### Question:
KVRepository { public Set<String> keys() { return backend.keySet(); }  String get(String key); void put(String key, String body); void delete(String key); Set<String> keys();  }### Answer:
@Test public void testKeys() throws Exception { underTest.put("a", "b"); underTest.put("b", "c"); Set<String> keys = underTest.keys(); assertEquals(2, keys.size()); assertTrue(keys.contains("a")); assertTrue(keys.contains("b")); }