@Override
  protected void set(String key, String value) {
    localProperties.put(
      requireNonNull(key, "key can't be null"),
      requireNonNull(value, "value can't be null").trim());
  }
@Test
  public void set_will_throw_NPE_if_key_is_null() {
    assertThatThrownBy(() -> underTest.set(null, ""))
      .isInstanceOf(NullPointerException.class)
      .hasMessage("key can't be null");
  }