Java Interview Programming Questions and Solutions
This page provides a collection of the most commonly asked Java interview programming questions, designed to help beginners as well as experienced Java developers strengthen their problem-solving skills. Each problem focuses on building a strong foundation in core Java concepts such as strings, arrays, loops, conditions, and basic data manipulation, which are frequently tested in technical interviews.
For every program, we have clearly defined the problem statement along with expected input and output to make understanding easier and learning more effective. These questions are especially useful for Java freshers, mid-level developers, and experienced professionals who are preparing for coding rounds, technical interviews, or revising fundamental Java concepts.
Top 20 Java String Programming Interview Questions
1. Reverse a String
Write a program to reverse a string without using built-in reverse methods.
Input: "Java"
Output: "avaJ"
2. Check if a String is Palindrome
Determine whether a given string is a palindrome.
Input: "madam"
Output: true
3. Count Occurrence of Characters
Count the number of times each character appears in a string.
Input: "apple"
Output: a=1, p=2, l=1, e=1
4. Count Vowels and Consonants
Write a program to count vowels and consonants in a string.
Input: "hello"
Output: Vowels=2, Consonants=3
5. Find Duplicate Characters
Print duplicate characters in a string.
Input: "programming"
Output: r, g, m
6. Remove Duplicate Characters
Remove duplicate characters and return a string with unique characters.
Input: "banana"
Output: "ban"
7. Find First Non-Repeated Character
Find the first non-repeating character in a string.
Input: "swiss"
Output: w
8. Check Anagram Strings
Check whether two strings are anagrams of each other.
Input: "listen", "silent"
Output: true
9. Count Words in a String
Write a program to count the number of words in a sentence.
Input: "Java is powerful"
Output: 3
10. Reverse Words in a Sentence
Reverse each word or reverse the entire sentence.
Input: "Java is fun"
Output: "fun is Java"
11. Find Longest Word in a String
Find the longest word from a given string.
Input: "Java makes development easier"
Output: development
12. Replace Characters Without replace()
Replace all spaces with %20 without using String.replace().
Input: "Java is easy"
Output: "Java%20is%20easy"
13. String Rotation Check
Check if one string is a rotation of another.
Input: "ABCD", "CDAB"
Output: true
14. Remove Special Characters
Remove all special characters from a string.
Input: "Ja@va#1$"
Output: "Java1"
15. Count Digits, Letters, and Special Characters
Count letters, digits, spaces, and special characters.
Input: "Java@123"
Output: Letters=4, Digits=3, Special=1
16. Find Substring Occurrences
Count how many times a substring appears in a string.
Input: "abababa", "aba"
Output: 2
17. Toggle Case of Characters
Convert lowercase characters to uppercase and uppercase to lowercase.
Input: "JaVa"
Output: "jAvA"
18. Check if String Contains Only Digits
Check if the string contains only numeric characters.
Input: "12345"
Output: true
19. Print All Permutations of a String
Print all permutations of a string.
Input: "ABC"
Output: ABC, ACB, BAC, BCA, CAB, CBA
20. String Compression
Compress a string like aaabbc → a3b2c1.
Input: "aaabbc"
Output: "a3b2c1"
Top 20 Java Array Programming Interview Questions
1. Find the Largest Element in an Array
4. Reverse an Array
Write a program to reverse the elements of an array.
Input: [1, 2, 3, 4]
Output: [4, 3, 2, 1]
5. Sort an Array
Write a program to sort an array in ascending order.
Input: [5, 2, 8, 1]
Output: [1, 2, 5, 8]
6. Remove Duplicate Elements
Remove duplicate elements from an array.
Input: [1, 2, 2, 3, 1]
Output: [1, 2, 3]
7. Find Duplicate Elements
Find and print duplicate elements in an array.
Input: [1, 2, 2, 3, 1]
Output: [1, 2]
8. Sum of Array Elements
Calculate the sum of all elements in an array.
Input: [1, 2, 3, 4]
Output: 10
9. Average of Array Elements
Find the average of numbers in an array.
Input: [2, 4, 6, 8]
Output: 5
10. Check if Array is Sorted
Check whether an array is sorted in ascending order.
Input: [1, 2, 3, 4]
Output: true
11. Merge Two Arrays
Merge two arrays into a single array.
Input: [1, 2], [3, 4]
Output: [1, 2, 3, 4]
12. Find Common Elements Between Two Arrays
Find common elements between two arrays.
Input: [1, 2, 3], [2, 3, 4]
Output: [2, 3]
13. Find Missing Number in an Array
Find a missing number from a given range.
Input: [1, 2, 4, 5]
Output: 3
14. Move Zeros to End
Move all zeros to the end of the array.
Input: [0, 1, 0, 3, 12]
Output: [1, 3, 12, 0, 0]
15. Rotate Array
Rotate an array to the right by one position.
Input: [1, 2, 3, 4]
Output: [4, 1, 2, 3]
16. Find Frequency of Elements
Count the frequency of each element in an array.
Input: [1, 1, 2, 3, 2]
Output: 1=2, 2=2, 3=1
17. Find Pair with Given Sum
Find a pair of elements whose sum equals a given number.
Input: [2, 4, 3, 5], Sum=7
Output: (2,5) or (4,3)
18. Find Maximum Product of Two Elements
Find the maximum product of two elements in an array.
Input: [2, 3, 5, 7]
Output: 35
19. Separate Even and Odd Numbers
Separate even and odd numbers in an array.
Input: [1, 2, 3, 4, 5]
Output: Even=[2,4], Odd=[1,3,5]
20. Check Equality of Two Arrays
Check if two arrays are equal.
Input: [1, 2, 3], [1, 2, 3]
Output: true