code: public static int substr_count(final String string, final String substring) {
          if(substring.length()==1) {
              return substr_count(string, substring.charAt(0));
          }
          
          int count = 0;
          int idx = 0;
  
          while ((idx = string.indexOf(substring, idx)) != -1) {
             ++idx;
             ++count;
          }
  
          return count;
      }
docs: Count the number of substring occurrences.
  
  @param string
  @param substring
  @return