content: class DifferAtOneBitPos {
      /**
       * Write a java function to check whether the two numbers differ at one bit position only or not.
       * > DifferAtOneBitPos.differAtOneBitPos(13, 9)
       * true
       * > DifferAtOneBitPos.differAtOneBitPos(15, 8)
       * false
       * > DifferAtOneBitPos.differAtOneBitPos(2, 4)
       * false
       */
      public static boolean differAtOneBitPos(int a, int b) {
          int diff = a ^ b;
          int count = 0;
          while (diff > 0) {
              diff = diff & (diff - 1);
              count++;
          }
          return count == 1;
      }
  }
  
  
  class TestDuplicate {
      /**
       * Write a function to find whether a given array of integers contains any duplicate element.
       * > TestDuplicate.testDuplicate([1, 2, 3, 4, 5])
       * false
       * > TestDuplicate.testDuplicate([1, 2, 3, 4, 4])
       * true
       * > TestDuplicate.testDuplicate([1, 1, 2, 2, 3, 3, 4, 4, 5])
       * true
       */
      public static boolean testDuplicate(List<Integer> arraynums) {
          Set<Integer> set = new HashSet<Integer>();
          for (int i = 0; i < arraynums.size(); i++) {
              if (set.contains(arraynums.get(i))) {
                  return true;
              }
              set.add(arraynums.get(i));
          }
          return false;
      }
  }
  
  class FreqElement {
      /**
       * Write a function to find frequency of the elements in a given list of lists using collections module.
       * > FreqElement.freqElement([[1, 2, 3, 2], [4, 5, 6, 2], [7, 1, 9, 5]])
       * {'2':3,'1':2,'5':2,'3':1,'4':1,'6':1,'7':1,'9':1}
       * > FreqElement.freqElement([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
       * {'1':1,'2':1,'3':1,'4':1,'5':1,'6':1,'7':1,'8':1,'9':1,'10':1,'11':1,'12':1}
       * > FreqElement.freqElement([[15, 20, 30, 40], [80, 90, 100, 110], [30, 30, 80, 90]])
       * {'30':3,'80':2,'90':2,'15':1,'20':1,'40':1,'100':1,'110':1}
       */
      public static HashMap<Integer, Integer> freqElement(List<List<Integer>> nums) {
          HashMap<Integer, Integer> freq = new HashMap<>();
          for (List<Integer> list : nums) {
              for (int i : list) {
                  freq.put(i, freq.getOrDefault(i, 0) + 1);
              }
          }
          return freq;
      }
  }