Examples of 15 Tricky Tasks You May Encounter During the Java Junior Interview

Answering technical questions is an essential part of any Java Junior interview. Yet, major tech companies often include unusual tasks and even require you to solve coding challenges to assess your skills. In this article, we give a selection of tricky coding challenges you may face during an interview. So, let's prepare to equip ourselves with the knowledge needed to tackle these challenges head-on.

Types of Coding Challenges

First, let's define what coding challenges are the most widespread among tech companies. If we refer to FAANG and other top IT companies, we'll see that they follow one of these five ways to assign coding challenges:

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Open Coding Challenges

Open coding challenges refer to coding exercises available to the public, typically on online platforms or websites, where individuals can attempt to solve them. These challenges are "open" in that they are accessible to anyone interested, and there are no restrictions on who can participate. Open coding challenges are an excellent way for “fresh” developers to practice and improve their coding skills, compete with others, and sometimes even showcase their abilities to potential employers. For instance, Amazon hires candidates through coding challenges on TechGig and HackerEarth, whereas Google conducts three coding challenges per year.

Take-home Coding Challenges

Here, recruiters give coding assignments that candidates can complete in their homes, usually within a specified time frame (ranging from a few hours to a few days). Unlike traditional on-site coding interviews, these challenges allow candidates to work on the problem at their own pace and in a familiar setting. What's excellent about take-home coding challenges is that they provide a more realistic and less time-pressured environment for candidates to demonstrate their abilities.

Pair Programming

In this interview coding challenge, you solve the problems by working with the interviewer. Pair programming can evaluate your ability to work in a team, yet such interview practice usually applies to senior Java developers only.

Whiteboarding

Whiteboarding is the most common interview practice when candidates are asked to solve coding problems or algorithms on a physical or virtual whiteboard tool. During whiteboarding, developers are typically required to write Java code by hand, explaining their thought process and discussing their approach with the interviewer. This exercise assesses a candidate's problem-solving skills, coding abilities, and ability to think through complex technical challenges under pressure.

Screening Questions

Screening questions are the initial set of questions or assessments often used by employers to filter and identify qualified candidates before proceeding to more in-depth interviews. These questions are short, so they can’t be called full-fledged coding challenges.

Unusual Technical Tasks or Questions

Now, let’s take a look at some samples of the Java interview coding challenges (typically performed via Whiteboard tests):

1. Create a Java Singleton class.
2. Write a simple phone number word decoder. Create a Java app that can accept a contact number with letters and then convert it to a number with digits only.
3. Create a Java app to reverse a string. Do not use the reverse method of Java's String class.
4. Print all permutations of String in both iterative and Recursive ways.
5. Design a vending machine by writing a usable code.
6. Write a Java program that can return an MD5 hash.
7. Reverse a String without using any built-in functions or libraries.
8. Create a custom class loader in Java.
9. Write a mini Java program to generate Fibonacci numbers using both recursion and memoization.
10. Design a thread-safe singleton class in Java using the Enum approach.
11. Create a program to detect palindromes within a Sentence.
12. Implement the unique Merge Names method. When passed two arrays of names, it should return an array containing the names that appear in either or both arrays. The returned array should have no duplicates.

As you see tasks may be quite comprehensive and aimed to assess your knowledge of design patterns, string manipulation, arrays, recursion, dynamic programming, optimization techniques, algorithmic thinking, and other aspects.

Samples of Tricky Interview Coding Challenges (with Answers)

We’d also like to give you some questions with answers to help you get a grasp of tricky tasks and the implementation process.

Detect if the List is Cyclic using Hash Table

Answer: To detect if a list is cyclic, we can check whether a node had been visited before. A natural way is to use a hash table. Algorithm: We go through each node one by one and record each node's reference (or memory address) in a hash table. If the current node is null, we have reached the end of the list and it must not be cyclic. If the current node's reference is in the hash table, then return true. Implementation:

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Implement Stack using Two Queues (with efficient push)

Problem: Given two queues with their standard operations (enqueue, dequeue, isempty, size), implement a stack with its standard operations (pop, push, isempty, size). The stack should be efficient when pushing an item. Answer: Given we have queue1 and queue2: push - 0(1):

• enqueue in queue1

pop - 0(n):

• while size of queue1 is bigger than 1, pipe (dequeue + enqueue) dequeued items from queue1 into queue2
• dequeue and return the last item of queue1, then switch the names of queue1 and queue2

Implementation:

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Implement a Queue using two Stacks

Problem: Suppose we have two stacks and no other temporary variable. Is it possible to "construct" a queue data structure using only the two stacks? Answer: Keep two stacks, let's call them inbox and outbox. Enqueue:

• Push the new element onto inbox

Dequeue:

• If outbox is empty, refill it by popping each element from inbox and pushing it onto outbox
• Pop and return the top element from outbox

Implementation:

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What to Do if You’ve Got Stuck during an Interview

Getting stuck during a coding interview, especially if it's not “take-home coding challenge” may be rough. So, what to do when you have an empty whiteboard and an interviewer looking at you expectantly? Tip #1. Write a sample input on the whiteboard and turn it into the correct output "by hand." For example, if you're dealing with reversing a String, you can just write “hello” on the board and then reverse it “by hand” by drawing arrows from each character's current position to the needed position. Tip # 2. Try to solve a simpler problem. You may remove one of the requirements of the problem to simplify it. Once you have a solution to a simpler problem, check if you can adapt the same approach to the original question. For example, when finding the k-largest element in a set, you can identify the largest element, the second-largest element, the third-largest element, etc. Generalizing from there to find the k-largest element may work. Tip #3. Start with an ineffective solution. Even when the only solution seems extremely inefficient, it may be really helpful to start with something. Once done, you only need to optimize your solution. In the process, you can explain to your interviewer that this is your first idea and you're now thinking about faster and more efficient solutions. The key is to start with something rather than getting lost at the beginning.

Conclusion

Summing up, you should be ready for your future recruiters will go beyond basic coding questions and require you to think critically and apply your knowledge to real-world scenarios. The unusual tasks mentioned in this article can help interviewers gauge their understanding of Java concepts, problem-solving skills, and creativity. CodeGym can equip you with the essential knowledge to crack a coding challenge and ace an interview. Just thoroughly go through each topic and practice as many coding problems as possible. Keep coding, stay curious, and you'll unlock endless opportunities in the tech world!