### Question:
CommonsConfigurationSource implements ConfigurationSource { @Override public boolean isEmpty() { return getConfiguration().isEmpty(); }  @Override @SuppressWarnings("unchecked") T getProperty(String key, T defaultValue); @Override @SuppressWarnings("unchecked") T getProperty(String key); @Override @SuppressWarnings("unchecked") T getProperty(String key, Class<T> valueClass); @Override List<String> getKeys(); @Override boolean containsKey(String key); @Override boolean isEmpty();  }### Answer:
@Test void testIsEmpty() { assertTrue(this.configuration.isEmpty()); this.configuration.addProperty("properties", "key2=value2"); assertFalse(this.configuration.isEmpty()); }