar.get(Calendar.MONTH) + 1; // month starts from 0
int year = calendar.get(Calendar.YEAR) + 1900; // year starts from 1900

13. How do you convert XMLGregorianCalendar to Date in Java? (answer)
I have discussed this problem before; please check the link for a detailed answer.

14. How do you get the current time in GMT in Java? (answer) The Java date is independent of time and displays time in a current time zone only if you print them. So, to convert the current time in GMT, you need to use a Calendar with a time zone as shown in the given answer.

15. Is the Date class be Immutable in Java? (answer
No, the Date is not immutable. You can change its internal to represent a different date. This is why when you have a member represent Date, you must be careful not to leak its reference outside using the getDate() method because then the client can modify the state of the Immutable object. Instead, a copy should be returned to the client as suggested in the Effective Java book by none other than Joshua Bloch, author of several key classes of JDK API.

Java Date, Time and Calendar Interview Questions Answers

16. How do you add and subtract a day, month, and year in a Date object? (answer) 
This is one of the common day-to-day tasks of a Java developer; you can use the code shared in the answer for adding or subtracting a day or month from the Date object.

17. How do you compare two dates in Java to check if they are equal? (answer) 
This is one of the easier questions you will see in terms of skill checking on Date classes. The java.util.Date class implements the equals() method, which returns true if both Dates are the same and false otherwise.

18. How do you find if one date comes before or after another date in Java? (answer) 
This is the follow-up question to the previous question. The interviewer further checks the candidate's date comparison skill by asking this question. The answer is simple, the java.util.Date class provides before() and after() method, which returns true and false if the date on which method is called comes before or after the other date passed as a method argument.

19. How do you convert java.util.Date to java.sql.Date in Java JDBC? (answer) 
It's the follow-up question of the 6th question. Since java.sql.The date is a subclass of java.util.Date you can simply get the time in a millisecond and passed it to SQL date as shown below:

Date now = new Date();
java.sql.Date sqlDate = new java.sql.Date(now.getTime());

Simple and easy, right? but there are some interesting follow-up questions when you write this code; I suggest you check the answer link for more detailed discussion, where I have answered those follow-up questions as well. You can further see Java Fundamentals: The Java Language to learn more about Java fundamental concepts.

Java Date and Calendar Interview Questions Answers

20. How to convert local time to GMT in Java? (answer) 
This is similar to one of the previous questions where we need to get the current time in GMT

21. How do you show timezone in formatted dates in Java? (answer)
see the answer link for a code example

22. What is the difference between java.util.Date and java.sql.Date in Java? (answer) 
There is much difference between them, but the most important one is that java.sql.The date is from the JDBC package, and it acts as converted between Date and Time data types in the database and Date in Java.

23. How do you calculate the difference between two dates in Java? (program) 
There are multiple ways to calculate the difference between two dates in Java e.g. you can get the long millisecond values from two dates and get the difference, but that's not very valuable; you mostly need the difference in days or hours. See the program to learn more about those differences.

24. How do you convert a string (YYYYMMDD) to date in Java? (answer) 
You can use SimpleDateFormat before JDK 8 and DateTimeFormatter along with the format and parse method to convert a string to date in Java.

That's all about the date, time, and Calendar based Java Interview Questions. A Java developer needs to have a solid understanding of Java's Date and Time API, both old and new, because you will not find Java 8 in every project you work on. You still need to know how to work with old Date and Calendar API while maintaining the legacy project for a couple of more years.

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Thanks for reading this article so far. If you like these questions, then please share them with your friends and colleagues too. If you have any questions or feedback, then please drop a note.

P. S. - If you are looking to fill gaps in your Java knowledge or want to learn Java from scratch, here are some useful free online Java courses to check out at your convenience; they are good for online learning from your office or home.

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