How to Parse Json generic array with inheritance items

Question

my question is not same is this question. if i want to explain lets assume we have these classes (A, B, C).

@Data
@NoArgsConstructor
class A {
    private Integer x;
    private String type;

    public A(String type, Integer x) {
        this.type = type;
        this.x = x;
    }
}

@Data
class B extends A {
    private Integer y;

    public B(Integer x, Integer y) {
        super("B", x);
        this.y = y;
    }
}

@Data
class C extends A {
    private Integer z;

    public C(Integer x, Integer z) {
        super("C", x);
        this.z = z;
    }
}

know i want to parse an array which their base classes is A. and with property named type i want to convert each item to its specific class. something like this code.

public static void main(String[] args) {
        Gson gson = new Gson();
        List<A> list = new ArrayList<>();
        A a = new A("A", 0);
        list.add(a);
        B b = new B(1, 2);
        list.add(b);
        C c = new C(3, 4);
        list.add(c);
        String serializedJson = gson.toJson(list);
        List<? extends A> deserializedList = gson.fromJson(serializedJson, new TypeToken<List<? extends A>>() {
        }.getType());
        for (A item : deserializedList) {
            System.out.println(item.getType());
            if (item instanceof B) {
                System.out.println(((B) item).getY());

            } else if (item instanceof C) {
                System.out.println(((C) item).getZ());

            }
        }
    }

serialized json is something like this

[{"x":0,"type":"A"},{"y":2,"x":1,"type":"B"},{"z":4,"x":3,"type":"C"}]

in real world, i have something like this json and want to parse it. but when i run the code y and z properties not printed and objects are not instance of B or C. how to achieve to this goal to parse and create each item with type property.

Isn't that because you have defined list as List<A> - instead of something like List<? extends A>? — JGFMK
– JGFMK, Commented Feb 14, 2020 at 13:46
I'd also say because you've tried tagging the class with a type property, you should have been able to use A["type"] or something like that as well as the instanceof stuff - maybe even A.type - not sure how Gson works. — JGFMK
– JGFMK, Commented Feb 14, 2020 at 13:57

Mahdiyar Zerehpoush · Accepted Answer · 2020-02-15 20:42:13Z

1

I found the solution via Gson TypeAdapterFactory mechanism. creating a class to implement com.google.gson.TypeAdapterFactory. for more detail see this link

answered Feb 15, 2020 at 20:42

Mahdiyar Zerehpoush

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donquih0te · Accepted Answer · 2020-02-14 14:00:07Z

0

for(A item : deserializedList) {
  switch(item.getType()) {
    case "B":
      System.out.println(((B) item).getY());
    break;
    case "C":
      System.out.println(((C) item).getZ());
    break;
  }
}

answered Feb 14, 2020 at 14:00

donquih0te

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1 Comment

Mahdiyar Zerehpoush Over a year ago

not working. java.lang.ClassCastException ! when our objects are not instance of B or C absolutely this exception occurs!

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