### Question:
SetExpression extends PrimitiveUpdateExpression { @Override public JsonNode toJson() { ObjectNode node = getFactory().objectNode(); ObjectNode values = getFactory().objectNode(); for (FieldAndRValue x : fields) { values.set(x.getField().toString(), x.getRValue().toJson()); } node.set(op.toString(), values); return node; } SetExpression(UpdateOperator op, List<FieldAndRValue> list);  SetExpression(UpdateOperator op, FieldAndRValue... l); List<FieldAndRValue> getFields(); UpdateOperator getOp(); @Override JsonNode toJson(); static SetExpression fromJson(ObjectNode node); @Override int hashCode(); @Override boolean equals(Object obj);  }### Answer:
@Test public void testToJson() { List<FieldAndRValue> list = new ArrayList<>(); FieldAndRValue fav = new FieldAndRValue(); fav.setField(Path.EMPTY); fav.setRValue(new RValueExpression(Path.EMPTY)); list.add(fav); SetExpression instance = null; instance = new SetExpression(UpdateOperator._set, list); JsonNode result = instance.toJson(); assertEquals("{\"$set\":{\"\":{\"$valueof\":\"\"}}}", result.toString()); }