### Question:
JsonObject implements Serializable { @Override public String toString() { return toJson().toString(); } JsonObject();  JsonObject(JsonNode node); static JsonNodeFactory getFactory(); JsonNode getSourceNode(); abstract JsonNode toJson(); @Override String toString();  }### Answer:
@Test public void testToString() { JsonObject instance = new JsonObjectImpl(); String expResult = "null"; String result = instance.toString(); assertEquals(expResult, result); }