### Question:
PropertiesBuilder { public static Properties of(String key, String value) { Properties properties = new Properties(); properties.setProperty(key, value); return properties; } private  PropertiesBuilder(); static Properties of(String key, String value);  }### Answer:
@Test void trivial_singleton_properties() { Properties properties = PropertiesBuilder.of("testKey", "testValue"); assertThat(properties).containsOnly(entry("testKey", "testValue")); }