Top 50 Java Programs for Coding Interviews

Here is a big list of Java programs for Job Interviews. As I said, it includes questions from problem-solving, linked list, array, string, matrix, bitwise operators, and other miscellaneous parts of programming. Once you are gone through these questions, you can handle a good number of questions in real Job interviews.

1. Fibonacci series
   Write a simple Java program that will print Fibonacci series, e.g. 1 1 2 3 5 8 13 … . up to a given number. We prepare for cross questions like using iteration over recursion and how to optimize the solution using caching and memoization.
   
   2. A prime number
   Write a Java program to check if a given number is prime or not. Remember, a prime number is a number which is not divisible by any other number, e.g. 3, 5, 7, 11, 13, 17, etc. Be prepared for cross, e.g. checking till the square root of a number, etc.
   
   3. String Palindrome
   You need to write a simple Java program to check if a given String is palindrome or not. A Palindrome is a String which is equal to the reverse of itself, e.g., “Bob” is a palindrome because of the reverse of “Bob” is also “Bob.”  Though be prepared with both recursive and iterative solution of this problem. The interviewer may ask you to solve without using any library method, e.g. indexOf() or subString() so be prepared for that.
   
   4. Integer Palindrome
   This is generally asked as follow-up or alternative of the previous program. This time you need to check if given Integer is palindrome or not. An integer is called palindrome if it’s equal to its reverse, e.g. 1001 is a palindrome, but 1234 is not because the reverse of 1234 is 4321 which is not equal to 1234. You can use divide by 10 to reduce the number and modulus 10 to get the last digit. This trick is used to solve this problem.
   
   5. Armstrong number
   A number is called an Armstrong number if it is equal to the cube of its every digit. For example, 153 is an Armstrong number because of 153= 1+ 125+27, which is equal to 1^3+5^3+3^3. You need to write a program to check if the given number is Armstrong number or not.
   
   6. Avoiding deadlock in Java
   This is one of the interesting programs from Java Interviews, mostly asked to 2 to 3 years of experienced programmers or higher. The interviewer simply asked you to write code where a resource is accessed by multiple threads. You need to write code in such a way that no deadlock should occur. The trick to solving this problem is acquiring resources in order and release them in reverse order, e.g. first acquire resource R1 and only if you have got R1 to go for R2. This way, you can avoid deadlock.
   
   7. Factorial
   This is one of the simplest programs you can expect in interviews. It is generally asked to see if you can code or not. Sometimes interviewer may also ask about changing a recursive solution to iterative one or vice-versa.
   
   8. Reverse a String
   This problem is similar to the String Palindrome problem we have discussed above. If you can solve that problem, you can solve this as well. You can use indexOf() or substring() to reverse a String or alternatively, convert the problem to reverse an array by operating on character array instead of String.
   
   9. Remove duplicates from an array
   Write a program to remove duplicates from an array in Java without using the Java Collection API. The array can be an array of String, Integer or Character, your solution should be independent of the type of array.
   
   10. Printing patterns
   
   11. Print repeated characters of String?
   
   12. GCD of two numbers.
   
   13. The square root of a number
   You need to write a program to calculate the square root of a number without using the Math.sqrt() function of JDK. You need to write your logic and method to calculate the square root. You can though use the popular algorithm, like Newton’s method.
   
   14. Reverse array in place
   
   15. Reverse words of a sentence
   
   16. Leap year
   
   17. Binary search
   
   18. String Anagram
   Write a program to check if two given String is Anagram of each other. Your function should return true if two Strings are Anagram, false otherwise. A string is said to be an anagram if it contains the same characters and same length, but in a different order, e.g. army and Mary are anagrams. You can ignore cases for this problem, but you should clarify that from your interview.
   
   19. Design a Vending Machine (solutionThis one of the popular OOAD (object-oriented analysis and design) question from Java Interviews. You will be given 3 hours to design and code a vending machine satisfying some of the business requirements.

20. Reverse a number

21. The first non-repeated character of String

22. Finding Middle element of linked list in one pass (

23. Pre-order traversal

24. Pre-order traversal without recursion

25. In order traversal

26. In order traversal without recursion

27. Post-order traversal

28. Postorder traversal without recursion

29. Print all leaves of a binary tree

30. Sort array using quicksort
You need to write a Java program to sort an array of integers using a quick sort algorithm. You cannot use any library method, e.g. JDK or a third party library, which means, you need to first implement the quicksort algorithm and then sort the array.

31. Insertion sort
Write a program to implement the insertion sort algorithm in Java. The program should take an unsorted array and sort it using insertion sort algorithm Also explain the best case and worst case time and space complexity of the Insertion sort algorithm.

32. Bubble sort
Write a program to implement the bubble sort algorithm in Java. You can use basic operators and functions, but sorting functions from Java API is not allowed.

33. Transpose a matrix

34. Print all permutations of String
Write a Java program to print all permutations of a given String. For example, if given String is “GOD” then your program should print all 6 permutations of this string, e.g. “GOD,” “OGD,” “DOG,” “GDO,” “ODG,” and “DGO.”

35. Reverse a String in place

36. Adding two matrices in Java

37. Matrix multiplication

38. Removal all white space from String

39. Reverse a linked list
Write a program to reverse a singly linked list in Java. You can use iteration and recursion to solve this problem, but you should reverse a linked list in place.

40. Find the length of the linked list
Just write a program in Java to find the length of a singly linked list in one pass, i.e. in just one iteration of a singly linked list.

41. Check if a linked list has a loop
Write a program to check if given linked list has a loop or not. Sometimes a linked list get corrupt, and two nodes point to the same node, which forms the loop or cycle in the linked list.

42. Find the start of loop in a linked list

43. Find the middle element of a linked list

44. Find the 3rd element from the tail linked list
You need to write a program to find the 3rd element from the tail of a singly linked list. You need to solve this problem without iterating twice.

45. Convert a linked list to a binary tree
It’s possible to convert a doubly-linked list to a binary tree, you need to write a Java program which takes a doubly-linked list and returns a binary tree.

45. Sort a linked list
You need to given an unsorted linked list, and you need to write a program in Java to sort them in ascending order of the values in each node.


46. Iterative Quicksort
You need to write a Java program to implement quicksort sorting algorithm without recursion. You can use essential JDK classes and programming constructs, but recursion is not allowed.

46. Bucket sort
This program is increasingly getting popular on Java interview because it sorts a given array in linear time. Though there is a lot of prerequisites, e.g. you must know the maximum value present in the array, it is a very interesting problem from interview point of view. You need to write a program to implement a bucket sort algorithm in Java.

47. Counting sort
This is another problem which is similar to the previous one because counting sort is also a linear sorting algorithm. Just remember that bucket sort, and counting sort are different algorithms, so it’s also good to state how they are different.

48. Check if two string rotation of each other
Write a program which accepts two given String and checks if they are the rotation of each. If they then return true otherwise return false. A String is said to be a rotation of other string if they contain same characters and the sequence is rotated across any character, e.g. “dabc” is a rotation of “abcd” but “dbac” is not. 

49. LRU cache in Java
Write a program to implement an LRU cache in Java.  An LRU cache means Least Recently Used Cache which removes the least recently used element if the cache is full.

50. Merge sort
Implement the merge sort algorithm in Java. You can write a recursive or iterative solution, whichever you like. You also need to explain the time and space complexity for the best, worst, and average case.

That’s all about top 50 programs from Java interviews. You can practice these Java programs even if you are not preparing for any Job interview. They not only help you to do well on your programming job interviews but also on learning how to code and developing your programming skill and coding sense.

These small programs touch several important areas, e.g. popular data structures like an array, linked list, binary tree, binary search tree, string, etc., popular algorithms, e.g. sieve of the Eratosthenes algorithm for generating primes, the Euclidean algorithm for calculating LCM, and GCF, Fibonacci sequence, printing patterns and so on.

These programs also touch base on useful operators like bit-shift and bitwise operators, modulus operators, and others. Overall, you get a good understanding of the Java programming world by practicing these Java programs from coding interviews.

Thanks for reading this article so far, if you like these Java programs, then please share with your friends and colleagues, if you have any question or problem, then please drop a comment.