Top Java Code questions and Answer

• Find the maximum number in an array: 

Java Code 

public static int findMax(int[] arr) { 
int max = arr[0]; 
for (int i = 1; i < arr.length; i++) { 
if (arr[i] > max) { 
max = arr[i]; 
} 
} 
return max; 
} 
 

• Reverse a string: 

Java Code 

public static String reverseString(String str) { 
return new StringBuilder(str).reverse().toString(); 
} 
 

• Check if a number is prime: 

Java Code 

public static boolean isPrime(int num) { 
if (num <= 1) return false; 
for (int i = 2; i <= Math.sqrt(num); i++) { 
if (num % i == 0) return false; 
} 
return true; 
} 
 

• Check if a string is palindrome: 

Java Code 

public static boolean isPalindrome(String str) { 
return str.equals(new StringBuilder(str).reverse().toString()); 
} 
 

• Find factorial of a number: 

Java Code 

public static int factorial(int n) { 
if (n == 0) return 1; 
return n * factorial(n – 1); 
} 
 

• Calculate Fibonacci series: 

Java Code 

public static int fibonacci(int n) { 
if (n <= 1) return n; 
return fibonacci(n – 1) + fibonacci(n – 2); 
} 
 

• Check if two strings are anagrams: 

Java Code 

public static boolean areAnagrams(String str1, String str2) { 
char[] charArray1 = str1.toCharArray(); 
char[] charArray2 = str2.toCharArray(); 
Arrays.sort(charArray1); 
Arrays.sort(charArray2); 
return Arrays.equals(charArray1, charArray2); 
} 
 

• Count the occurrence of a character in a string: 

Java Code 

public static int countCharOccurrence(String str, char c) { 
int count = 0; 
for (char ch : str.toCharArray()) { 
if (ch == c) count++; 
} 
return count; 
} 
 

• Remove duplicates from an array: 

Java Code 

public static int[] removeDuplicates(int[] arr) { 
Set<Integer> set = new LinkedHashSet<>(); 
for (int num : arr) { 
set.add(num); 
} 
return set.stream().mapToInt(Integer::intValue).toArray(); 
} 
 

• Find intersection of two arrays: 

Java Code 

public static int[] findIntersection(int[] arr1, int[] arr2) { 
Set<Integer> set1 = new HashSet<>(); 
Set<Integer> set2 = new HashSet<>(); 
for (int num : arr1) { 
set1.add(num); 
} 
for (int num : arr2) { 
if (set1.contains(num)) { 
set2.add(num); 
} 
} 
return set2.stream().mapToInt(Integer::intValue).toArray(); 
} 
 

• Reverse words in a string: 

Java Code 

public static String reverseWords(String str) { 
String[] words = str.split(“\\s+”); 
StringBuilder reversed = new StringBuilder(); 
for (int i = words.length – 1; i >= 0; i–) { 
reversed.append(words[i]).append(” “); 
} 
return reversed.toString().trim(); 
} 
 

• Check if a number is Armstrong: 

Java Code 

public static boolean isArmstrong(int num) { 
int original = num; 
int sum = 0; 
while (num > 0) { 
int digit = num % 10; 
sum += Math.pow(digit, 3); // For 3-digit Armstrong number 
num /= 10; 
} 
return sum == original; 
} 
 

• Find the second largest number in an array: 

Java Code 

public static int findSecondLargest(int[] arr) { 
Arrays.sort(arr); 
return arr[arr.length – 2]; 
} 
 

• Check if a number is even or odd: 

Java Code 

public static String checkEvenOdd(int num) { 
return (num % 2 == 0) ? “Even” : “Odd”; 
} 
 

• Check if a year is a leap year: 

Java Code 

public static boolean isLeapYear(int year) { 
return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); 
} 
 

• Find GCD (Greatest Common Divisor) of two numbers: 

Java Code 

public static int findGCD(int a, int b) { 
if (b == 0) return a; 
return findGCD(b, a % b); 
} 
 

• Convert decimal to binary: 

Java Code 

public static String decimalToBinary(int num) { 
return Integer.toBinaryString(num); 
} 
 

• Convert binary to decimal: 

Java Code 

public static int binaryToDecimal(String binary) { 
return Integer.parseInt(binary, 2); 
} 
 

• Check if a string contains only digits: 

Java Code 

public static boolean containsOnlyDigits(String str) { 
return str.matches(“[0-9]+”); 
} 
 

• Check if a string contains only alphabets: 

Java Code 

public static boolean containsOnlyAlphabets(String str) { 
return str.matches(“[a-zA-Z]+”); 
}