error happend when converting object to json

What steps will reproduce the problem?
1.class A declared a field age
2.class B extends A and also declared a field age 
3.new an instance of class B
4.convert the newed B instance to json
5.java.lang.IllegalArgumentException: class B declares multiple JSON fields 
named age

What is the expected output? What do you see instead?


What version of the product are you using? On what operating system?
2.1

Please provide any additional information below.

public class Main {
    public static void main(String[] args) {
        try {
            A a = new A();
            a.setAge(2);
            a.setName("someone");
            B b = new B();
            b.setAge(2);
            Gson gson = new Gson();
            System.out.println(gson.toJson(a));
            System.out.println(gson.toJson(b));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
class A {
    String name;
    int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

class B extends A {
    Date birthday;
    int age;

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Original issue reported on code.google.com by [e-mail] on 11 Jan 2012 at 3:50

[wxi]

I recently got below error: ERROR:class java.text.DecimalFormat declares multiple JSON fields named maximumIntegerDigits

The reason is I used “private final java.text.DecimalFormat” in a class which is deserialized by Gson. And DecimalFormat and its base class NumberFormat, both define “private int maximumIntegerDigits”. I fixed the problem by removing the field.

But my point is, as gson knows duplicate fields, if they are same type, can gson just deserialize the value to subclass instead of throwing an exception?

[JakeWharton]

I just told you the workaround: a type adapter which doesn’t serialize implementation details of classes outside your control. Use their public APIs to encode their data.

This is less of a workaround and more just the correct thing to do.

On Fri, Feb 5, 2016 at 11:39 AM William [e-mail] wrote:

Agree. @JakeWharton https://github.com/JakeWharton. Not a good way to have duplicate fields.

But the condition is if we cannot modify those classes and we have to use them, it’s better that gson provides a work around.

— Reply to this email directly or view it on GitHub https://github.com/google/gson/issues/399#issuecomment-180431956.