Interview Java Q's Flashcards
How do you create a Scanner object to read input from console?
Scanner scanner = new Scanner(System.in);
What Scanner method reads an entire line as a String?
scanner.nextLine()
What Scanner method reads the next integer value?
scanner.nextInt()
How do you print output without a new line in Java?
System.out.print()
How do you print output with a new line in Java?
System.out.println()
How do you get the length of a String?
string.length()
How do you get a character at a specific index in a String?
string.charAt(index)
How do you extract a portion of a String from start index (inclusive) to end index (exclusive)?
string.substring(startIndex, endIndex)
How do you convert a String to uppercase?
string.toUpperCase()
How do you convert a String to lowercase?
string.toLowerCase()
How do you remove whitespace from beginning and end of a String?
string.trim()
How do you split a String into an array using a delimiter?
string.split(delimiter)
How do you convert a String to an integer?
Integer.parseInt(string)
How do you convert a String to a double?
Double.parseDouble(string)
How do you convert a number to a String?
String.valueOf(number)
How do you create an empty ArrayList?
ArrayList<Type> list = new ArrayList<>();
ArrayList<String> cars = new ArrayList<String>();
How do you add an element to a List?
list.add(element)
How do you remove an element at a specific index from a List?
list.remove(index)
How do you get the size of a List?
list.size()
How do you check if a List contains an element?
list.contains(element)
How do you get an element at a specific index from a List?
list.get(index)
How do you find the maximum number between two numbers?
Math.max(a, b)
How do you find the minimum of two numbers?
Math.min(a, b)
How do you calculate the absolute value? (ab=+ve number)
Math.abs(number)
How do you calculate a power?
Math.pow(base, exponent)
How do you calculate a square root?
Math.sqrt(number)
How do you create a LocalDate object for a specific date?
LocalDate date = LocalDate.of(year, month, day);
How do you get the day of week from a LocalDate?
DayOfWeek day = date.getDayOfWeek();
How do you parse a date string with a specific format?
LocalDate parsed = LocalDate.parse(dateString, DateTimeFormatter.ofPattern(“dd/MM/yyyy”));
How do you get a currency formatter for a specific locale?
NumberFormat formatter = NumberFormat.getCurrencyInstance(Locale.US);
How do you format a number with specific decimal places?
String.format(“%.2f”, number)
How do you throw an exception with a custom message?
throw new Exception(“Your message here”);
How do you write a basic try-catch block?
try { // code that may throw exception } catch (Exception e) { // handle exception }
How do you sort an array?
Arrays.sort(array)
How do you convert an array to a List?
Arrays.asList(array)
How do you check if a number is odd using bitwise operations?
number & 1
How do you check if a number is a power of 2 using bitwise operations?
(n & (n - 1)) == 0
How do you sort a List in natural order?
Collections.sort(list)
How do you sort a List in reverse order?
list.sort(Comparator.reverseOrder())
How do you check if Scanner has an integer input available?
scanner.hasNextInt()
How do you safely parse a String to Integer with exception handling?
try { Integer.parseInt(stringValue); } catch (NumberFormatException e) { // handle invalid number }
How do you implement a two-pointer technique for a sorted array?
int left = 0; int right = array.length - 1; while (left < right) { // Process elements from both ends left++; right–; }
How do you find an element in a sorted array using binary search?
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; }
How do you implement the sliding window pattern?
int windowStart = 0; for (int windowEnd = 0; windowEnd < array.length; windowEnd++) { // Add element to window while (/* condition */) { // Remove element from window windowStart++; } }
How do you traverse a linked list using fast and slow pointers?
ListNode slow = head; ListNode fast = head; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; }
How do you create and use a HashMap for counting elements?
HashMap<ElementType, Integer> map = new HashMap<>(); map.put(element, map.getOrDefault(element, 0) + 1);
How do you iterate over HashMap entries?
for (Map.Entry<K, V> entry : map.entrySet()) { K key = entry.getKey(); V value = entry.getValue(); }
How do you find all pairs in an array that sum to a target in O(n) time?
HashSet<Integer> set = new HashSet<>(); for (int num : array) { if (set.contains(target - num)) { // Found pair } set.add(num); }</Integer>
How do you find the majority element using Boyer-Moore algorithm?
int count = 0; int candidate = 0; for (int num : array) { if (count == 0) candidate = num; count += (num == candidate) ? 1 : -1; }
How do you check if a string is a palindrome efficiently?
int left = 0; int right = s.length() - 1; while (left < right) { if (s.charAt(left++) != s.charAt(right–)) return false; } return true;
How do you implement a character frequency counter?
int[] freq = new int[26]; // for lowercase letters for (char c : s.toCharArray()) { freq[c - ‘a’]++; }
What is the difference between ArrayList and LinkedList?
ArrayList: O(1) access, O(n) insertion/deletion. LinkedList: O(n) access, O(1) insertion/deletion at known position
What is the time complexity of HashMap operations?
Average case: O(1) for put/get/remove/containsKey. Worst case: O(n) if many collisions
