@Test public void testToString() throws Exception { Map<String, Object> v1 = new HashMap<>(); v1.put("k1", "v1"); Map<String, Object> v2 = new HashMap<>(); v2.put("k2", "v2"); v2.put("k3", null); Map<String, Object> union = new HashMap<String, Object>() {{ putAll(v1); put("k2", "v2"); }}; ConcatMap c = create(v1, v2); Assert.assertEquals(c.toString(), union.toString()); }