@Override
  public void storeProperty(String key, String value) {
    checkArgument(key != null, "Key of context property must not be null");
    checkArgument(value != null, "Value of context property must not be null");
    contextProperties.put(key, value);
  }
@Test
  public void storeProperty_throws_IAE_if_key_is_null() {
    assertThatThrownBy(() -> underTest.storeProperty(null, "bar"))
      .isInstanceOf(IllegalArgumentException.class)
      .hasMessage("Key of context property must not be null");
  }