Why does Predicates.instanceOf returns false?

Question

I have a

String s =
    {
      "code1" : {
        "price" : 100,
        "type" : null
      },
      "code2" : {
        "price" : 110,
        "type" : null
      }
    }

Then I do:

Object p = Mapper.readValue(s, Person.class);

So it executes the method annotated with @JsonCreator in Person.class:

@JsonCreator
static Person create(Map<String, Object> s) {
        s = Maps.filterValues(s, Predicates.instanceOf(Person.class));
        ...
    }

My problem is s is always empty. I checked and the values have a price and a type. But when I do ps.get("code1").getClass(), it gives me LinkedHashMap.

I don't understand what is happening... Do you have any clue?

This is my class Person (it's an inner class):

public static class Person{

        private int price;
        private String type;
        public Person(int price) {
            this.price = price;
        }
        public int getPrice() {
            return price;
        }
        public String getType() {
            return type;
        }
    }

Thanks!

This is a chicken and egg problem. You are literally writing the creator for the Person class. How would expect that the Object values in the Map could be Person instances yet? — rmlan, Oct 06 '16 at 15:36
@jordaniac89 Huh? That's not what that statement means at all. — rmlan, Oct 06 '16 at 15:37
@rmlan: that's why the filter is here for. In this particular case, I have Person instances, but it doesn't work... — dardy, Oct 06 '16 at 15:43
No, you don't. You are _creating_ person instances. As such, your Object instances are still Maps. `ps.get("code1").get("price");` will return an Integer with the value of 100. — rmlan, Oct 06 '16 at 15:49
@OlivierGrégoire It's Google Guava: https://mvnrepository.com/artifact/com.google.guava/guava — eGoLai, Oct 06 '16 at 20:53
I'm rather familiar with Guava (the question was even tagged). I actually meant the other library, the one with `@JsonCreator` and `ObjectMapper`. Is it Jackson or another fancy library? This might explain some things. — Olivier Grégoire, Oct 06 '16 at 20:56

eGoLai · Accepted Answer · 2016-10-06T18:26:48.140

The problem is that you are deserializing json String to Object and you will always have LinkedHashMap there, because java.lang.Object doesn't have any custom field.

Just try a different way:

  public  class Demo {
        public static void main(String[] args) throws IOException {
            String s = "{" +
                    "  \"code1\" : {" +
                    "    \"price\" : 100," +
                    "    \"type\" : null" +
                    "  }," +
                    "  \"code3\" : {" +
                    "    \"somethingElsse\" : false," +
                    "    \"otherType\" : 1" +
                    "  }," +
                    "  \"code2\" : {" +
                    "    \"price\" : 110," +
                    "    \"type\" : null" +
                    "  }" +
                    "}";


            ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
            Map<String, Person> mapPerson = mapper.readValue(s, MapPerson.class);

            Map<String, Person> filteredMap = Maps.filterValues(mapPerson, new Predicate<Person>() {
                @Override
                public boolean apply(Person person) {
                    return person.isNotEmpty();
                }
            });

            System.out.println(filteredMap);


        }

        public static class MapPerson extends HashMap<String, Person> {}

        public static class Person{

            private int price;
            private String type;

            public Person() {
            }

            public boolean isNotEmpty() {
                return !(0 == price && null ==type);
            }

            @Override
            public String toString() {
                return "Person{" +
                        "price=" + price +
                        ", type='" + type + '\'' +
                        '}';
            }

            public int getPrice() {
                return price;
            }

            public void setPrice(int price) {
                this.price = price;
            }

            public String getType() {
                return type;
            }

            public void setType(String type) {
                this.type = type;
            }
        }
    }

When you configure your objec mapper with configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false) it will just add an empty instance of Person to your map instead of throwing an exception. So you should also define a method that will answer if an instance of Person is empty and then filter your map using it.

If you use java 8 you can have a less code when you filter the map:

Map<String, Person> filteredMap = Maps.filterValues(mapPerson, Person::isNotEmpty);

BTW, it will work wine even you have some extra fields inside key values fro you JSON:

 {
      "code1" : {
        "price" : 100,
        "type" : null,
        "uselessExtraField": "Hi Stack"
      },
      "code2" : {
        "price" : 110,
        "type" : null,
        "anotherAccidentalField": "What?"
      }
    }

You will have the same result as if that fields never existed.

Ok I see why I always have `LinkedHashMap`... Thanks! – dardy Oct 07 '16 at 07:17 — dardy, Oct 07 '16 at 07:17

Why does Predicates.instanceOf returns false?

1 Answers1