java.lang.IllegalStateException: Nesting problem.

I want to use custom TypeAdapter to serialize a nest class. But an exception occured:

Exception in thread "main" java.lang.IllegalStateException: Nesting problem.
	at com.google.gson.stream.JsonWriter.beforeValue(JsonWriter.java:655)
	at com.google.gson.stream.JsonWriter.open(JsonWriter.java:326)
	at com.google.gson.stream.JsonWriter.beginObject(JsonWriter.java:309)
	at com.example.gsonlib._01_basic_use.Test$AddressTypeAdapter.write(Test.java:30)
	at com.example.gsonlib._01_basic_use.Test$StudentTypeAdapter.write(Test.java:53)
	at com.example.gsonlib._01_basic_use.Test$StudentTypeAdapter.write(Test.java:41)
	at com.google.gson.Gson.toJson(Gson.java:704)
	at com.google.gson.Gson.toJson(Gson.java:683)
	at com.google.gson.Gson.toJson(Gson.java:638)
	at com.google.gson.Gson.toJson(Gson.java:618)
	at com.example.gsonlib._01_basic_use.Test.main(Test.java:22)

My code is the following:

public class Test {
    public static void main(String[] args) {
        Student student = new Student("wzc", 18, new Student.Address("earth"));
        Gson gson2 = new GsonBuilder()
                .serializeNulls()
                .registerTypeAdapter(Student.class, new StudentTypeAdapter(new AddressTypeAdapter()))
                .create();
        String json2 = gson2.toJson(student);
        System.out.println(json2);
    }

    static class AddressTypeAdapter extends TypeAdapter<Student.Address> {

        @Override
        public void write(JsonWriter out, Student.Address value) throws IOException {
            out.beginObject();
            out.name("address").value(value.nation);
            out.endObject();
        }

        @Override
        public Student.Address read(JsonReader in) throws IOException {
            return null;
        }
    }

    static class StudentTypeAdapter extends TypeAdapter<Student> {
        private AddressTypeAdapter mAddressTypeAdapter;

        public StudentTypeAdapter(AddressTypeAdapter addressTypeAdapter) {
            mAddressTypeAdapter = addressTypeAdapter;
        }

        @Override
        public void write(JsonWriter out, Student student) throws IOException {
            out.beginObject()
                    .name("name").value(student.name)
                    .name("age").value(student.age);
            mAddressTypeAdapter.write(out, student.adress);
            out.endObject();
        }

        @Override
        public Student read(JsonReader in) throws IOException {
            return null;
        }
    }
}

public class Student {
    public String name;
    public int age;
    public Address adress;

    public Student() {
    }

    public Student(String name, int age, Address adress) {
        this.name = name;
        this.age = age;
        this.adress = adress;
    }

    public static class Address {
        public String nation;

        public Address(String nation) {
            this.nation = nation;
        }
    }
}

[jhwsx]

Thanks a lot.

[lyubomyr-shaydariv]

@jhwsx yes, if you’re writing a JSON object (in curly braces {...}), you must write key/value pairs. You can’t put anonymous values in objects as this is disallowed by the JSON grammar, not Gson (any sane JSON tool would reject such a JSON document).