public interface UnaryOperator<T> extends Function<T, T> {
  /**
   * Returns a unary operator that always returns its input argument.
   *
   * @param <T> the type of the input and output of the operator
   * @return a unary operator that always returns its input argument
   */
  static <T> UnaryOperator<T> identity() {
    return t -> t;
  }
}

@Test
  public void test(){
    super.print(UnaryOperator.identity().apply(1));//输出1
    super.print(UnaryOperator.identity().apply(false));//输出false
    super.print(UnaryOperator.identity().apply("string"));//输出string
  }