### Question:
ConfigSourceMap extends AbstractMap<String, String> implements Map<String, String>, Serializable { public boolean containsKey(final Object key) { return delegate.getPropertyNames().contains(key); } ConfigSourceMap(final ConfigSource delegate); int size(); boolean isEmpty(); boolean containsKey(final Object key); String get(final Object key); Set<String> keySet(); Collection<String> values(); Set<Entry<String, String>> entrySet(); void forEach(final BiConsumer<? super String, ? super String> action);  }### Answer:
@Test public void testContainsKey() { final ConfigSourceMap csm = new ConfigSourceMap(MANY_CONF_SRC); assertTrue(csm.containsKey("test")); assertTrue(csm.containsKey("null-valued-key")); assertFalse(csm.containsKey("missing-key")); }