### Question:
ConfigValueMapView extends AbstractMap<String, String> { @Override public Set<String> keySet() { return delegate.keySet(); } ConfigValueMapView(final Map<String, ConfigValue> delegate); @Override int size(); @Override boolean isEmpty(); @Override boolean containsKey(final Object key); @Override boolean containsValue(final Object value); @Override String get(final Object key); @Override Set<String> keySet(); @Override Set<Map.Entry<String, String>> entrySet(); @Override Collection<String> values();  }### Answer:
@Test public void keySet() { final Set<String> keys = sampleMap().keySet(); assertEquals(3, keys.size()); assertTrue(keys.contains("my.prop")); assertTrue(keys.contains("my.null")); assertTrue(keys.contains("my.null.value")); assertThrows(UnsupportedOperationException.class, () -> keys.remove("my.prop")); }