code: public static String repeat(String string, int count) {
      checkNotNull(string); // eager for GWT.
  
      if (count <= 1) {
        checkArgument(count >= 0, "invalid count: %s", count);
        return (count == 0) ? "" : string;
      }
  
      // IF YOU MODIFY THE CODE HERE, you must update StringsRepeatBenchmark
      final int len = string.length();
      final long longSize = (long) len * (long) count;
      final int size = (int) longSize;
      if (size != longSize) {
        throw new ArrayIndexOutOfBoundsException("Required array size too large: " + longSize);
      }
  
      final char[] array = new char[size];
      string.getChars(0, len, array, 0);
      int n;
      for (n = len; n < size - n; n <<= 1) {
        System.arraycopy(array, 0, array, n, n);
      }
      System.arraycopy(array, 0, array, n, size - n);
      return new String(array);
    }
docs: Returns a string consisting of a specific number of concatenated copies of an input string. For
  example, {@code repeat("hey", 3)} returns the string {@code "heyheyhey"}.
  
  @param string any non-null string
  @param count the number of times to repeat it; a nonnegative integer
  @return a string containing {@code string} repeated {@code count} times (the empty string if
  {@code count} is zero)
  @throws IllegalArgumentException if {@code count} is negative