1. Reverse a String in Java
Problem Statement:
Write a Java program to reverse a given string using built-in methods.
Code Example:
java
public class StringReversal {
public static void main(String[] args) {
String original = “automation”;
String reversed = new StringBuilder(original).reverse().toString();
System.out.println(“Reversed: ” + reversed);
}
}
Output:
makefile
Reversed: noitamotua
Explanation:
This program uses StringBuilder’s reverse() method which is efficient and avoids manual iteration. Ideal for interviews asking how to reverse a string in Java using built-in functions.
2. Palindrome Checker in Java
Problem Statement:
Check whether a given string is a palindrome (reads the same forward and backward).
Code Example:
java
public class PalindromeCheck {
public static void main(String[] args) {
String str = “madam”;
boolean isPalindrome = true;
for (int i = 0; i < str.length() / 2; i++) {
if (str.charAt(i) != str.charAt(str.length() – i – 1)) {
isPalindrome = false;
break;
}
}
System.out.println(str + ” is palindrome: ” + isPalindrome);
}
}
Output:
csharp
madam is palindrome: true
Explanation:
This logic compares the characters from the start and end moving towards the center. It’s efficient and works in O(n) time.
3. Swap Two Numbers Without Temp Variable
Problem Statement:
Swap two integer variables without using a temporary variable.
Code Example:
java
public class SwapNumbers {
public static void main(String[] args) {
int a = 10, b = 20;
System.out.println(“Before: a=” + a + “, b=” + b);
a = a + b;
b = a – b;
a = a – b;
System.out.println(“After: a=” + a + “, b=” + b);
}
}
Output:
makefile
Before: a=10, b=20
After: a=20, b=10
Explanation:
This technique uses arithmetic operations to swap values without a third variable. This is a commonly asked beginner logic question in coding interviews.
4. Finding the Largest Number in an Array
Problem Statement:
Find the largest number in a given integer array.
Code Example:
java
public class LargestInArray {
public static void main(String[] args) {
int[] numbers = {10, 5, 25, 8, 15, 3};
int max = numbers[0];
for (int i = 1; i < numbers.length; i++) {
if (numbers[i] > max) {
max = numbers[i];
}
}
System.out.println(“Largest number: ” + max);
}
}
Output:
yaml
Largest number: 25
Explanation:
The loop updates the maximum value as it encounters larger numbers. Simple yet fundamental for array-based interview questions.
5. Finding the Smallest Number in an Array
Problem Statement:
Write a Java program to find the minimum value in an integer array.
Code Example:
java
public class SmallestInArray {
public static void main(String[] args) {
int[] numbers = {10, 5, 25, 8, 15, 3};
int min = numbers[0];
for (int i = 1; i < numbers.length; i++) {
if (numbers[i] < min) {
min = numbers[i];
}
}
System.out.println(“Smallest number: ” + min);
}
}
Output:
yaml
Smallest number: 3
Explanation:
Just like finding the max, this solution checks each number and keeps the smallest. A classic problem for data structure interviews.
6. Count Vowels and Consonants in a String
Problem Statement:
Create a Java program to count vowels and consonants in a string.
Code Example:
java
public class VowelConsonantCounter {
public static void main(String[] args) {
String str = “Automation World”;
str = str.toLowerCase();
int vowels = 0, consonants = 0;
for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);
if (ch >= ‘a’ && ch <= ‘z’) {
if (“aeiou”.indexOf(ch) != -1) {
vowels++;
} else {
consonants++;
}
}
}
System.out.println(“Vowels: ” + vowels + “, Consonants: ” + consonants);
}
}
Output:
yaml
Vowels: 6, Consonants: 10
Question 7: Character Occurrence Counter in Java
Q: How do you count character occurrences in a string using Java?
A:
We can use a HashMap to store each character as a key and its count as a value.
Code Example:
java
import java.util.HashMap;
public class CharOccurrence {
public static void main(String[] args) {
String str = “automation”;
HashMap<Character, Integer> charCount = new HashMap<>();
for (char ch : str.toCharArray()) {
charCount.put(ch, charCount.getOrDefault(ch, 0) + 1);
}
System.out.println(charCount);
}
}
Output: {a=2, u=2, t=2, o=1, m=1, i=1, n=1}
Time Complexity: O(n)
Application: Useful in data analysis, string processing, and frequency counting.
Question 8: Print Fibonacci Series in Java
Q: How do you print the first N Fibonacci numbers?
Code Example:
java
public class Fibonacci {
public static void main(String[] args) {
int n = 10;
int a = 0, b = 1;
System.out.print(a + ” ” + b + ” “);
for (int i = 2; i < n; i++) {
int c = a + b;
System.out.print(c + ” “);
a = b;
b = c;
}
}
}
Output: 0 1 1 2 3 5 8 13 21 34
Concept: Each number is the sum of the two preceding numbers.
Tip: Often asked in logic-based Java interviews.
Question 9: Factorial Calculation (Loop & Recursion)
Q: How do you calculate factorial in Java?
Code Example:
java
public class Factorial {
public static void main(String[] args) {
int num = 5;
int factorial = 1;
for (int i = 1; i <= num; i++) {
factorial *= i;
}
System.out.println(“Factorial (loop): ” + factorial);
System.out.println(“Factorial (recursive): ” + factorialRecursive(num));
}
public static int factorialRecursive(int n) {
if (n == 0 || n == 1) return 1;
return n * factorialRecursive(n – 1);
}
}
Output: 120 (loop & recursive)
Loop and Recursion: Know both approaches for interviews.
Question 10: Prime Number Checker
Q: How to check if a number is prime in Java?
Code Example:
java
public class PrimeChecker {
public static void main(String[] args) {
int num = 17;
boolean isPrime = num > 1;
for (int i = 2; i <= Math.sqrt(num); i++) {
if (num % i == 0) {
isPrime = false;
break;
}
}
System.out.println(num + ” is prime: ” + isPrime);
}
}
👉The Next 10 Questions-I: JAVA STRING
