Properties { public boolean containsKey(String key) { return properties.containsKey(key); }  void load(String classpath); Optional<String> get(String key); void set(String key, String value); Set<String> keys(); boolean containsKey(String key);  }
@Test void containsKey() { properties.properties.put("key", ""); assertThat(properties.containsKey("key")).isTrue(); }