/**
   * Joins array elements to string.
   *
   * @param arr Array.
   * @return String.
   */
  @Nullable
  public static String compactArray(Object[] arr) {
    if (arr == null || arr.length == 0) return null;

    String sep = ", ";

    StringBuilder sb = new StringBuilder();

    for (Object s : arr) sb.append(s).append(sep);

    if (sb.length() > 0) sb.setLength(sb.length() - sep.length());

    return U.compact(sb.toString());
  }
  /**
   * Returns compact class host.
   *
   * @param obj Object to compact.
   * @return String.
   */
  @Nullable
  public static Object compactObject(Object obj) {
    if (obj == null) return null;

    if (obj instanceof Enum) return obj.toString();

    if (obj instanceof String || obj instanceof Boolean || obj instanceof Number) return obj;

    if (obj instanceof Collection) {
      Collection col = (Collection) obj;

      Object[] res = new Object[col.size()];

      int i = 0;

      for (Object elm : col) res[i++] = compactObject(elm);

      return res;
    }

    if (obj.getClass().isArray()) {
      Class<?> arrType = obj.getClass().getComponentType();

      if (arrType.isPrimitive()) {
        if (obj instanceof boolean[]) return Arrays.toString((boolean[]) obj);
        if (obj instanceof byte[]) return Arrays.toString((byte[]) obj);
        if (obj instanceof short[]) return Arrays.toString((short[]) obj);
        if (obj instanceof int[]) return Arrays.toString((int[]) obj);
        if (obj instanceof long[]) return Arrays.toString((long[]) obj);
        if (obj instanceof float[]) return Arrays.toString((float[]) obj);
        if (obj instanceof double[]) return Arrays.toString((double[]) obj);
      }

      Object[] arr = (Object[]) obj;

      int iMax = arr.length - 1;

      StringBuilder sb = new StringBuilder("[");

      for (int i = 0; i <= iMax; i++) {
        sb.append(compactObject(arr[i]));

        if (i != iMax) sb.append(", ");
      }

      sb.append("]");

      return sb.toString();
    }

    return U.compact(obj.getClass().getName());
  }
  /**
   * Compact class names.
   *
   * @param obj Object for compact.
   * @return Compacted string.
   */
  @Nullable
  public static String compactClass(@Nullable Object obj) {
    if (obj == null) return null;

    return U.compact(obj.getClass().getName());
  }