Interview Java Q's

Question 1

How do you create a Scanner object to read input from console?

Answer 1

Scanner scanner = new Scanner(System.in);

Question 2

What Scanner method reads an entire line as a String?

Answer 2

scanner.nextLine()

Question 3

What Scanner method reads the next integer value?

Answer 3

scanner.nextInt()

Question 4

How do you print output without a new line in Java?

Answer 4

System.out.print()

Question 5

How do you print output with a new line in Java?

Answer 5

System.out.println()

Question 6

How do you get the length of a String?

Answer 6

string.length()

Question 7

How do you get a character at a specific index in a String?

Answer 7

string.charAt(index)

Question 8

How do you extract a portion of a String from start index (inclusive) to end index (exclusive)?

Answer 8

string.substring(startIndex, endIndex)

Question 9

How do you convert a String to uppercase?

Answer 9

string.toUpperCase()

Question 10

How do you convert a String to lowercase?

Answer 10

string.toLowerCase()

Question 11

How do you remove whitespace from beginning and end of a String?

Answer 11

string.trim()

Question 12

How do you split a String into an array using a delimiter?

Answer 12

string.split(delimiter)

Question 13

How do you convert a String to an integer?

Answer 13

Integer.parseInt(string)

Question 14

How do you convert a String to a double?

Answer 14

Double.parseDouble(string)

Question 15

How do you convert a number to a String?

Answer 15

String.valueOf(number)

Question 16

How do you create an empty ArrayList?

Answer 16

ArrayList<Type> list = new ArrayList<>();

ArrayList<String> cars = new ArrayList<String>();

Question 17

How do you add an element to a List?

Answer 17

list.add(element)

Question 18

How do you remove an element at a specific index from a List?

Answer 18

list.remove(index)

Question 19

How do you get the size of a List?

Answer 19

list.size()

Question 20

How do you check if a List contains an element?

Answer 20

list.contains(element)

Question 21

How do you get an element at a specific index from a List?

Answer 21

list.get(index)

Question 22

How do you find the maximum number between two numbers?

Answer 22

Math.max(a, b)

Question 23

How do you find the minimum of two numbers?

Answer 23

Math.min(a, b)

Question 24

How do you calculate the absolute value? (ab=+ve number)

Answer 24

Math.abs(number)

Question 25

How do you calculate a power?

Answer 25

Math.pow(base, exponent)

Question 26

How do you calculate a square root?

Answer 26

Math.sqrt(number)

Question 27

How do you create a LocalDate object for a specific date?

Answer 27

LocalDate date = LocalDate.of(year, month, day);

Question 28

How do you get the day of week from a LocalDate?

Answer 28

DayOfWeek day = date.getDayOfWeek();

Question 29

How do you parse a date string with a specific format?

Answer 29

LocalDate parsed = LocalDate.parse(dateString, DateTimeFormatter.ofPattern("dd/MM/yyyy"));

Question 30

How do you get a currency formatter for a specific locale?

Answer 30

NumberFormat formatter = NumberFormat.getCurrencyInstance(Locale.US);

Question 31

How do you format a number with specific decimal places?

Answer 31

String.format("%.2f", number)

Question 32

How do you throw an exception with a custom message?

Answer 32

throw new Exception("Your message here");

Question 33

How do you write a basic try-catch block?

Answer 33

try { // code that may throw exception } catch (Exception e) { // handle exception }

Question 34

How do you sort an array?

Answer 34

Arrays.sort(array)

Question 35

How do you convert an array to a List?

Answer 35

Arrays.asList(array)

Question 36

How do you check if a number is odd using bitwise operations?

Answer 36

number & 1

Question 37

How do you check if a number is a power of 2 using bitwise operations?

Answer 37

(n & (n - 1)) == 0

Question 38

How do you sort a List in natural order?

Answer 38

Collections.sort(list)

Question 39

How do you sort a List in reverse order?

Answer 39

list.sort(Comparator.reverseOrder())

Question 40

How do you check if Scanner has an integer input available?

Answer 40

scanner.hasNextInt()

Question 41

How do you safely parse a String to Integer with exception handling?

Answer 41

try { Integer.parseInt(stringValue); } catch (NumberFormatException e) { // handle invalid number }

Question 42

How do you implement a two-pointer technique for a sorted array?

Answer 42

int left = 0; int right = array.length - 1; while (left < right) { // Process elements from both ends left++; right--; }

Question 43

How do you find an element in a sorted array using binary search?

Answer 43

int left = 0; int right = array.length - 1; while (left <= right) { int mid = left + (right - left) / 2; if (array[mid] == target) return mid; else if (array[mid] < target) left = mid + 1; else right = mid - 1; }

Question 44

How do you implement the sliding window pattern?

Answer 44

int windowStart = 0; for (int windowEnd = 0; windowEnd < array.length; windowEnd++) { // Add element to window while (/* condition */) { // Remove element from window windowStart++; } }

Question 45

How do you traverse a linked list using fast and slow pointers?

Answer 45

ListNode slow = head; ListNode fast = head; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; }

Question 46

How do you create and use a HashMap for counting elements?

Answer 46

HashMap map = new HashMap<>(); map.put(element, map.getOrDefault(element, 0) + 1);

Question 47

How do you iterate over HashMap entries?

Answer 47

for (Map.Entry entry : map.entrySet()) { K key = entry.getKey(); V value = entry.getValue(); }

Question 48

How do you find all pairs in an array that sum to a target in O(n) time?

Answer 48

HashSet set = new HashSet<>(); for (int num : array) { if (set.contains(target - num)) { // Found pair } set.add(num); }

Question 49

How do you find the majority element using Boyer-Moore algorithm?

Answer 49

int count = 0; int candidate = 0; for (int num : array) { if (count == 0) candidate = num; count += (num == candidate) ? 1 : -1; }

Question 50

How do you check if a string is a palindrome efficiently?

Answer 50

int left = 0; int right = s.length() - 1; while (left < right) { if (s.charAt(left++) != s.charAt(right--)) return false; } return true;

Question 51

How do you implement a character frequency counter?

Answer 51

int[] freq = new int[26]; // for lowercase letters for (char c : s.toCharArray()) { freq[c - 'a']++; }

Question 52

What is the difference between ArrayList and LinkedList?

Answer 52

ArrayList: O(1) access, O(n) insertion/deletion. LinkedList: O(n) access, O(1) insertion/deletion at known position

Question 53

What is the time complexity of HashMap operations?

Answer 53

Average case: O(1) for put/get/remove/containsKey. Worst case: O(n) if many collisions